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Vulnerability of Northern Prairie Wetlands to Climate ChangeAuthor(s) W CARTER JOHNSON BRUCE V MILLETT TAGIR GILMANOV RICHARD AVOLDSETH GLENN R GUNTENSPERGEN and DAVID E NAUGLESource BioScience 55(10)863-872 2005Published By American Institute of Biological SciencesDOI httpdxdoiorg1016410006-3568(2005)055[0863VONPWT]20CO2URL httpwwwbiooneorgdoifull1016410006-3568282005290555B08633AVONPWT5D20CO3B2

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October 2005 Vol 55 No 10 bull BioScience 863

Articles

Northern prairie wetlands occupy the expansiveprairie pothole region (PPR) in central North America

(figure 1) The PPR covers approximately 800000 squarekilometers (km2) and is delineated by two elements (1) latePleistocene (Wisconsin) glaciation that left millions of smalldepressions on the landscape and (2) a relatively dry andstrongly seasonal climate supporting grassland vegetation(Tiner 2003)

The PPR and the larger Great Plains region extendingsouthward have a notoriously extreme and variable climate(Woodhouse and Overpeck 1998) The climate is punctuatedby severe droughts and deluges that influence both natural andhuman-dominated ecosystems For example drought in the1930s produced major geographic shifts of grassland com-munities killed millions of riparian trees and caused mas-sive economic losses to agriculture (Albertson and Weaver1942) Conversely deluges in the 1990s filled lakes and wet-lands and flooded farms towns and roads (Winter andRosenberry 1998 Johnson et al 2004 Shapley et al 2005)

These weather extremes are particularly important for thelong-term productivity and biodiversity of semipermanentprairie wetlands that occupy deeper basins than associated sea-sonal and temporary wetland classes (van der Valk and Davis1978) Low water and occasional drying of the wetland bot-tom during droughts (dry marsh phase) stimulate plant re-cruitment from a diverse seed bank and increase productivityby mobilizing nutrients In contrast high water during del-uges (lake marsh phase) causes turnover in plant populations

and creates greater interspersion of emergent cover and openwater but lowers overall productivity During a cover cycle thatranges from open water to complete vegetation cover annualnet primary productivity may vary 20-fold

The PPR is the single most productive habitat for water-fowl in the world In particular demographic analyses for mid-continent populations of mallard (Anas platyrhynchos) showthat approximately 90 of variation in population growth rateis associated with breeding activities that occur within the PPR(Austin 2002 Hoekman et al 2002) Wetland availability andemergent cover conditions are the primary factors that de-termine the number and diversity of breeding waterfowl thatwill settle in the PPR (Weller and Spatcher 1965) Waterfowlare adapted to exploit periodic shifts in wetland conditionsand are known to migrate past drought-stricken areas to set-tle in landscapes with an abundance of ponded wetlands

W Carter Johnson (e-mail carterjohnsonsdstateedu) is a professor of ecol-

ogy in the Department of Horticulture Forestry Landscape and Parks Bruce

V Millett is an instructor in the Department of Geography and Tagir

Gilmanov is a professor of biology in the Department of Biology and Micro-

biology at South Dakota State University Brookings SD 57007 Richard A

Voldseth is a research ecologist at the USDA Forest Servicersquos North-Central For-

est Experiment Station Grand Rapids MN 55744 Glenn R Guntenspergen

is a research ecologist with the US Geological Survey Patuxent Wildlife Re-

search Center Laurel MD 20708 David E Naugle is a professor of wildlife

at the University of Montana Missoula MT 59812 copy 2005 American Institute

of Biological Sciences

Vulnerability of Northern PrairieWetlands to Climate Change

W CARTER JOHNSON BRUCE V MILLETT TAGIR GILMANOV RICHARD A VOLDSETHGLENN R GUNTENSPERGEN AND DAVID E NAUGLE

The prairie pothole region (PPR) lies in the heart of North America and contains millions of glacially formed depressional wetlands embedded in a landscape matrix of natural grassland and agriculture These wetlands provide valuable ecosystem services and produce 50 to 80 of the continentrsquos ducks We explored the broad spatial and temporal patterns across the PPR between climate and wetland water levels and vegetation byapplying a wetland simulation model (WETSIM) to 18 stations with 95-year weather records Simulations suggest that the most productive habitatfor breeding waterfowl would shift under a drier climate from the center of the PPR (the Dakotas and southeastern Saskatchewan) to the wetter eastern and northern fringes areas currently less productive or where most wetlands have been drained Unless these wetlands are protected and restored there is little insurance for waterfowl against future climate warming WETSIM can assist wetland managers in allocating restoration dollars in an uncertain climate future

Keywords prairie wetlands climate change potholes waterfowl wetland restoration

(wetland basins with standing water) During times of wide-spread drought waterfowl may only find favorable conditionsnear the wetter northern and eastern fringes of the PPR orbeyond in northern Canada where wetlands are less pro-ductive but water levels are more stable

North American duck numbers correspond well with thenumber of ponded wetlands in the PPR at the start of thebreeding season (figure 2) The number of mallards hasranged from approximately seven million birds during wet pe-riods to two million birds during droughts Factors otherthan weather that also regulate continental waterfowl num-bers include harvest land use and density-dependent controls(Viljugrein et al 2005)

Climate is not uniform across the PPR Strong northndashsouthtemperature and eastndashwest precipitation gradients producedistinct regional climates ranging from relatively wet and sta-ble conditions in Iowa to the unstable dry climates of Albertaand Saskatchewan As a result wetlands often exist in differ-ent stages of the cover cycle across the PPR at any singlepoint in time Moreover the return time of the cover cycle (iethe time to complete one cycle) varies across the PPR accordingto climate variability Central portions of the PPR with mod-erate precipitation and temperature have the fastest returntimes

The well-established sensitivity of prairie wetlands to cur-rent climate variability portends a similarly sensitive response

to climate changeWeather extremes and climatic fluctuationsdrive hydrology which in turn drives key ecological processesin glaciated prairie wetlands These include wetland hy-droperiod ratio of emergent plant cover to open waterspecies composition water permanence class (ie temporaryseasonal semipermanent) and primary and secondary pro-ductivity among others (van der Valk 1989) Thus addi-tional climate variability of the magnitude suggested by globalclimate change models would profoundly affect wetland hy-drology and many other linked processes and attributes

The latest assessment from the Intergovernmental Panel onClimate Change based on several different models predictsincreases in global average surface temperatures ranging from14 degrees Celsius (ordmC) to 58ordmC by the year 2100 (Houghtonet al 2001) The temperatures increase most in the mid to highlatitudes of the Northern Hemisphere Precipitation is gen-erally predicted to increase in the northern latitudes and de-crease in the mid latitudes

Increased drought conditions in the PPR are forecast to occur under nearly all global circulation model scenariosRegional climate assessments (Ojima and Lackett 2002) suggest that the central and northern Great Plains of theUnited States may experience a 36ordmC to 61ordmC increase in meanair temperature over the next 100 years Longer growing seasons milder winters in the north hotter summers in thesouth and extreme drought are projected to be a more

864 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 1 The prairie pothole region of central North America (inset map) and six ecoregion subdivi-sions (adapted from Omernik 1995) Yellow symbols represent weather stations (three per ecoregion)used in WETSIM analyses

common occurrence over the PPR Trends in mean annualprecipitation are more difficult to predict and range from nochange to an increase of 10 to 20 concentrated in the fallwinter and spring accompanied by decreased summer pre-cipitation and a higher frequency of extreme spring and fallprecipitation events

In the past century temperatures across parts of the north-ern and central Great Plains have risen more than 3ordmC whileannual precipitation over the last 100 years has decreased by10 in eastern Montana and North Dakota (National As-sessment Synthesis Team 2000) Winter (2000) assessed thevulnerability of wetlands in glacial landscapes such as the PPRwhere wetland hydrology is dependent on interactions withatmospheric moisture and groundwater He predicted that inareas where groundwater movement through the glacial tillis slow wetlands will be highly vulnerable to climate changeIncreased summer temperatures in these midcontinental re-gions also will result in higher summer evapotranspirationrates putting increased demands on groundwater and re-sulting in earlier drying of wetlands Our research indicatesthat trends in the Palmer Drought Severity Index (PDSI) forthe PPR during the 20th century reflected increasing mois-ture availability for most weather stations however several sta-tions in the western Canadian Prairies recorded effectively drierconditions

Poiani and Johnson (1991) used a wetland simulationmodel (WETSIM) to show that a likely future climate (muchwarmer and slightly wetter) would produce poorer breedingconditions for waterfowl Their simulations for a semiper-manent wetland in east-central North Dakota showed ahigher frequency of dry basins with too much emergentcover for optimal breeding Larson (1995) and Sorenson andcolleagues (1998) projected similar habitat degradation forbreeding waterfowl in central North America under future cli-mate warming scenarios using statistical models based on his-toric relationships between PDSI and breeding waterfowlsurveys Sorenson and colleagues (1998) estimated that un-der a doubling of carbon dioxide by 2060 the north-centralUS duck population would be cut in half

No geographically extensive analyses of the quantitative re-lationships between climate and prairie wetland structureand function have been conducted for the PPR Most of theresearch progress in prairie wetland ecology has come fromintensively studied but geographically restricted sites Thebroader extrasite patterns have not been determined

We devised an analytical approach to determine the long-term geographic patterns of wetland conditions across the PPRdriven by historic and future climates This was accomplishedby making improvements to WETSIM (Poiani et al 1996) re-calibrating the model and quantifying geographic variabil-ity in wetland condition by applying the model to 18 PPRweather stations with 95-year records This methodology al-lowed us to compute various measures of wetland conditionsuch as hydroperiod drought frequency cover ratio cover cy-cle return time and water depth and variability across the cli-

mate space of the PPR under both historic and possible fu-ture conditions

The main goals of our research were to use WETSIM tocharacterize the historic temporal and geographic variabilityof wetland conditions across the PPR to identify in whichecoregions of the PPR wetlands are the most vulnerable to cli-mate variability and to determine how the most productivewaterfowl breeding areas may shift geographically under dif-ferent climates It was assumed that wetlands across the PPRare not equally vulnerable to climate variability The ecolog-ical and management implications of a geographic shift in theproductivity of wetlands across the PPR in response to climatechange would be considerable

Simulating wetland dynamicsWETSIM is a process-oriented deterministic model thatsimulates watershed and wetland surface processes watershedgroundwater and wetland vegetation dynamics The modeluses daily precipitation and mean daily temperature to estimate wetland water balance wetland stage and wetlandvegetation dynamics Simulations were conducted usingWETSIM 31 a next-generation wetland model upgradedfrom WETSIM 10 and 20 (Poiani et al 1996) The WETSIM31 upgrade included (a) conversion of the multiple plat-form WETSIM 20 to a single platform using Mathematicasoftware (b) replacement of the Blaney-Criddle equation tocalculate evapotranspiration by the Hargreaves (1994) equa-tion and latitude adjustment using the maximum possible solar radiation equation (Williams et al 1990) (c) replace-ment of estimates of groundwater discharge to the wetlandfrom EPIC (Sharpley and Williams 1990 Williams et al1990) with those from a simplified groundwater submodel derived from MODFLOW-96 with boundary conditions obtained from piezometer data (d) refinement of estimatesof wetland evapotranspiration into three cover categories(open water flooded emergent cover and emergent cover inwet unflooded soil) (e) development of an empirically based

October 2005 Vol 55 No 10 bull BioScience 865

Articles

Figure 2 Number of mallard ducks and May ponds forthe North American prairie pothole region 1975ndash2004Source US Fish and Wildlife Service (wwwfwsgovbirddata)

seasonally adjusted curve of leaf area index to estimate uplandevapotranspiration to replace EPICrsquos crop growth model (f)programming of the bucket model for soil water content inthe rooting zone and use of advanced numerical capabilitiesof Mathematica to iteratively solve the nonlinear soil mois-ture dynamics equation and (g) increased wetland cell sizefrom 9 square meters (m2) to 25 m2 to better match themonthly resolution of the vegetation submodel

WETSIM 31 was calibrated and tested using field data fromsemipermanent wetland P1 at the Cottonwood Lake study areain east-central North Dakota (figure 3) the same wetland usedto parameterize and evaluate earlier versions of WETSIMModel bathymetry was held constant during simulationsWetland P1 has the longest and most detailed hydrologicalrecord of any wetland in the PPR (Winter 2003) Systematicmeasurements and monitoring began in 1979 and continueto the present The testing of WETSIM 31 was more rigor-ous than for earlier model versions because the longer P1 dataset included much greater climatic variability by capturing therecord high water levels in the mid-1990s

The revised model accurately simulated the spring risesummer drawdown and interannual variability typical ofprairie wetlands and of P1 in particular (figure 4) The veg-

etation responded adequately to water-level dynamics judg-ing from historic photographs of P1

Moving the model geographically to other weather stationsrequired making adjustments The depth at which waterwould begin to flow out of the P1 basin was reduced from 52 m (not reached in modern times) to 14 m to enable themodel to potentially pass through most phases of the covercycle in wetter PPR climates Also the Hargreaves evapo-transpiration equation was adjusted by latitude to account fordifferences among stations in day length

We tested the geographic mobility of WETSIM 31 by us-ing data from another long-term prairie wetland monitoringsite in South Dakota (Johnson et al 2004) with a climatewarmer and wetter than that of wetland P1 in North DakotaThe model closely captured the historic water level and veg-etation dynamics of a semipermanent wetland at this site

The P1 basin from the Missouri Coteau may not be rep-resentative of wetland basin structure in other parts of the PPRUse of different basin structure for different ecoregions if dif-ferences were found to exist could accentuate or lessen thesimulated differences among weather stations and ecore-gions produced by the single-basin approach Connections be-tween groundwater and surface water are also known todiffer even among adjacent wetlands if there are systematic

866 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 3 The Cottonwood Lake study area in east-central North Dakota managed by the US Fishand Wildlife Service WETSIM a wetland simulation model was developed and tested using long-term monitoring data from wetland P1 located in the center of the scene (note white instrumentbarge) See Winter (2003) for site description Photograph courtesy of George Swanson (retired)Northern Prairie Wildlife Research Center

differences in groundwater fluxes among ecoregions thesecould temper the WETSIM results

Ecoregions of the prairie pothole region We subdivided the PPR into six ecoregions by combining USand Canadian approaches (figure 1) The ecoregions were usedas strata from which to select weather stations We chosethree widely separated weather stations from each ecoregionon the basis of their length and completeness of record Mostecoregions were oriented northndashsouth thus weather sta-tions were selected from northern central and southern lo-cations

We compiled a 95-year data set comprising 104097 records(daily precipitation minimum daily temperature and max-imum daily temperature) for each of the 18 weather sta-tions Missing data were replaced by interpolating from threenearby stations where possible We conducted accuracy as-

sessments by estimating known data from nearby stations Es-timates of temperature were more strongly correlated toknown values (86 moderate to strong correlations) thanwere estimates of precipitation (71 moderate to strongcorrelations) Error was higher for stations in less populatedareas with fewer nearby stations

Historic water levelsSimulated long-term water depths ranged widely amongPPR stations during the historic period (figure 5) Mean wa-ter depth for the historic period was greatest and nearest theoutlet level along the eastern and northern fringes of the PPR(Webster City and Algona Iowa Morris Minnesota RanfurlyAlberta) Average water levels were lowest along the north-western fringes of the PPR (032 m at Poplar Montana)Mean water depths at most weather stations ranged from 08to 11 m

Drought conditions followed a similar pattern across thePPR Droughts in the model wetland defined as less than 01m of standing water were shortest (5 to 15 days) or absentalong the eastern and northern fringes of the PPR (figure 6)Drought length was greatest (15 to 30 days) at northwesterlystations Muenster Saskatchewan was an outlier (nearly 80-day average drought length) caused by two exceptionallylong droughts Overall the length of the longest drought pe-riod correlated positively with mean length (except for Muen-ster) as did the percentage of time dry The Medicine HatAlberta and Poplar Montana stations were especiallydroughty being dry 15 and 25 of the time respectively(figure 6)

Geographic variability in wetland water budgets was evi-dent when we compared outflow volume among the PPRweather stations Outflow from the model wetland was in-dicative of the relative wetness of the PPR climates The high-

October 2005 Vol 55 No 10 bull BioScience 867

Articles

Figure 4 Simulations of stage levels of wetland P1 usingWETSIM (a) Comparison of simulated (solid line) andobserved (red dots) wetland stage levels (1960ndash2002) us-ing local precipitation data from the Woodworth NorthDakota weather station and temperature data from theJamestown North Dakota station (b) Stage levels simu-lated for 101 years using all weather data from theJamestown station and a 14-meter (m) outlet level

Figure 5 Mean daily wetland stage for 95 years at 18weather stations using WETSIM with a 14-meter (m)outlet level Ecoregion abbreviations CAP Canadian as-pen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern shortgrasslands NTG northern tall grasslands PC PrairieCoteau

est cumulative outflow of 2099366 m3 occurred at AlgonaOverflow at this station occurred in 87 of 95 years Overflowwas a rare event at Poplar and Medicine Hat each had onlyone overflow event in 95 years of about 4000 and 6000 m3respectively Large overflow volumes in the southeastern PPRmay have contributed to the development of an integrateddrainage network Smaller water surpluses in the central andnorthwestern PPR have maintained closed wetland basins withless integrated surface drainage

Historic vegetation response Spatial and temporal variability in modeled water condi-tions across the PPR produced correspondingly variable andcomplex responses from vegetation Temporal variability in-cluding droughts and deluges produced rapid changes inthe ratios between cover and open water at a single station

while spatial variability in weather across the PPR producedwidely differing cover ratios at a single point in time At thestation in Watertown South Dakota for example wetlanddrawdown during drought in the 1930s stimulated rapid ex-pansion of emergent cover and shrinkage of open water areaduring just eight consecutive growing-season months (figure7) A deluge period in the 1940s at Minot North Dakota pro-duced the opposite effect large increases in open water areaover a four-year period (figure 7)

868 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 6 Mean length and maximum length of dry peri-ods estimated by WETSIM model for prairie pothole region weather stations from 1906 to 2000 Size of circle corresponds to percent time dry Ranfurly (Alberta)Algona (Iowa) and Webster City (Iowa) had no dry periods Ecoregion abbreviations CAP Canadian aspenforests and parklands CTG central tall grasslands NMGnorthern mixed grasslands NSG northern short grass-lands NTG northern tall grasslands PC Prairie CoteauState and province abbreviations AB Alberta IA IowaMB Manitoba MN Minnesota MT Montana NDNorth Dakota SD South Dakota SK Saskatchewan

Figure 7 Response of WETSIM to temporal and spatialweather extremes Monthly water and cover conditionsare included for three periods drought (July 1933 to July1934 for the Watertown South Dakota weather station)deluge (August 1942 to July 1945 for the Minot NorthDakota weather station) and spatial variability (sixecoregion weather stations at a single point in time May1912) State and province abbreviations AB Alberta IAIowa MN Minnesota ND North Dakota SD SouthDakota SK Saskatchewan

Spatial variability was as striking when com-paring simulated cover ratios for six stations onein each ecoregion in May 1912 (figure 7) Themodel wetland indicated that at a single point intime dry marsh conditions existed in the west-ern Canadian prairies (Medicine Hat) balancedratios of cover to open water in the aspen park-lands of Canada (Muenster) and in the Dakotas(Minot and Watertown) and lake marsh con-ditions along the eastern PPR boundary (Crook-ston Minnesota and Algona)

Emergent cover averaged for each stationduring the 20th century ranged widely across thePPR from only 11 at Webster City to 99 atPoplar (figure 8) The most cover occurred in thewest-lying northern shortgrass ecoregion whilethe least cover occurred along the northern andeastern fringe of ecoregions bordering forestecosystems Moderate cover percentages most fa-vorable for waterfowl breeding formed a broadarc in the middle of the PPR

The simulated number of completions of thewetland cover cycle (return times) during the 95-year historic period ranged from zero to threeNearly half (48) of the PPR remained stuck inone or two of the four cover stages and did notcomplete a single cycle (figure 9) Nearly asmuch of the PPR (40) completed one cycle(95-year return time) while 10 and 2 of thePPR completed two (475-year return time) andthree (317-year return time) cycles respectively

Simulations of hydrology and vegetationclearly identified a broad northwestndashsoutheastrunning arc in the middle of the PPR as themost dynamic and as a result the most sup-portive for overall biodiversity in general and waterfowl breeding in particular Areas fartherwest become productive only rarely during es-pecially wet periods while wetlands farthernorth and east become so during dry periodsThe eastern Dakotas and southeasternSaskatchewan stand out in this analysis as hav-ing been the heart of the PPRrsquos ldquoduck factoryrdquoduring the 20th century

Future climate and wetlandsThe historic patterns of wetland dynamics and favorability forwaterfowl breeding across the PPR may shift in the futuredepending on the extent and magnitude of climate changeWe found that the PPR climate changed during the 20thcentury nearly all major weather stations examined becamewarmer but western stations became drier and eastern sta-tions wetter These results suggest that the historically strongwest-to-east moisture gradient across the PPR has steep-ened Since it cannot be known with certainty whether or notthe trends of the past century will continue we adopted the

equilibrium scenario approach to examine the possible effectsof climate change on PPR wetlands

The effects of three combinations of temperature andmoisture on wetland conditions were compared to the his-torical reference using WETSIM 31 for 6 of the 18 weatherstations one from nearest the center of each ecoregion Thesethree scenarios were (1) a 3ordmC temperature increase with nochange in precipitation (2) a 3ordmC temperature increase witha 20 increase in precipitation and (3) a 3ordmC temperature increase with a 20 decrease in precipitation The climate scenarios were applied uniformly across seasons to the historic weather data files

October 2005 Vol 55 No 10 bull BioScience 869

Articles

Figure 8 Simulated historic pattern of wetland emergent cover (a) acrossthe prairie pothole region and (b) by weather station Cover percentageswere scaled to a maximum potential open water area of 28125 square me-ters (m2) with an outlet level of 14 m Ecoregion abbreviations CAPCanadian aspen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern short grasslands NTGnorthern tall grasslands PC Prairie Coteau

The model was highly sensitive to alternative future climates A temperature increase alone produced moreemergent cover at the relatively wet stations in Iowaand South Dakota (figure 10) Cover ratios shiftedstrongly toward dense emergent cover under the warmertemperatures at the drier stations such as Medicine Hatand Minot Most stations spent more time in the drymarsh phase under the warmer-only climate scenario

Increasing both temperature and precipitation had acounterbalancing effect on water budgets producingonly a small change in cover compared with historic sim-ulations at most stationsWarmer and wetter conditionsat Algona Watertown and Medicine Hat producedcover ratios nearly identical to historic conditions whilehemimarsh conditions (ie conditions in which themix of emergent cover and open water is roughly even)declined slightly at the other stations (figure 10) It is ap-parent from this simulation that a 20 increase in pre-cipitation would generally compensate for a 3ordmC rise intemperature if applied uniformly Altering seasonal pat-terns of temperature and precipitation in WETSIMwould produce different results

Increased temperature and decreased precipitationhad the greatest effect on wetland conditions The modelwetland at five of the six stations became completelydominated by dry marsh conditions because of more fre-quent and longer drought (figure 10) Only Algonathe wettest station formed hemimarsh conditions moreoften under this temperature and precipitation regimethan under the two other climate scenarios Cover dy-namics at Algona under this driest condition were nearly

identical to the historic reference at Crookston lo-cated 250 km to the northwest

The geographic pattern of return times shiftedmarkedly with changes in temperature and pre-cipitation A change in temperature alone pushedthe region of fastest return times eastward wherethe generally wetter climate could accommodatethe greater evaporative demand (figure 9c) Theportion of the PPR with the fastest return timesshifted geographically from the eastern Dakotasand southeastern Saskatchewan to western Min-nesota and Iowa Return times in this easternfringe of the PPR increased two- to threefold Thewarmer temperatures increased drawdown fre-quency and magnitude thereby stimulating veg-etation regeneration

Increases in both temperature and precipitationproduced the most spatially dynamic result (fig-ure 9b) This scenario generated an area of four

870 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 9 Geographic patterns of the speed of thewetland cover cycle simulated for the prairie pot-hole region under historic (a) and alternative future(b c and d) climatic conditions

Figure 10 Simulated percentages of emergent cover and open waterfor six weather stations each in a different ecoregion under historicand alternative future climatic conditions ldquoClosedrdquo represents morethan 75 emergent cover ldquohemirdquo (hemimarsh) represents 25ndash75 emergent cover ldquoopenrdquo represents less than 25 emergent cover State and province abbreviations AB Alberta IA Iowa MN Minnesota ND North Dakota SD South Dakota SKSaskatchewan

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

(wetland basins with standing water) During times of wide-spread drought waterfowl may only find favorable conditionsnear the wetter northern and eastern fringes of the PPR orbeyond in northern Canada where wetlands are less pro-ductive but water levels are more stable

North American duck numbers correspond well with thenumber of ponded wetlands in the PPR at the start of thebreeding season (figure 2) The number of mallards hasranged from approximately seven million birds during wet pe-riods to two million birds during droughts Factors otherthan weather that also regulate continental waterfowl num-bers include harvest land use and density-dependent controls(Viljugrein et al 2005)

Climate is not uniform across the PPR Strong northndashsouthtemperature and eastndashwest precipitation gradients producedistinct regional climates ranging from relatively wet and sta-ble conditions in Iowa to the unstable dry climates of Albertaand Saskatchewan As a result wetlands often exist in differ-ent stages of the cover cycle across the PPR at any singlepoint in time Moreover the return time of the cover cycle (iethe time to complete one cycle) varies across the PPR accordingto climate variability Central portions of the PPR with mod-erate precipitation and temperature have the fastest returntimes

The well-established sensitivity of prairie wetlands to cur-rent climate variability portends a similarly sensitive response

to climate changeWeather extremes and climatic fluctuationsdrive hydrology which in turn drives key ecological processesin glaciated prairie wetlands These include wetland hy-droperiod ratio of emergent plant cover to open waterspecies composition water permanence class (ie temporaryseasonal semipermanent) and primary and secondary pro-ductivity among others (van der Valk 1989) Thus addi-tional climate variability of the magnitude suggested by globalclimate change models would profoundly affect wetland hy-drology and many other linked processes and attributes

The latest assessment from the Intergovernmental Panel onClimate Change based on several different models predictsincreases in global average surface temperatures ranging from14 degrees Celsius (ordmC) to 58ordmC by the year 2100 (Houghtonet al 2001) The temperatures increase most in the mid to highlatitudes of the Northern Hemisphere Precipitation is gen-erally predicted to increase in the northern latitudes and de-crease in the mid latitudes

Increased drought conditions in the PPR are forecast to occur under nearly all global circulation model scenariosRegional climate assessments (Ojima and Lackett 2002) suggest that the central and northern Great Plains of theUnited States may experience a 36ordmC to 61ordmC increase in meanair temperature over the next 100 years Longer growing seasons milder winters in the north hotter summers in thesouth and extreme drought are projected to be a more

864 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 1 The prairie pothole region of central North America (inset map) and six ecoregion subdivi-sions (adapted from Omernik 1995) Yellow symbols represent weather stations (three per ecoregion)used in WETSIM analyses

common occurrence over the PPR Trends in mean annualprecipitation are more difficult to predict and range from nochange to an increase of 10 to 20 concentrated in the fallwinter and spring accompanied by decreased summer pre-cipitation and a higher frequency of extreme spring and fallprecipitation events

In the past century temperatures across parts of the north-ern and central Great Plains have risen more than 3ordmC whileannual precipitation over the last 100 years has decreased by10 in eastern Montana and North Dakota (National As-sessment Synthesis Team 2000) Winter (2000) assessed thevulnerability of wetlands in glacial landscapes such as the PPRwhere wetland hydrology is dependent on interactions withatmospheric moisture and groundwater He predicted that inareas where groundwater movement through the glacial tillis slow wetlands will be highly vulnerable to climate changeIncreased summer temperatures in these midcontinental re-gions also will result in higher summer evapotranspirationrates putting increased demands on groundwater and re-sulting in earlier drying of wetlands Our research indicatesthat trends in the Palmer Drought Severity Index (PDSI) forthe PPR during the 20th century reflected increasing mois-ture availability for most weather stations however several sta-tions in the western Canadian Prairies recorded effectively drierconditions

Poiani and Johnson (1991) used a wetland simulationmodel (WETSIM) to show that a likely future climate (muchwarmer and slightly wetter) would produce poorer breedingconditions for waterfowl Their simulations for a semiper-manent wetland in east-central North Dakota showed ahigher frequency of dry basins with too much emergentcover for optimal breeding Larson (1995) and Sorenson andcolleagues (1998) projected similar habitat degradation forbreeding waterfowl in central North America under future cli-mate warming scenarios using statistical models based on his-toric relationships between PDSI and breeding waterfowlsurveys Sorenson and colleagues (1998) estimated that un-der a doubling of carbon dioxide by 2060 the north-centralUS duck population would be cut in half

No geographically extensive analyses of the quantitative re-lationships between climate and prairie wetland structureand function have been conducted for the PPR Most of theresearch progress in prairie wetland ecology has come fromintensively studied but geographically restricted sites Thebroader extrasite patterns have not been determined

We devised an analytical approach to determine the long-term geographic patterns of wetland conditions across the PPRdriven by historic and future climates This was accomplishedby making improvements to WETSIM (Poiani et al 1996) re-calibrating the model and quantifying geographic variabil-ity in wetland condition by applying the model to 18 PPRweather stations with 95-year records This methodology al-lowed us to compute various measures of wetland conditionsuch as hydroperiod drought frequency cover ratio cover cy-cle return time and water depth and variability across the cli-

mate space of the PPR under both historic and possible fu-ture conditions

The main goals of our research were to use WETSIM tocharacterize the historic temporal and geographic variabilityof wetland conditions across the PPR to identify in whichecoregions of the PPR wetlands are the most vulnerable to cli-mate variability and to determine how the most productivewaterfowl breeding areas may shift geographically under dif-ferent climates It was assumed that wetlands across the PPRare not equally vulnerable to climate variability The ecolog-ical and management implications of a geographic shift in theproductivity of wetlands across the PPR in response to climatechange would be considerable

Simulating wetland dynamicsWETSIM is a process-oriented deterministic model thatsimulates watershed and wetland surface processes watershedgroundwater and wetland vegetation dynamics The modeluses daily precipitation and mean daily temperature to estimate wetland water balance wetland stage and wetlandvegetation dynamics Simulations were conducted usingWETSIM 31 a next-generation wetland model upgradedfrom WETSIM 10 and 20 (Poiani et al 1996) The WETSIM31 upgrade included (a) conversion of the multiple plat-form WETSIM 20 to a single platform using Mathematicasoftware (b) replacement of the Blaney-Criddle equation tocalculate evapotranspiration by the Hargreaves (1994) equa-tion and latitude adjustment using the maximum possible solar radiation equation (Williams et al 1990) (c) replace-ment of estimates of groundwater discharge to the wetlandfrom EPIC (Sharpley and Williams 1990 Williams et al1990) with those from a simplified groundwater submodel derived from MODFLOW-96 with boundary conditions obtained from piezometer data (d) refinement of estimatesof wetland evapotranspiration into three cover categories(open water flooded emergent cover and emergent cover inwet unflooded soil) (e) development of an empirically based

October 2005 Vol 55 No 10 bull BioScience 865

Articles

Figure 2 Number of mallard ducks and May ponds forthe North American prairie pothole region 1975ndash2004Source US Fish and Wildlife Service (wwwfwsgovbirddata)

seasonally adjusted curve of leaf area index to estimate uplandevapotranspiration to replace EPICrsquos crop growth model (f)programming of the bucket model for soil water content inthe rooting zone and use of advanced numerical capabilitiesof Mathematica to iteratively solve the nonlinear soil mois-ture dynamics equation and (g) increased wetland cell sizefrom 9 square meters (m2) to 25 m2 to better match themonthly resolution of the vegetation submodel

WETSIM 31 was calibrated and tested using field data fromsemipermanent wetland P1 at the Cottonwood Lake study areain east-central North Dakota (figure 3) the same wetland usedto parameterize and evaluate earlier versions of WETSIMModel bathymetry was held constant during simulationsWetland P1 has the longest and most detailed hydrologicalrecord of any wetland in the PPR (Winter 2003) Systematicmeasurements and monitoring began in 1979 and continueto the present The testing of WETSIM 31 was more rigor-ous than for earlier model versions because the longer P1 dataset included much greater climatic variability by capturing therecord high water levels in the mid-1990s

The revised model accurately simulated the spring risesummer drawdown and interannual variability typical ofprairie wetlands and of P1 in particular (figure 4) The veg-

etation responded adequately to water-level dynamics judg-ing from historic photographs of P1

Moving the model geographically to other weather stationsrequired making adjustments The depth at which waterwould begin to flow out of the P1 basin was reduced from 52 m (not reached in modern times) to 14 m to enable themodel to potentially pass through most phases of the covercycle in wetter PPR climates Also the Hargreaves evapo-transpiration equation was adjusted by latitude to account fordifferences among stations in day length

We tested the geographic mobility of WETSIM 31 by us-ing data from another long-term prairie wetland monitoringsite in South Dakota (Johnson et al 2004) with a climatewarmer and wetter than that of wetland P1 in North DakotaThe model closely captured the historic water level and veg-etation dynamics of a semipermanent wetland at this site

The P1 basin from the Missouri Coteau may not be rep-resentative of wetland basin structure in other parts of the PPRUse of different basin structure for different ecoregions if dif-ferences were found to exist could accentuate or lessen thesimulated differences among weather stations and ecore-gions produced by the single-basin approach Connections be-tween groundwater and surface water are also known todiffer even among adjacent wetlands if there are systematic

866 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 3 The Cottonwood Lake study area in east-central North Dakota managed by the US Fishand Wildlife Service WETSIM a wetland simulation model was developed and tested using long-term monitoring data from wetland P1 located in the center of the scene (note white instrumentbarge) See Winter (2003) for site description Photograph courtesy of George Swanson (retired)Northern Prairie Wildlife Research Center

differences in groundwater fluxes among ecoregions thesecould temper the WETSIM results

Ecoregions of the prairie pothole region We subdivided the PPR into six ecoregions by combining USand Canadian approaches (figure 1) The ecoregions were usedas strata from which to select weather stations We chosethree widely separated weather stations from each ecoregionon the basis of their length and completeness of record Mostecoregions were oriented northndashsouth thus weather sta-tions were selected from northern central and southern lo-cations

We compiled a 95-year data set comprising 104097 records(daily precipitation minimum daily temperature and max-imum daily temperature) for each of the 18 weather sta-tions Missing data were replaced by interpolating from threenearby stations where possible We conducted accuracy as-

sessments by estimating known data from nearby stations Es-timates of temperature were more strongly correlated toknown values (86 moderate to strong correlations) thanwere estimates of precipitation (71 moderate to strongcorrelations) Error was higher for stations in less populatedareas with fewer nearby stations

Historic water levelsSimulated long-term water depths ranged widely amongPPR stations during the historic period (figure 5) Mean wa-ter depth for the historic period was greatest and nearest theoutlet level along the eastern and northern fringes of the PPR(Webster City and Algona Iowa Morris Minnesota RanfurlyAlberta) Average water levels were lowest along the north-western fringes of the PPR (032 m at Poplar Montana)Mean water depths at most weather stations ranged from 08to 11 m

Drought conditions followed a similar pattern across thePPR Droughts in the model wetland defined as less than 01m of standing water were shortest (5 to 15 days) or absentalong the eastern and northern fringes of the PPR (figure 6)Drought length was greatest (15 to 30 days) at northwesterlystations Muenster Saskatchewan was an outlier (nearly 80-day average drought length) caused by two exceptionallylong droughts Overall the length of the longest drought pe-riod correlated positively with mean length (except for Muen-ster) as did the percentage of time dry The Medicine HatAlberta and Poplar Montana stations were especiallydroughty being dry 15 and 25 of the time respectively(figure 6)

Geographic variability in wetland water budgets was evi-dent when we compared outflow volume among the PPRweather stations Outflow from the model wetland was in-dicative of the relative wetness of the PPR climates The high-

October 2005 Vol 55 No 10 bull BioScience 867

Articles

Figure 4 Simulations of stage levels of wetland P1 usingWETSIM (a) Comparison of simulated (solid line) andobserved (red dots) wetland stage levels (1960ndash2002) us-ing local precipitation data from the Woodworth NorthDakota weather station and temperature data from theJamestown North Dakota station (b) Stage levels simu-lated for 101 years using all weather data from theJamestown station and a 14-meter (m) outlet level

Figure 5 Mean daily wetland stage for 95 years at 18weather stations using WETSIM with a 14-meter (m)outlet level Ecoregion abbreviations CAP Canadian as-pen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern shortgrasslands NTG northern tall grasslands PC PrairieCoteau

est cumulative outflow of 2099366 m3 occurred at AlgonaOverflow at this station occurred in 87 of 95 years Overflowwas a rare event at Poplar and Medicine Hat each had onlyone overflow event in 95 years of about 4000 and 6000 m3respectively Large overflow volumes in the southeastern PPRmay have contributed to the development of an integrateddrainage network Smaller water surpluses in the central andnorthwestern PPR have maintained closed wetland basins withless integrated surface drainage

Historic vegetation response Spatial and temporal variability in modeled water condi-tions across the PPR produced correspondingly variable andcomplex responses from vegetation Temporal variability in-cluding droughts and deluges produced rapid changes inthe ratios between cover and open water at a single station

while spatial variability in weather across the PPR producedwidely differing cover ratios at a single point in time At thestation in Watertown South Dakota for example wetlanddrawdown during drought in the 1930s stimulated rapid ex-pansion of emergent cover and shrinkage of open water areaduring just eight consecutive growing-season months (figure7) A deluge period in the 1940s at Minot North Dakota pro-duced the opposite effect large increases in open water areaover a four-year period (figure 7)

868 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 6 Mean length and maximum length of dry peri-ods estimated by WETSIM model for prairie pothole region weather stations from 1906 to 2000 Size of circle corresponds to percent time dry Ranfurly (Alberta)Algona (Iowa) and Webster City (Iowa) had no dry periods Ecoregion abbreviations CAP Canadian aspenforests and parklands CTG central tall grasslands NMGnorthern mixed grasslands NSG northern short grass-lands NTG northern tall grasslands PC Prairie CoteauState and province abbreviations AB Alberta IA IowaMB Manitoba MN Minnesota MT Montana NDNorth Dakota SD South Dakota SK Saskatchewan

Figure 7 Response of WETSIM to temporal and spatialweather extremes Monthly water and cover conditionsare included for three periods drought (July 1933 to July1934 for the Watertown South Dakota weather station)deluge (August 1942 to July 1945 for the Minot NorthDakota weather station) and spatial variability (sixecoregion weather stations at a single point in time May1912) State and province abbreviations AB Alberta IAIowa MN Minnesota ND North Dakota SD SouthDakota SK Saskatchewan

Spatial variability was as striking when com-paring simulated cover ratios for six stations onein each ecoregion in May 1912 (figure 7) Themodel wetland indicated that at a single point intime dry marsh conditions existed in the west-ern Canadian prairies (Medicine Hat) balancedratios of cover to open water in the aspen park-lands of Canada (Muenster) and in the Dakotas(Minot and Watertown) and lake marsh con-ditions along the eastern PPR boundary (Crook-ston Minnesota and Algona)

Emergent cover averaged for each stationduring the 20th century ranged widely across thePPR from only 11 at Webster City to 99 atPoplar (figure 8) The most cover occurred in thewest-lying northern shortgrass ecoregion whilethe least cover occurred along the northern andeastern fringe of ecoregions bordering forestecosystems Moderate cover percentages most fa-vorable for waterfowl breeding formed a broadarc in the middle of the PPR

The simulated number of completions of thewetland cover cycle (return times) during the 95-year historic period ranged from zero to threeNearly half (48) of the PPR remained stuck inone or two of the four cover stages and did notcomplete a single cycle (figure 9) Nearly asmuch of the PPR (40) completed one cycle(95-year return time) while 10 and 2 of thePPR completed two (475-year return time) andthree (317-year return time) cycles respectively

Simulations of hydrology and vegetationclearly identified a broad northwestndashsoutheastrunning arc in the middle of the PPR as themost dynamic and as a result the most sup-portive for overall biodiversity in general and waterfowl breeding in particular Areas fartherwest become productive only rarely during es-pecially wet periods while wetlands farthernorth and east become so during dry periodsThe eastern Dakotas and southeasternSaskatchewan stand out in this analysis as hav-ing been the heart of the PPRrsquos ldquoduck factoryrdquoduring the 20th century

Future climate and wetlandsThe historic patterns of wetland dynamics and favorability forwaterfowl breeding across the PPR may shift in the futuredepending on the extent and magnitude of climate changeWe found that the PPR climate changed during the 20thcentury nearly all major weather stations examined becamewarmer but western stations became drier and eastern sta-tions wetter These results suggest that the historically strongwest-to-east moisture gradient across the PPR has steep-ened Since it cannot be known with certainty whether or notthe trends of the past century will continue we adopted the

equilibrium scenario approach to examine the possible effectsof climate change on PPR wetlands

The effects of three combinations of temperature andmoisture on wetland conditions were compared to the his-torical reference using WETSIM 31 for 6 of the 18 weatherstations one from nearest the center of each ecoregion Thesethree scenarios were (1) a 3ordmC temperature increase with nochange in precipitation (2) a 3ordmC temperature increase witha 20 increase in precipitation and (3) a 3ordmC temperature increase with a 20 decrease in precipitation The climate scenarios were applied uniformly across seasons to the historic weather data files

October 2005 Vol 55 No 10 bull BioScience 869

Articles

Figure 8 Simulated historic pattern of wetland emergent cover (a) acrossthe prairie pothole region and (b) by weather station Cover percentageswere scaled to a maximum potential open water area of 28125 square me-ters (m2) with an outlet level of 14 m Ecoregion abbreviations CAPCanadian aspen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern short grasslands NTGnorthern tall grasslands PC Prairie Coteau

The model was highly sensitive to alternative future climates A temperature increase alone produced moreemergent cover at the relatively wet stations in Iowaand South Dakota (figure 10) Cover ratios shiftedstrongly toward dense emergent cover under the warmertemperatures at the drier stations such as Medicine Hatand Minot Most stations spent more time in the drymarsh phase under the warmer-only climate scenario

Increasing both temperature and precipitation had acounterbalancing effect on water budgets producingonly a small change in cover compared with historic sim-ulations at most stationsWarmer and wetter conditionsat Algona Watertown and Medicine Hat producedcover ratios nearly identical to historic conditions whilehemimarsh conditions (ie conditions in which themix of emergent cover and open water is roughly even)declined slightly at the other stations (figure 10) It is ap-parent from this simulation that a 20 increase in pre-cipitation would generally compensate for a 3ordmC rise intemperature if applied uniformly Altering seasonal pat-terns of temperature and precipitation in WETSIMwould produce different results

Increased temperature and decreased precipitationhad the greatest effect on wetland conditions The modelwetland at five of the six stations became completelydominated by dry marsh conditions because of more fre-quent and longer drought (figure 10) Only Algonathe wettest station formed hemimarsh conditions moreoften under this temperature and precipitation regimethan under the two other climate scenarios Cover dy-namics at Algona under this driest condition were nearly

identical to the historic reference at Crookston lo-cated 250 km to the northwest

The geographic pattern of return times shiftedmarkedly with changes in temperature and pre-cipitation A change in temperature alone pushedthe region of fastest return times eastward wherethe generally wetter climate could accommodatethe greater evaporative demand (figure 9c) Theportion of the PPR with the fastest return timesshifted geographically from the eastern Dakotasand southeastern Saskatchewan to western Min-nesota and Iowa Return times in this easternfringe of the PPR increased two- to threefold Thewarmer temperatures increased drawdown fre-quency and magnitude thereby stimulating veg-etation regeneration

Increases in both temperature and precipitationproduced the most spatially dynamic result (fig-ure 9b) This scenario generated an area of four

870 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 9 Geographic patterns of the speed of thewetland cover cycle simulated for the prairie pot-hole region under historic (a) and alternative future(b c and d) climatic conditions

Figure 10 Simulated percentages of emergent cover and open waterfor six weather stations each in a different ecoregion under historicand alternative future climatic conditions ldquoClosedrdquo represents morethan 75 emergent cover ldquohemirdquo (hemimarsh) represents 25ndash75 emergent cover ldquoopenrdquo represents less than 25 emergent cover State and province abbreviations AB Alberta IA Iowa MN Minnesota ND North Dakota SD South Dakota SKSaskatchewan

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

common occurrence over the PPR Trends in mean annualprecipitation are more difficult to predict and range from nochange to an increase of 10 to 20 concentrated in the fallwinter and spring accompanied by decreased summer pre-cipitation and a higher frequency of extreme spring and fallprecipitation events

In the past century temperatures across parts of the north-ern and central Great Plains have risen more than 3ordmC whileannual precipitation over the last 100 years has decreased by10 in eastern Montana and North Dakota (National As-sessment Synthesis Team 2000) Winter (2000) assessed thevulnerability of wetlands in glacial landscapes such as the PPRwhere wetland hydrology is dependent on interactions withatmospheric moisture and groundwater He predicted that inareas where groundwater movement through the glacial tillis slow wetlands will be highly vulnerable to climate changeIncreased summer temperatures in these midcontinental re-gions also will result in higher summer evapotranspirationrates putting increased demands on groundwater and re-sulting in earlier drying of wetlands Our research indicatesthat trends in the Palmer Drought Severity Index (PDSI) forthe PPR during the 20th century reflected increasing mois-ture availability for most weather stations however several sta-tions in the western Canadian Prairies recorded effectively drierconditions

Poiani and Johnson (1991) used a wetland simulationmodel (WETSIM) to show that a likely future climate (muchwarmer and slightly wetter) would produce poorer breedingconditions for waterfowl Their simulations for a semiper-manent wetland in east-central North Dakota showed ahigher frequency of dry basins with too much emergentcover for optimal breeding Larson (1995) and Sorenson andcolleagues (1998) projected similar habitat degradation forbreeding waterfowl in central North America under future cli-mate warming scenarios using statistical models based on his-toric relationships between PDSI and breeding waterfowlsurveys Sorenson and colleagues (1998) estimated that un-der a doubling of carbon dioxide by 2060 the north-centralUS duck population would be cut in half

No geographically extensive analyses of the quantitative re-lationships between climate and prairie wetland structureand function have been conducted for the PPR Most of theresearch progress in prairie wetland ecology has come fromintensively studied but geographically restricted sites Thebroader extrasite patterns have not been determined

We devised an analytical approach to determine the long-term geographic patterns of wetland conditions across the PPRdriven by historic and future climates This was accomplishedby making improvements to WETSIM (Poiani et al 1996) re-calibrating the model and quantifying geographic variabil-ity in wetland condition by applying the model to 18 PPRweather stations with 95-year records This methodology al-lowed us to compute various measures of wetland conditionsuch as hydroperiod drought frequency cover ratio cover cy-cle return time and water depth and variability across the cli-

mate space of the PPR under both historic and possible fu-ture conditions

The main goals of our research were to use WETSIM tocharacterize the historic temporal and geographic variabilityof wetland conditions across the PPR to identify in whichecoregions of the PPR wetlands are the most vulnerable to cli-mate variability and to determine how the most productivewaterfowl breeding areas may shift geographically under dif-ferent climates It was assumed that wetlands across the PPRare not equally vulnerable to climate variability The ecolog-ical and management implications of a geographic shift in theproductivity of wetlands across the PPR in response to climatechange would be considerable

Simulating wetland dynamicsWETSIM is a process-oriented deterministic model thatsimulates watershed and wetland surface processes watershedgroundwater and wetland vegetation dynamics The modeluses daily precipitation and mean daily temperature to estimate wetland water balance wetland stage and wetlandvegetation dynamics Simulations were conducted usingWETSIM 31 a next-generation wetland model upgradedfrom WETSIM 10 and 20 (Poiani et al 1996) The WETSIM31 upgrade included (a) conversion of the multiple plat-form WETSIM 20 to a single platform using Mathematicasoftware (b) replacement of the Blaney-Criddle equation tocalculate evapotranspiration by the Hargreaves (1994) equa-tion and latitude adjustment using the maximum possible solar radiation equation (Williams et al 1990) (c) replace-ment of estimates of groundwater discharge to the wetlandfrom EPIC (Sharpley and Williams 1990 Williams et al1990) with those from a simplified groundwater submodel derived from MODFLOW-96 with boundary conditions obtained from piezometer data (d) refinement of estimatesof wetland evapotranspiration into three cover categories(open water flooded emergent cover and emergent cover inwet unflooded soil) (e) development of an empirically based

October 2005 Vol 55 No 10 bull BioScience 865

Articles

Figure 2 Number of mallard ducks and May ponds forthe North American prairie pothole region 1975ndash2004Source US Fish and Wildlife Service (wwwfwsgovbirddata)

seasonally adjusted curve of leaf area index to estimate uplandevapotranspiration to replace EPICrsquos crop growth model (f)programming of the bucket model for soil water content inthe rooting zone and use of advanced numerical capabilitiesof Mathematica to iteratively solve the nonlinear soil mois-ture dynamics equation and (g) increased wetland cell sizefrom 9 square meters (m2) to 25 m2 to better match themonthly resolution of the vegetation submodel

WETSIM 31 was calibrated and tested using field data fromsemipermanent wetland P1 at the Cottonwood Lake study areain east-central North Dakota (figure 3) the same wetland usedto parameterize and evaluate earlier versions of WETSIMModel bathymetry was held constant during simulationsWetland P1 has the longest and most detailed hydrologicalrecord of any wetland in the PPR (Winter 2003) Systematicmeasurements and monitoring began in 1979 and continueto the present The testing of WETSIM 31 was more rigor-ous than for earlier model versions because the longer P1 dataset included much greater climatic variability by capturing therecord high water levels in the mid-1990s

The revised model accurately simulated the spring risesummer drawdown and interannual variability typical ofprairie wetlands and of P1 in particular (figure 4) The veg-

etation responded adequately to water-level dynamics judg-ing from historic photographs of P1

Moving the model geographically to other weather stationsrequired making adjustments The depth at which waterwould begin to flow out of the P1 basin was reduced from 52 m (not reached in modern times) to 14 m to enable themodel to potentially pass through most phases of the covercycle in wetter PPR climates Also the Hargreaves evapo-transpiration equation was adjusted by latitude to account fordifferences among stations in day length

We tested the geographic mobility of WETSIM 31 by us-ing data from another long-term prairie wetland monitoringsite in South Dakota (Johnson et al 2004) with a climatewarmer and wetter than that of wetland P1 in North DakotaThe model closely captured the historic water level and veg-etation dynamics of a semipermanent wetland at this site

The P1 basin from the Missouri Coteau may not be rep-resentative of wetland basin structure in other parts of the PPRUse of different basin structure for different ecoregions if dif-ferences were found to exist could accentuate or lessen thesimulated differences among weather stations and ecore-gions produced by the single-basin approach Connections be-tween groundwater and surface water are also known todiffer even among adjacent wetlands if there are systematic

866 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 3 The Cottonwood Lake study area in east-central North Dakota managed by the US Fishand Wildlife Service WETSIM a wetland simulation model was developed and tested using long-term monitoring data from wetland P1 located in the center of the scene (note white instrumentbarge) See Winter (2003) for site description Photograph courtesy of George Swanson (retired)Northern Prairie Wildlife Research Center

differences in groundwater fluxes among ecoregions thesecould temper the WETSIM results

Ecoregions of the prairie pothole region We subdivided the PPR into six ecoregions by combining USand Canadian approaches (figure 1) The ecoregions were usedas strata from which to select weather stations We chosethree widely separated weather stations from each ecoregionon the basis of their length and completeness of record Mostecoregions were oriented northndashsouth thus weather sta-tions were selected from northern central and southern lo-cations

We compiled a 95-year data set comprising 104097 records(daily precipitation minimum daily temperature and max-imum daily temperature) for each of the 18 weather sta-tions Missing data were replaced by interpolating from threenearby stations where possible We conducted accuracy as-

sessments by estimating known data from nearby stations Es-timates of temperature were more strongly correlated toknown values (86 moderate to strong correlations) thanwere estimates of precipitation (71 moderate to strongcorrelations) Error was higher for stations in less populatedareas with fewer nearby stations

Historic water levelsSimulated long-term water depths ranged widely amongPPR stations during the historic period (figure 5) Mean wa-ter depth for the historic period was greatest and nearest theoutlet level along the eastern and northern fringes of the PPR(Webster City and Algona Iowa Morris Minnesota RanfurlyAlberta) Average water levels were lowest along the north-western fringes of the PPR (032 m at Poplar Montana)Mean water depths at most weather stations ranged from 08to 11 m

Drought conditions followed a similar pattern across thePPR Droughts in the model wetland defined as less than 01m of standing water were shortest (5 to 15 days) or absentalong the eastern and northern fringes of the PPR (figure 6)Drought length was greatest (15 to 30 days) at northwesterlystations Muenster Saskatchewan was an outlier (nearly 80-day average drought length) caused by two exceptionallylong droughts Overall the length of the longest drought pe-riod correlated positively with mean length (except for Muen-ster) as did the percentage of time dry The Medicine HatAlberta and Poplar Montana stations were especiallydroughty being dry 15 and 25 of the time respectively(figure 6)

Geographic variability in wetland water budgets was evi-dent when we compared outflow volume among the PPRweather stations Outflow from the model wetland was in-dicative of the relative wetness of the PPR climates The high-

October 2005 Vol 55 No 10 bull BioScience 867

Articles

Figure 4 Simulations of stage levels of wetland P1 usingWETSIM (a) Comparison of simulated (solid line) andobserved (red dots) wetland stage levels (1960ndash2002) us-ing local precipitation data from the Woodworth NorthDakota weather station and temperature data from theJamestown North Dakota station (b) Stage levels simu-lated for 101 years using all weather data from theJamestown station and a 14-meter (m) outlet level

Figure 5 Mean daily wetland stage for 95 years at 18weather stations using WETSIM with a 14-meter (m)outlet level Ecoregion abbreviations CAP Canadian as-pen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern shortgrasslands NTG northern tall grasslands PC PrairieCoteau

est cumulative outflow of 2099366 m3 occurred at AlgonaOverflow at this station occurred in 87 of 95 years Overflowwas a rare event at Poplar and Medicine Hat each had onlyone overflow event in 95 years of about 4000 and 6000 m3respectively Large overflow volumes in the southeastern PPRmay have contributed to the development of an integrateddrainage network Smaller water surpluses in the central andnorthwestern PPR have maintained closed wetland basins withless integrated surface drainage

Historic vegetation response Spatial and temporal variability in modeled water condi-tions across the PPR produced correspondingly variable andcomplex responses from vegetation Temporal variability in-cluding droughts and deluges produced rapid changes inthe ratios between cover and open water at a single station

while spatial variability in weather across the PPR producedwidely differing cover ratios at a single point in time At thestation in Watertown South Dakota for example wetlanddrawdown during drought in the 1930s stimulated rapid ex-pansion of emergent cover and shrinkage of open water areaduring just eight consecutive growing-season months (figure7) A deluge period in the 1940s at Minot North Dakota pro-duced the opposite effect large increases in open water areaover a four-year period (figure 7)

868 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 6 Mean length and maximum length of dry peri-ods estimated by WETSIM model for prairie pothole region weather stations from 1906 to 2000 Size of circle corresponds to percent time dry Ranfurly (Alberta)Algona (Iowa) and Webster City (Iowa) had no dry periods Ecoregion abbreviations CAP Canadian aspenforests and parklands CTG central tall grasslands NMGnorthern mixed grasslands NSG northern short grass-lands NTG northern tall grasslands PC Prairie CoteauState and province abbreviations AB Alberta IA IowaMB Manitoba MN Minnesota MT Montana NDNorth Dakota SD South Dakota SK Saskatchewan

Figure 7 Response of WETSIM to temporal and spatialweather extremes Monthly water and cover conditionsare included for three periods drought (July 1933 to July1934 for the Watertown South Dakota weather station)deluge (August 1942 to July 1945 for the Minot NorthDakota weather station) and spatial variability (sixecoregion weather stations at a single point in time May1912) State and province abbreviations AB Alberta IAIowa MN Minnesota ND North Dakota SD SouthDakota SK Saskatchewan

Spatial variability was as striking when com-paring simulated cover ratios for six stations onein each ecoregion in May 1912 (figure 7) Themodel wetland indicated that at a single point intime dry marsh conditions existed in the west-ern Canadian prairies (Medicine Hat) balancedratios of cover to open water in the aspen park-lands of Canada (Muenster) and in the Dakotas(Minot and Watertown) and lake marsh con-ditions along the eastern PPR boundary (Crook-ston Minnesota and Algona)

Emergent cover averaged for each stationduring the 20th century ranged widely across thePPR from only 11 at Webster City to 99 atPoplar (figure 8) The most cover occurred in thewest-lying northern shortgrass ecoregion whilethe least cover occurred along the northern andeastern fringe of ecoregions bordering forestecosystems Moderate cover percentages most fa-vorable for waterfowl breeding formed a broadarc in the middle of the PPR

The simulated number of completions of thewetland cover cycle (return times) during the 95-year historic period ranged from zero to threeNearly half (48) of the PPR remained stuck inone or two of the four cover stages and did notcomplete a single cycle (figure 9) Nearly asmuch of the PPR (40) completed one cycle(95-year return time) while 10 and 2 of thePPR completed two (475-year return time) andthree (317-year return time) cycles respectively

Simulations of hydrology and vegetationclearly identified a broad northwestndashsoutheastrunning arc in the middle of the PPR as themost dynamic and as a result the most sup-portive for overall biodiversity in general and waterfowl breeding in particular Areas fartherwest become productive only rarely during es-pecially wet periods while wetlands farthernorth and east become so during dry periodsThe eastern Dakotas and southeasternSaskatchewan stand out in this analysis as hav-ing been the heart of the PPRrsquos ldquoduck factoryrdquoduring the 20th century

Future climate and wetlandsThe historic patterns of wetland dynamics and favorability forwaterfowl breeding across the PPR may shift in the futuredepending on the extent and magnitude of climate changeWe found that the PPR climate changed during the 20thcentury nearly all major weather stations examined becamewarmer but western stations became drier and eastern sta-tions wetter These results suggest that the historically strongwest-to-east moisture gradient across the PPR has steep-ened Since it cannot be known with certainty whether or notthe trends of the past century will continue we adopted the

equilibrium scenario approach to examine the possible effectsof climate change on PPR wetlands

The effects of three combinations of temperature andmoisture on wetland conditions were compared to the his-torical reference using WETSIM 31 for 6 of the 18 weatherstations one from nearest the center of each ecoregion Thesethree scenarios were (1) a 3ordmC temperature increase with nochange in precipitation (2) a 3ordmC temperature increase witha 20 increase in precipitation and (3) a 3ordmC temperature increase with a 20 decrease in precipitation The climate scenarios were applied uniformly across seasons to the historic weather data files

October 2005 Vol 55 No 10 bull BioScience 869

Articles

Figure 8 Simulated historic pattern of wetland emergent cover (a) acrossthe prairie pothole region and (b) by weather station Cover percentageswere scaled to a maximum potential open water area of 28125 square me-ters (m2) with an outlet level of 14 m Ecoregion abbreviations CAPCanadian aspen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern short grasslands NTGnorthern tall grasslands PC Prairie Coteau

The model was highly sensitive to alternative future climates A temperature increase alone produced moreemergent cover at the relatively wet stations in Iowaand South Dakota (figure 10) Cover ratios shiftedstrongly toward dense emergent cover under the warmertemperatures at the drier stations such as Medicine Hatand Minot Most stations spent more time in the drymarsh phase under the warmer-only climate scenario

Increasing both temperature and precipitation had acounterbalancing effect on water budgets producingonly a small change in cover compared with historic sim-ulations at most stationsWarmer and wetter conditionsat Algona Watertown and Medicine Hat producedcover ratios nearly identical to historic conditions whilehemimarsh conditions (ie conditions in which themix of emergent cover and open water is roughly even)declined slightly at the other stations (figure 10) It is ap-parent from this simulation that a 20 increase in pre-cipitation would generally compensate for a 3ordmC rise intemperature if applied uniformly Altering seasonal pat-terns of temperature and precipitation in WETSIMwould produce different results

Increased temperature and decreased precipitationhad the greatest effect on wetland conditions The modelwetland at five of the six stations became completelydominated by dry marsh conditions because of more fre-quent and longer drought (figure 10) Only Algonathe wettest station formed hemimarsh conditions moreoften under this temperature and precipitation regimethan under the two other climate scenarios Cover dy-namics at Algona under this driest condition were nearly

identical to the historic reference at Crookston lo-cated 250 km to the northwest

The geographic pattern of return times shiftedmarkedly with changes in temperature and pre-cipitation A change in temperature alone pushedthe region of fastest return times eastward wherethe generally wetter climate could accommodatethe greater evaporative demand (figure 9c) Theportion of the PPR with the fastest return timesshifted geographically from the eastern Dakotasand southeastern Saskatchewan to western Min-nesota and Iowa Return times in this easternfringe of the PPR increased two- to threefold Thewarmer temperatures increased drawdown fre-quency and magnitude thereby stimulating veg-etation regeneration

Increases in both temperature and precipitationproduced the most spatially dynamic result (fig-ure 9b) This scenario generated an area of four

870 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 9 Geographic patterns of the speed of thewetland cover cycle simulated for the prairie pot-hole region under historic (a) and alternative future(b c and d) climatic conditions

Figure 10 Simulated percentages of emergent cover and open waterfor six weather stations each in a different ecoregion under historicand alternative future climatic conditions ldquoClosedrdquo represents morethan 75 emergent cover ldquohemirdquo (hemimarsh) represents 25ndash75 emergent cover ldquoopenrdquo represents less than 25 emergent cover State and province abbreviations AB Alberta IA Iowa MN Minnesota ND North Dakota SD South Dakota SKSaskatchewan

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

seasonally adjusted curve of leaf area index to estimate uplandevapotranspiration to replace EPICrsquos crop growth model (f)programming of the bucket model for soil water content inthe rooting zone and use of advanced numerical capabilitiesof Mathematica to iteratively solve the nonlinear soil mois-ture dynamics equation and (g) increased wetland cell sizefrom 9 square meters (m2) to 25 m2 to better match themonthly resolution of the vegetation submodel

WETSIM 31 was calibrated and tested using field data fromsemipermanent wetland P1 at the Cottonwood Lake study areain east-central North Dakota (figure 3) the same wetland usedto parameterize and evaluate earlier versions of WETSIMModel bathymetry was held constant during simulationsWetland P1 has the longest and most detailed hydrologicalrecord of any wetland in the PPR (Winter 2003) Systematicmeasurements and monitoring began in 1979 and continueto the present The testing of WETSIM 31 was more rigor-ous than for earlier model versions because the longer P1 dataset included much greater climatic variability by capturing therecord high water levels in the mid-1990s

The revised model accurately simulated the spring risesummer drawdown and interannual variability typical ofprairie wetlands and of P1 in particular (figure 4) The veg-

etation responded adequately to water-level dynamics judg-ing from historic photographs of P1

Moving the model geographically to other weather stationsrequired making adjustments The depth at which waterwould begin to flow out of the P1 basin was reduced from 52 m (not reached in modern times) to 14 m to enable themodel to potentially pass through most phases of the covercycle in wetter PPR climates Also the Hargreaves evapo-transpiration equation was adjusted by latitude to account fordifferences among stations in day length

We tested the geographic mobility of WETSIM 31 by us-ing data from another long-term prairie wetland monitoringsite in South Dakota (Johnson et al 2004) with a climatewarmer and wetter than that of wetland P1 in North DakotaThe model closely captured the historic water level and veg-etation dynamics of a semipermanent wetland at this site

The P1 basin from the Missouri Coteau may not be rep-resentative of wetland basin structure in other parts of the PPRUse of different basin structure for different ecoregions if dif-ferences were found to exist could accentuate or lessen thesimulated differences among weather stations and ecore-gions produced by the single-basin approach Connections be-tween groundwater and surface water are also known todiffer even among adjacent wetlands if there are systematic

866 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 3 The Cottonwood Lake study area in east-central North Dakota managed by the US Fishand Wildlife Service WETSIM a wetland simulation model was developed and tested using long-term monitoring data from wetland P1 located in the center of the scene (note white instrumentbarge) See Winter (2003) for site description Photograph courtesy of George Swanson (retired)Northern Prairie Wildlife Research Center

differences in groundwater fluxes among ecoregions thesecould temper the WETSIM results

Ecoregions of the prairie pothole region We subdivided the PPR into six ecoregions by combining USand Canadian approaches (figure 1) The ecoregions were usedas strata from which to select weather stations We chosethree widely separated weather stations from each ecoregionon the basis of their length and completeness of record Mostecoregions were oriented northndashsouth thus weather sta-tions were selected from northern central and southern lo-cations

We compiled a 95-year data set comprising 104097 records(daily precipitation minimum daily temperature and max-imum daily temperature) for each of the 18 weather sta-tions Missing data were replaced by interpolating from threenearby stations where possible We conducted accuracy as-

sessments by estimating known data from nearby stations Es-timates of temperature were more strongly correlated toknown values (86 moderate to strong correlations) thanwere estimates of precipitation (71 moderate to strongcorrelations) Error was higher for stations in less populatedareas with fewer nearby stations

Historic water levelsSimulated long-term water depths ranged widely amongPPR stations during the historic period (figure 5) Mean wa-ter depth for the historic period was greatest and nearest theoutlet level along the eastern and northern fringes of the PPR(Webster City and Algona Iowa Morris Minnesota RanfurlyAlberta) Average water levels were lowest along the north-western fringes of the PPR (032 m at Poplar Montana)Mean water depths at most weather stations ranged from 08to 11 m

Drought conditions followed a similar pattern across thePPR Droughts in the model wetland defined as less than 01m of standing water were shortest (5 to 15 days) or absentalong the eastern and northern fringes of the PPR (figure 6)Drought length was greatest (15 to 30 days) at northwesterlystations Muenster Saskatchewan was an outlier (nearly 80-day average drought length) caused by two exceptionallylong droughts Overall the length of the longest drought pe-riod correlated positively with mean length (except for Muen-ster) as did the percentage of time dry The Medicine HatAlberta and Poplar Montana stations were especiallydroughty being dry 15 and 25 of the time respectively(figure 6)

Geographic variability in wetland water budgets was evi-dent when we compared outflow volume among the PPRweather stations Outflow from the model wetland was in-dicative of the relative wetness of the PPR climates The high-

October 2005 Vol 55 No 10 bull BioScience 867

Articles

Figure 4 Simulations of stage levels of wetland P1 usingWETSIM (a) Comparison of simulated (solid line) andobserved (red dots) wetland stage levels (1960ndash2002) us-ing local precipitation data from the Woodworth NorthDakota weather station and temperature data from theJamestown North Dakota station (b) Stage levels simu-lated for 101 years using all weather data from theJamestown station and a 14-meter (m) outlet level

Figure 5 Mean daily wetland stage for 95 years at 18weather stations using WETSIM with a 14-meter (m)outlet level Ecoregion abbreviations CAP Canadian as-pen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern shortgrasslands NTG northern tall grasslands PC PrairieCoteau

est cumulative outflow of 2099366 m3 occurred at AlgonaOverflow at this station occurred in 87 of 95 years Overflowwas a rare event at Poplar and Medicine Hat each had onlyone overflow event in 95 years of about 4000 and 6000 m3respectively Large overflow volumes in the southeastern PPRmay have contributed to the development of an integrateddrainage network Smaller water surpluses in the central andnorthwestern PPR have maintained closed wetland basins withless integrated surface drainage

Historic vegetation response Spatial and temporal variability in modeled water condi-tions across the PPR produced correspondingly variable andcomplex responses from vegetation Temporal variability in-cluding droughts and deluges produced rapid changes inthe ratios between cover and open water at a single station

while spatial variability in weather across the PPR producedwidely differing cover ratios at a single point in time At thestation in Watertown South Dakota for example wetlanddrawdown during drought in the 1930s stimulated rapid ex-pansion of emergent cover and shrinkage of open water areaduring just eight consecutive growing-season months (figure7) A deluge period in the 1940s at Minot North Dakota pro-duced the opposite effect large increases in open water areaover a four-year period (figure 7)

868 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 6 Mean length and maximum length of dry peri-ods estimated by WETSIM model for prairie pothole region weather stations from 1906 to 2000 Size of circle corresponds to percent time dry Ranfurly (Alberta)Algona (Iowa) and Webster City (Iowa) had no dry periods Ecoregion abbreviations CAP Canadian aspenforests and parklands CTG central tall grasslands NMGnorthern mixed grasslands NSG northern short grass-lands NTG northern tall grasslands PC Prairie CoteauState and province abbreviations AB Alberta IA IowaMB Manitoba MN Minnesota MT Montana NDNorth Dakota SD South Dakota SK Saskatchewan

Figure 7 Response of WETSIM to temporal and spatialweather extremes Monthly water and cover conditionsare included for three periods drought (July 1933 to July1934 for the Watertown South Dakota weather station)deluge (August 1942 to July 1945 for the Minot NorthDakota weather station) and spatial variability (sixecoregion weather stations at a single point in time May1912) State and province abbreviations AB Alberta IAIowa MN Minnesota ND North Dakota SD SouthDakota SK Saskatchewan

Spatial variability was as striking when com-paring simulated cover ratios for six stations onein each ecoregion in May 1912 (figure 7) Themodel wetland indicated that at a single point intime dry marsh conditions existed in the west-ern Canadian prairies (Medicine Hat) balancedratios of cover to open water in the aspen park-lands of Canada (Muenster) and in the Dakotas(Minot and Watertown) and lake marsh con-ditions along the eastern PPR boundary (Crook-ston Minnesota and Algona)

Emergent cover averaged for each stationduring the 20th century ranged widely across thePPR from only 11 at Webster City to 99 atPoplar (figure 8) The most cover occurred in thewest-lying northern shortgrass ecoregion whilethe least cover occurred along the northern andeastern fringe of ecoregions bordering forestecosystems Moderate cover percentages most fa-vorable for waterfowl breeding formed a broadarc in the middle of the PPR

The simulated number of completions of thewetland cover cycle (return times) during the 95-year historic period ranged from zero to threeNearly half (48) of the PPR remained stuck inone or two of the four cover stages and did notcomplete a single cycle (figure 9) Nearly asmuch of the PPR (40) completed one cycle(95-year return time) while 10 and 2 of thePPR completed two (475-year return time) andthree (317-year return time) cycles respectively

Simulations of hydrology and vegetationclearly identified a broad northwestndashsoutheastrunning arc in the middle of the PPR as themost dynamic and as a result the most sup-portive for overall biodiversity in general and waterfowl breeding in particular Areas fartherwest become productive only rarely during es-pecially wet periods while wetlands farthernorth and east become so during dry periodsThe eastern Dakotas and southeasternSaskatchewan stand out in this analysis as hav-ing been the heart of the PPRrsquos ldquoduck factoryrdquoduring the 20th century

Future climate and wetlandsThe historic patterns of wetland dynamics and favorability forwaterfowl breeding across the PPR may shift in the futuredepending on the extent and magnitude of climate changeWe found that the PPR climate changed during the 20thcentury nearly all major weather stations examined becamewarmer but western stations became drier and eastern sta-tions wetter These results suggest that the historically strongwest-to-east moisture gradient across the PPR has steep-ened Since it cannot be known with certainty whether or notthe trends of the past century will continue we adopted the

equilibrium scenario approach to examine the possible effectsof climate change on PPR wetlands

The effects of three combinations of temperature andmoisture on wetland conditions were compared to the his-torical reference using WETSIM 31 for 6 of the 18 weatherstations one from nearest the center of each ecoregion Thesethree scenarios were (1) a 3ordmC temperature increase with nochange in precipitation (2) a 3ordmC temperature increase witha 20 increase in precipitation and (3) a 3ordmC temperature increase with a 20 decrease in precipitation The climate scenarios were applied uniformly across seasons to the historic weather data files

October 2005 Vol 55 No 10 bull BioScience 869

Articles

Figure 8 Simulated historic pattern of wetland emergent cover (a) acrossthe prairie pothole region and (b) by weather station Cover percentageswere scaled to a maximum potential open water area of 28125 square me-ters (m2) with an outlet level of 14 m Ecoregion abbreviations CAPCanadian aspen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern short grasslands NTGnorthern tall grasslands PC Prairie Coteau

The model was highly sensitive to alternative future climates A temperature increase alone produced moreemergent cover at the relatively wet stations in Iowaand South Dakota (figure 10) Cover ratios shiftedstrongly toward dense emergent cover under the warmertemperatures at the drier stations such as Medicine Hatand Minot Most stations spent more time in the drymarsh phase under the warmer-only climate scenario

Increasing both temperature and precipitation had acounterbalancing effect on water budgets producingonly a small change in cover compared with historic sim-ulations at most stationsWarmer and wetter conditionsat Algona Watertown and Medicine Hat producedcover ratios nearly identical to historic conditions whilehemimarsh conditions (ie conditions in which themix of emergent cover and open water is roughly even)declined slightly at the other stations (figure 10) It is ap-parent from this simulation that a 20 increase in pre-cipitation would generally compensate for a 3ordmC rise intemperature if applied uniformly Altering seasonal pat-terns of temperature and precipitation in WETSIMwould produce different results

Increased temperature and decreased precipitationhad the greatest effect on wetland conditions The modelwetland at five of the six stations became completelydominated by dry marsh conditions because of more fre-quent and longer drought (figure 10) Only Algonathe wettest station formed hemimarsh conditions moreoften under this temperature and precipitation regimethan under the two other climate scenarios Cover dy-namics at Algona under this driest condition were nearly

identical to the historic reference at Crookston lo-cated 250 km to the northwest

The geographic pattern of return times shiftedmarkedly with changes in temperature and pre-cipitation A change in temperature alone pushedthe region of fastest return times eastward wherethe generally wetter climate could accommodatethe greater evaporative demand (figure 9c) Theportion of the PPR with the fastest return timesshifted geographically from the eastern Dakotasand southeastern Saskatchewan to western Min-nesota and Iowa Return times in this easternfringe of the PPR increased two- to threefold Thewarmer temperatures increased drawdown fre-quency and magnitude thereby stimulating veg-etation regeneration

Increases in both temperature and precipitationproduced the most spatially dynamic result (fig-ure 9b) This scenario generated an area of four

870 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 9 Geographic patterns of the speed of thewetland cover cycle simulated for the prairie pot-hole region under historic (a) and alternative future(b c and d) climatic conditions

Figure 10 Simulated percentages of emergent cover and open waterfor six weather stations each in a different ecoregion under historicand alternative future climatic conditions ldquoClosedrdquo represents morethan 75 emergent cover ldquohemirdquo (hemimarsh) represents 25ndash75 emergent cover ldquoopenrdquo represents less than 25 emergent cover State and province abbreviations AB Alberta IA Iowa MN Minnesota ND North Dakota SD South Dakota SKSaskatchewan

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

differences in groundwater fluxes among ecoregions thesecould temper the WETSIM results

Ecoregions of the prairie pothole region We subdivided the PPR into six ecoregions by combining USand Canadian approaches (figure 1) The ecoregions were usedas strata from which to select weather stations We chosethree widely separated weather stations from each ecoregionon the basis of their length and completeness of record Mostecoregions were oriented northndashsouth thus weather sta-tions were selected from northern central and southern lo-cations

We compiled a 95-year data set comprising 104097 records(daily precipitation minimum daily temperature and max-imum daily temperature) for each of the 18 weather sta-tions Missing data were replaced by interpolating from threenearby stations where possible We conducted accuracy as-

sessments by estimating known data from nearby stations Es-timates of temperature were more strongly correlated toknown values (86 moderate to strong correlations) thanwere estimates of precipitation (71 moderate to strongcorrelations) Error was higher for stations in less populatedareas with fewer nearby stations

Historic water levelsSimulated long-term water depths ranged widely amongPPR stations during the historic period (figure 5) Mean wa-ter depth for the historic period was greatest and nearest theoutlet level along the eastern and northern fringes of the PPR(Webster City and Algona Iowa Morris Minnesota RanfurlyAlberta) Average water levels were lowest along the north-western fringes of the PPR (032 m at Poplar Montana)Mean water depths at most weather stations ranged from 08to 11 m

Drought conditions followed a similar pattern across thePPR Droughts in the model wetland defined as less than 01m of standing water were shortest (5 to 15 days) or absentalong the eastern and northern fringes of the PPR (figure 6)Drought length was greatest (15 to 30 days) at northwesterlystations Muenster Saskatchewan was an outlier (nearly 80-day average drought length) caused by two exceptionallylong droughts Overall the length of the longest drought pe-riod correlated positively with mean length (except for Muen-ster) as did the percentage of time dry The Medicine HatAlberta and Poplar Montana stations were especiallydroughty being dry 15 and 25 of the time respectively(figure 6)

Geographic variability in wetland water budgets was evi-dent when we compared outflow volume among the PPRweather stations Outflow from the model wetland was in-dicative of the relative wetness of the PPR climates The high-

October 2005 Vol 55 No 10 bull BioScience 867

Articles

Figure 4 Simulations of stage levels of wetland P1 usingWETSIM (a) Comparison of simulated (solid line) andobserved (red dots) wetland stage levels (1960ndash2002) us-ing local precipitation data from the Woodworth NorthDakota weather station and temperature data from theJamestown North Dakota station (b) Stage levels simu-lated for 101 years using all weather data from theJamestown station and a 14-meter (m) outlet level

Figure 5 Mean daily wetland stage for 95 years at 18weather stations using WETSIM with a 14-meter (m)outlet level Ecoregion abbreviations CAP Canadian as-pen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern shortgrasslands NTG northern tall grasslands PC PrairieCoteau

est cumulative outflow of 2099366 m3 occurred at AlgonaOverflow at this station occurred in 87 of 95 years Overflowwas a rare event at Poplar and Medicine Hat each had onlyone overflow event in 95 years of about 4000 and 6000 m3respectively Large overflow volumes in the southeastern PPRmay have contributed to the development of an integrateddrainage network Smaller water surpluses in the central andnorthwestern PPR have maintained closed wetland basins withless integrated surface drainage

Historic vegetation response Spatial and temporal variability in modeled water condi-tions across the PPR produced correspondingly variable andcomplex responses from vegetation Temporal variability in-cluding droughts and deluges produced rapid changes inthe ratios between cover and open water at a single station

while spatial variability in weather across the PPR producedwidely differing cover ratios at a single point in time At thestation in Watertown South Dakota for example wetlanddrawdown during drought in the 1930s stimulated rapid ex-pansion of emergent cover and shrinkage of open water areaduring just eight consecutive growing-season months (figure7) A deluge period in the 1940s at Minot North Dakota pro-duced the opposite effect large increases in open water areaover a four-year period (figure 7)

868 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 6 Mean length and maximum length of dry peri-ods estimated by WETSIM model for prairie pothole region weather stations from 1906 to 2000 Size of circle corresponds to percent time dry Ranfurly (Alberta)Algona (Iowa) and Webster City (Iowa) had no dry periods Ecoregion abbreviations CAP Canadian aspenforests and parklands CTG central tall grasslands NMGnorthern mixed grasslands NSG northern short grass-lands NTG northern tall grasslands PC Prairie CoteauState and province abbreviations AB Alberta IA IowaMB Manitoba MN Minnesota MT Montana NDNorth Dakota SD South Dakota SK Saskatchewan

Figure 7 Response of WETSIM to temporal and spatialweather extremes Monthly water and cover conditionsare included for three periods drought (July 1933 to July1934 for the Watertown South Dakota weather station)deluge (August 1942 to July 1945 for the Minot NorthDakota weather station) and spatial variability (sixecoregion weather stations at a single point in time May1912) State and province abbreviations AB Alberta IAIowa MN Minnesota ND North Dakota SD SouthDakota SK Saskatchewan

Spatial variability was as striking when com-paring simulated cover ratios for six stations onein each ecoregion in May 1912 (figure 7) Themodel wetland indicated that at a single point intime dry marsh conditions existed in the west-ern Canadian prairies (Medicine Hat) balancedratios of cover to open water in the aspen park-lands of Canada (Muenster) and in the Dakotas(Minot and Watertown) and lake marsh con-ditions along the eastern PPR boundary (Crook-ston Minnesota and Algona)

Emergent cover averaged for each stationduring the 20th century ranged widely across thePPR from only 11 at Webster City to 99 atPoplar (figure 8) The most cover occurred in thewest-lying northern shortgrass ecoregion whilethe least cover occurred along the northern andeastern fringe of ecoregions bordering forestecosystems Moderate cover percentages most fa-vorable for waterfowl breeding formed a broadarc in the middle of the PPR

The simulated number of completions of thewetland cover cycle (return times) during the 95-year historic period ranged from zero to threeNearly half (48) of the PPR remained stuck inone or two of the four cover stages and did notcomplete a single cycle (figure 9) Nearly asmuch of the PPR (40) completed one cycle(95-year return time) while 10 and 2 of thePPR completed two (475-year return time) andthree (317-year return time) cycles respectively

Simulations of hydrology and vegetationclearly identified a broad northwestndashsoutheastrunning arc in the middle of the PPR as themost dynamic and as a result the most sup-portive for overall biodiversity in general and waterfowl breeding in particular Areas fartherwest become productive only rarely during es-pecially wet periods while wetlands farthernorth and east become so during dry periodsThe eastern Dakotas and southeasternSaskatchewan stand out in this analysis as hav-ing been the heart of the PPRrsquos ldquoduck factoryrdquoduring the 20th century

Future climate and wetlandsThe historic patterns of wetland dynamics and favorability forwaterfowl breeding across the PPR may shift in the futuredepending on the extent and magnitude of climate changeWe found that the PPR climate changed during the 20thcentury nearly all major weather stations examined becamewarmer but western stations became drier and eastern sta-tions wetter These results suggest that the historically strongwest-to-east moisture gradient across the PPR has steep-ened Since it cannot be known with certainty whether or notthe trends of the past century will continue we adopted the

equilibrium scenario approach to examine the possible effectsof climate change on PPR wetlands

The effects of three combinations of temperature andmoisture on wetland conditions were compared to the his-torical reference using WETSIM 31 for 6 of the 18 weatherstations one from nearest the center of each ecoregion Thesethree scenarios were (1) a 3ordmC temperature increase with nochange in precipitation (2) a 3ordmC temperature increase witha 20 increase in precipitation and (3) a 3ordmC temperature increase with a 20 decrease in precipitation The climate scenarios were applied uniformly across seasons to the historic weather data files

October 2005 Vol 55 No 10 bull BioScience 869

Articles

Figure 8 Simulated historic pattern of wetland emergent cover (a) acrossthe prairie pothole region and (b) by weather station Cover percentageswere scaled to a maximum potential open water area of 28125 square me-ters (m2) with an outlet level of 14 m Ecoregion abbreviations CAPCanadian aspen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern short grasslands NTGnorthern tall grasslands PC Prairie Coteau

The model was highly sensitive to alternative future climates A temperature increase alone produced moreemergent cover at the relatively wet stations in Iowaand South Dakota (figure 10) Cover ratios shiftedstrongly toward dense emergent cover under the warmertemperatures at the drier stations such as Medicine Hatand Minot Most stations spent more time in the drymarsh phase under the warmer-only climate scenario

Increasing both temperature and precipitation had acounterbalancing effect on water budgets producingonly a small change in cover compared with historic sim-ulations at most stationsWarmer and wetter conditionsat Algona Watertown and Medicine Hat producedcover ratios nearly identical to historic conditions whilehemimarsh conditions (ie conditions in which themix of emergent cover and open water is roughly even)declined slightly at the other stations (figure 10) It is ap-parent from this simulation that a 20 increase in pre-cipitation would generally compensate for a 3ordmC rise intemperature if applied uniformly Altering seasonal pat-terns of temperature and precipitation in WETSIMwould produce different results

Increased temperature and decreased precipitationhad the greatest effect on wetland conditions The modelwetland at five of the six stations became completelydominated by dry marsh conditions because of more fre-quent and longer drought (figure 10) Only Algonathe wettest station formed hemimarsh conditions moreoften under this temperature and precipitation regimethan under the two other climate scenarios Cover dy-namics at Algona under this driest condition were nearly

identical to the historic reference at Crookston lo-cated 250 km to the northwest

The geographic pattern of return times shiftedmarkedly with changes in temperature and pre-cipitation A change in temperature alone pushedthe region of fastest return times eastward wherethe generally wetter climate could accommodatethe greater evaporative demand (figure 9c) Theportion of the PPR with the fastest return timesshifted geographically from the eastern Dakotasand southeastern Saskatchewan to western Min-nesota and Iowa Return times in this easternfringe of the PPR increased two- to threefold Thewarmer temperatures increased drawdown fre-quency and magnitude thereby stimulating veg-etation regeneration

Increases in both temperature and precipitationproduced the most spatially dynamic result (fig-ure 9b) This scenario generated an area of four

870 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 9 Geographic patterns of the speed of thewetland cover cycle simulated for the prairie pot-hole region under historic (a) and alternative future(b c and d) climatic conditions

Figure 10 Simulated percentages of emergent cover and open waterfor six weather stations each in a different ecoregion under historicand alternative future climatic conditions ldquoClosedrdquo represents morethan 75 emergent cover ldquohemirdquo (hemimarsh) represents 25ndash75 emergent cover ldquoopenrdquo represents less than 25 emergent cover State and province abbreviations AB Alberta IA Iowa MN Minnesota ND North Dakota SD South Dakota SKSaskatchewan

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

est cumulative outflow of 2099366 m3 occurred at AlgonaOverflow at this station occurred in 87 of 95 years Overflowwas a rare event at Poplar and Medicine Hat each had onlyone overflow event in 95 years of about 4000 and 6000 m3respectively Large overflow volumes in the southeastern PPRmay have contributed to the development of an integrateddrainage network Smaller water surpluses in the central andnorthwestern PPR have maintained closed wetland basins withless integrated surface drainage

Historic vegetation response Spatial and temporal variability in modeled water condi-tions across the PPR produced correspondingly variable andcomplex responses from vegetation Temporal variability in-cluding droughts and deluges produced rapid changes inthe ratios between cover and open water at a single station

while spatial variability in weather across the PPR producedwidely differing cover ratios at a single point in time At thestation in Watertown South Dakota for example wetlanddrawdown during drought in the 1930s stimulated rapid ex-pansion of emergent cover and shrinkage of open water areaduring just eight consecutive growing-season months (figure7) A deluge period in the 1940s at Minot North Dakota pro-duced the opposite effect large increases in open water areaover a four-year period (figure 7)

868 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 6 Mean length and maximum length of dry peri-ods estimated by WETSIM model for prairie pothole region weather stations from 1906 to 2000 Size of circle corresponds to percent time dry Ranfurly (Alberta)Algona (Iowa) and Webster City (Iowa) had no dry periods Ecoregion abbreviations CAP Canadian aspenforests and parklands CTG central tall grasslands NMGnorthern mixed grasslands NSG northern short grass-lands NTG northern tall grasslands PC Prairie CoteauState and province abbreviations AB Alberta IA IowaMB Manitoba MN Minnesota MT Montana NDNorth Dakota SD South Dakota SK Saskatchewan

Figure 7 Response of WETSIM to temporal and spatialweather extremes Monthly water and cover conditionsare included for three periods drought (July 1933 to July1934 for the Watertown South Dakota weather station)deluge (August 1942 to July 1945 for the Minot NorthDakota weather station) and spatial variability (sixecoregion weather stations at a single point in time May1912) State and province abbreviations AB Alberta IAIowa MN Minnesota ND North Dakota SD SouthDakota SK Saskatchewan

Spatial variability was as striking when com-paring simulated cover ratios for six stations onein each ecoregion in May 1912 (figure 7) Themodel wetland indicated that at a single point intime dry marsh conditions existed in the west-ern Canadian prairies (Medicine Hat) balancedratios of cover to open water in the aspen park-lands of Canada (Muenster) and in the Dakotas(Minot and Watertown) and lake marsh con-ditions along the eastern PPR boundary (Crook-ston Minnesota and Algona)

Emergent cover averaged for each stationduring the 20th century ranged widely across thePPR from only 11 at Webster City to 99 atPoplar (figure 8) The most cover occurred in thewest-lying northern shortgrass ecoregion whilethe least cover occurred along the northern andeastern fringe of ecoregions bordering forestecosystems Moderate cover percentages most fa-vorable for waterfowl breeding formed a broadarc in the middle of the PPR

The simulated number of completions of thewetland cover cycle (return times) during the 95-year historic period ranged from zero to threeNearly half (48) of the PPR remained stuck inone or two of the four cover stages and did notcomplete a single cycle (figure 9) Nearly asmuch of the PPR (40) completed one cycle(95-year return time) while 10 and 2 of thePPR completed two (475-year return time) andthree (317-year return time) cycles respectively

Simulations of hydrology and vegetationclearly identified a broad northwestndashsoutheastrunning arc in the middle of the PPR as themost dynamic and as a result the most sup-portive for overall biodiversity in general and waterfowl breeding in particular Areas fartherwest become productive only rarely during es-pecially wet periods while wetlands farthernorth and east become so during dry periodsThe eastern Dakotas and southeasternSaskatchewan stand out in this analysis as hav-ing been the heart of the PPRrsquos ldquoduck factoryrdquoduring the 20th century

Future climate and wetlandsThe historic patterns of wetland dynamics and favorability forwaterfowl breeding across the PPR may shift in the futuredepending on the extent and magnitude of climate changeWe found that the PPR climate changed during the 20thcentury nearly all major weather stations examined becamewarmer but western stations became drier and eastern sta-tions wetter These results suggest that the historically strongwest-to-east moisture gradient across the PPR has steep-ened Since it cannot be known with certainty whether or notthe trends of the past century will continue we adopted the

equilibrium scenario approach to examine the possible effectsof climate change on PPR wetlands

The effects of three combinations of temperature andmoisture on wetland conditions were compared to the his-torical reference using WETSIM 31 for 6 of the 18 weatherstations one from nearest the center of each ecoregion Thesethree scenarios were (1) a 3ordmC temperature increase with nochange in precipitation (2) a 3ordmC temperature increase witha 20 increase in precipitation and (3) a 3ordmC temperature increase with a 20 decrease in precipitation The climate scenarios were applied uniformly across seasons to the historic weather data files

October 2005 Vol 55 No 10 bull BioScience 869

Articles

Figure 8 Simulated historic pattern of wetland emergent cover (a) acrossthe prairie pothole region and (b) by weather station Cover percentageswere scaled to a maximum potential open water area of 28125 square me-ters (m2) with an outlet level of 14 m Ecoregion abbreviations CAPCanadian aspen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern short grasslands NTGnorthern tall grasslands PC Prairie Coteau

The model was highly sensitive to alternative future climates A temperature increase alone produced moreemergent cover at the relatively wet stations in Iowaand South Dakota (figure 10) Cover ratios shiftedstrongly toward dense emergent cover under the warmertemperatures at the drier stations such as Medicine Hatand Minot Most stations spent more time in the drymarsh phase under the warmer-only climate scenario

Increasing both temperature and precipitation had acounterbalancing effect on water budgets producingonly a small change in cover compared with historic sim-ulations at most stationsWarmer and wetter conditionsat Algona Watertown and Medicine Hat producedcover ratios nearly identical to historic conditions whilehemimarsh conditions (ie conditions in which themix of emergent cover and open water is roughly even)declined slightly at the other stations (figure 10) It is ap-parent from this simulation that a 20 increase in pre-cipitation would generally compensate for a 3ordmC rise intemperature if applied uniformly Altering seasonal pat-terns of temperature and precipitation in WETSIMwould produce different results

Increased temperature and decreased precipitationhad the greatest effect on wetland conditions The modelwetland at five of the six stations became completelydominated by dry marsh conditions because of more fre-quent and longer drought (figure 10) Only Algonathe wettest station formed hemimarsh conditions moreoften under this temperature and precipitation regimethan under the two other climate scenarios Cover dy-namics at Algona under this driest condition were nearly

identical to the historic reference at Crookston lo-cated 250 km to the northwest

The geographic pattern of return times shiftedmarkedly with changes in temperature and pre-cipitation A change in temperature alone pushedthe region of fastest return times eastward wherethe generally wetter climate could accommodatethe greater evaporative demand (figure 9c) Theportion of the PPR with the fastest return timesshifted geographically from the eastern Dakotasand southeastern Saskatchewan to western Min-nesota and Iowa Return times in this easternfringe of the PPR increased two- to threefold Thewarmer temperatures increased drawdown fre-quency and magnitude thereby stimulating veg-etation regeneration

Increases in both temperature and precipitationproduced the most spatially dynamic result (fig-ure 9b) This scenario generated an area of four

870 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 9 Geographic patterns of the speed of thewetland cover cycle simulated for the prairie pot-hole region under historic (a) and alternative future(b c and d) climatic conditions

Figure 10 Simulated percentages of emergent cover and open waterfor six weather stations each in a different ecoregion under historicand alternative future climatic conditions ldquoClosedrdquo represents morethan 75 emergent cover ldquohemirdquo (hemimarsh) represents 25ndash75 emergent cover ldquoopenrdquo represents less than 25 emergent cover State and province abbreviations AB Alberta IA Iowa MN Minnesota ND North Dakota SD South Dakota SKSaskatchewan

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

Spatial variability was as striking when com-paring simulated cover ratios for six stations onein each ecoregion in May 1912 (figure 7) Themodel wetland indicated that at a single point intime dry marsh conditions existed in the west-ern Canadian prairies (Medicine Hat) balancedratios of cover to open water in the aspen park-lands of Canada (Muenster) and in the Dakotas(Minot and Watertown) and lake marsh con-ditions along the eastern PPR boundary (Crook-ston Minnesota and Algona)

Emergent cover averaged for each stationduring the 20th century ranged widely across thePPR from only 11 at Webster City to 99 atPoplar (figure 8) The most cover occurred in thewest-lying northern shortgrass ecoregion whilethe least cover occurred along the northern andeastern fringe of ecoregions bordering forestecosystems Moderate cover percentages most fa-vorable for waterfowl breeding formed a broadarc in the middle of the PPR

The simulated number of completions of thewetland cover cycle (return times) during the 95-year historic period ranged from zero to threeNearly half (48) of the PPR remained stuck inone or two of the four cover stages and did notcomplete a single cycle (figure 9) Nearly asmuch of the PPR (40) completed one cycle(95-year return time) while 10 and 2 of thePPR completed two (475-year return time) andthree (317-year return time) cycles respectively

Simulations of hydrology and vegetationclearly identified a broad northwestndashsoutheastrunning arc in the middle of the PPR as themost dynamic and as a result the most sup-portive for overall biodiversity in general and waterfowl breeding in particular Areas fartherwest become productive only rarely during es-pecially wet periods while wetlands farthernorth and east become so during dry periodsThe eastern Dakotas and southeasternSaskatchewan stand out in this analysis as hav-ing been the heart of the PPRrsquos ldquoduck factoryrdquoduring the 20th century

Future climate and wetlandsThe historic patterns of wetland dynamics and favorability forwaterfowl breeding across the PPR may shift in the futuredepending on the extent and magnitude of climate changeWe found that the PPR climate changed during the 20thcentury nearly all major weather stations examined becamewarmer but western stations became drier and eastern sta-tions wetter These results suggest that the historically strongwest-to-east moisture gradient across the PPR has steep-ened Since it cannot be known with certainty whether or notthe trends of the past century will continue we adopted the

equilibrium scenario approach to examine the possible effectsof climate change on PPR wetlands

The effects of three combinations of temperature andmoisture on wetland conditions were compared to the his-torical reference using WETSIM 31 for 6 of the 18 weatherstations one from nearest the center of each ecoregion Thesethree scenarios were (1) a 3ordmC temperature increase with nochange in precipitation (2) a 3ordmC temperature increase witha 20 increase in precipitation and (3) a 3ordmC temperature increase with a 20 decrease in precipitation The climate scenarios were applied uniformly across seasons to the historic weather data files

October 2005 Vol 55 No 10 bull BioScience 869

Articles

Figure 8 Simulated historic pattern of wetland emergent cover (a) acrossthe prairie pothole region and (b) by weather station Cover percentageswere scaled to a maximum potential open water area of 28125 square me-ters (m2) with an outlet level of 14 m Ecoregion abbreviations CAPCanadian aspen forests and parklands CTG central tall grasslandsNMG northern mixed grasslands NSG northern short grasslands NTGnorthern tall grasslands PC Prairie Coteau

The model was highly sensitive to alternative future climates A temperature increase alone produced moreemergent cover at the relatively wet stations in Iowaand South Dakota (figure 10) Cover ratios shiftedstrongly toward dense emergent cover under the warmertemperatures at the drier stations such as Medicine Hatand Minot Most stations spent more time in the drymarsh phase under the warmer-only climate scenario

Increasing both temperature and precipitation had acounterbalancing effect on water budgets producingonly a small change in cover compared with historic sim-ulations at most stationsWarmer and wetter conditionsat Algona Watertown and Medicine Hat producedcover ratios nearly identical to historic conditions whilehemimarsh conditions (ie conditions in which themix of emergent cover and open water is roughly even)declined slightly at the other stations (figure 10) It is ap-parent from this simulation that a 20 increase in pre-cipitation would generally compensate for a 3ordmC rise intemperature if applied uniformly Altering seasonal pat-terns of temperature and precipitation in WETSIMwould produce different results

Increased temperature and decreased precipitationhad the greatest effect on wetland conditions The modelwetland at five of the six stations became completelydominated by dry marsh conditions because of more fre-quent and longer drought (figure 10) Only Algonathe wettest station formed hemimarsh conditions moreoften under this temperature and precipitation regimethan under the two other climate scenarios Cover dy-namics at Algona under this driest condition were nearly

identical to the historic reference at Crookston lo-cated 250 km to the northwest

The geographic pattern of return times shiftedmarkedly with changes in temperature and pre-cipitation A change in temperature alone pushedthe region of fastest return times eastward wherethe generally wetter climate could accommodatethe greater evaporative demand (figure 9c) Theportion of the PPR with the fastest return timesshifted geographically from the eastern Dakotasand southeastern Saskatchewan to western Min-nesota and Iowa Return times in this easternfringe of the PPR increased two- to threefold Thewarmer temperatures increased drawdown fre-quency and magnitude thereby stimulating veg-etation regeneration

Increases in both temperature and precipitationproduced the most spatially dynamic result (fig-ure 9b) This scenario generated an area of four

870 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 9 Geographic patterns of the speed of thewetland cover cycle simulated for the prairie pot-hole region under historic (a) and alternative future(b c and d) climatic conditions

Figure 10 Simulated percentages of emergent cover and open waterfor six weather stations each in a different ecoregion under historicand alternative future climatic conditions ldquoClosedrdquo represents morethan 75 emergent cover ldquohemirdquo (hemimarsh) represents 25ndash75 emergent cover ldquoopenrdquo represents less than 25 emergent cover State and province abbreviations AB Alberta IA Iowa MN Minnesota ND North Dakota SD South Dakota SKSaskatchewan

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

The model was highly sensitive to alternative future climates A temperature increase alone produced moreemergent cover at the relatively wet stations in Iowaand South Dakota (figure 10) Cover ratios shiftedstrongly toward dense emergent cover under the warmertemperatures at the drier stations such as Medicine Hatand Minot Most stations spent more time in the drymarsh phase under the warmer-only climate scenario

Increasing both temperature and precipitation had acounterbalancing effect on water budgets producingonly a small change in cover compared with historic sim-ulations at most stationsWarmer and wetter conditionsat Algona Watertown and Medicine Hat producedcover ratios nearly identical to historic conditions whilehemimarsh conditions (ie conditions in which themix of emergent cover and open water is roughly even)declined slightly at the other stations (figure 10) It is ap-parent from this simulation that a 20 increase in pre-cipitation would generally compensate for a 3ordmC rise intemperature if applied uniformly Altering seasonal pat-terns of temperature and precipitation in WETSIMwould produce different results

Increased temperature and decreased precipitationhad the greatest effect on wetland conditions The modelwetland at five of the six stations became completelydominated by dry marsh conditions because of more fre-quent and longer drought (figure 10) Only Algonathe wettest station formed hemimarsh conditions moreoften under this temperature and precipitation regimethan under the two other climate scenarios Cover dy-namics at Algona under this driest condition were nearly

identical to the historic reference at Crookston lo-cated 250 km to the northwest

The geographic pattern of return times shiftedmarkedly with changes in temperature and pre-cipitation A change in temperature alone pushedthe region of fastest return times eastward wherethe generally wetter climate could accommodatethe greater evaporative demand (figure 9c) Theportion of the PPR with the fastest return timesshifted geographically from the eastern Dakotasand southeastern Saskatchewan to western Min-nesota and Iowa Return times in this easternfringe of the PPR increased two- to threefold Thewarmer temperatures increased drawdown fre-quency and magnitude thereby stimulating veg-etation regeneration

Increases in both temperature and precipitationproduced the most spatially dynamic result (fig-ure 9b) This scenario generated an area of four

870 BioScience bull October 2005 Vol 55 No 10

Articles

Figure 9 Geographic patterns of the speed of thewetland cover cycle simulated for the prairie pot-hole region under historic (a) and alternative future(b c and d) climatic conditions

Figure 10 Simulated percentages of emergent cover and open waterfor six weather stations each in a different ecoregion under historicand alternative future climatic conditions ldquoClosedrdquo represents morethan 75 emergent cover ldquohemirdquo (hemimarsh) represents 25ndash75 emergent cover ldquoopenrdquo represents less than 25 emergent cover State and province abbreviations AB Alberta IA Iowa MN Minnesota ND North Dakota SD South Dakota SKSaskatchewan

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

return-time cycles in east-central SouthDakota four cycles did not appear in theother simulations (figure 9) These resultssuggest that a warmer future climate sup-plied with sufficient additional moisturemay provide more favorable cover andwater conditions for waterfowl breedingacross the PPR than the climate during the20th century In particular the high historicfavorability of the central PPR for water-fowl production was strengthened underthis scenario

Reduced precipitation and warmer airtemperatures however produced a nearlyfeatureless map with no complete cover cy-cles except in a small area in north-centralIowa (figure 9d) The dry conditions failedto generate sufficient water depths acrossthe PPR to produce and maintain suffi-ciently large areas of open water to qualifyas hemimarsh Overall the three scenariosprovide evidence that the cover cycle dy-namics that are linked to the breeding op-portunities for waterfowl provided by prairiewetlands are highly sensitive to climate

Integrating return time and the occur-rence of hemimarsh conditions produced a map of near op-timal cover and water conditions for waterfowl breedingacross the PPR (figure 11) As shown earlier the central PPRproduced the most favorable conditions during the historicperiod Also the largest proportion of the PPR under highlyfavorable conditions occurred during historic times

Geographic shifts in the most favorable region for water-fowl breeding were marked under all three scenarios (figure11) A temperature increase of 3ordmC and any decrease in precipitation shifted most favorable conditions to a muchsmaller area on the eastern fringe of the PPR With an in-creasingly dry climate this zone would shrink even furtherand move from western Minnesota south to Iowa The maincause of the simulated reductions in highly favorable condi-tions for waterfowl breeding is the longer duration of low water levels droughts and choked marsh conditions devel-oping in a warmer and drier future climate

Vulnerability to climate changeThe observed sensitivity of the model to climate variabilitysuggests that wetlands in the drier portions of the PPR suchas the US and Canadian High Plains would be especially vul-nerable to climate warming even if precipitation were tocontinue at historic levels Only a substantial increase in pre-cipitation would counterbalance the effects of a warmer cli-mate Additionally the most productive wetlands currentlycentrally located in the PPR may become marginally pro-ductive in a warmer drier future climate Historically a main-stay for waterfowl the region including the Dakotas andsoutheastern Saskatchewan would become a more episodic

and less reliable region for waterfowl production much as areas farther west have been during the past century

Continental waterfowl populations are characterized byboom and bust cycles that are largely dictated by regional wetland conditions Under historic conditions population declines were commonplace during drought because re-cruitment was limited to a few remaining regions with suit-able wetland conditions Populations would then rebound to previous levels when water returned to drier regionsbecause waterfowl quickly colonize favorable habitats Undera warmer and drier climate however we estimate that populations would decline below historic levels because wetlands in the central PPR that used to provide ample habi-tat would be too dry for most waterfowl in most yearsWe alsoestimate that populations would remain below historic levels because favorable habitat conditions on which thesebirds depend would occur even less frequently in the driestregions of the PPR

A logical question is whether the favorable water and coverconditions in the eastern PPR that we simulated can com-pensate for habitat losses in the western and central PPR His-torically the eastern PPR and northern parklands served asa safe haven for waterfowl during periodic droughts Todayhowever options are limited because more than 90 of eastern PPR wetlands have been drained for agriculturalproduction (Tiner 2003) Although wetland restoration programs have been under way since the mid-1980s lessthan 1 of basins drained in Minnesota and Iowa have beenrestored (Susan Galatowitsch University of Minnesota StPaul personal communication 1 July 2005) Restoration

October 2005 Vol 55 No 10 bull BioScience 871

Articles

Figure 11 Simulated occurrence of highly favorable water and cover conditionsfor waterfowl breeding (occurrence of at least one return time and hemimarshconditions at more than 30 frequency) across the prairie pothole region underhistoric (a) and alternative (b c and d) future climatic conditions

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles

efforts in the east have developed slowly largely because of thehigh cost of farmland easements

The results of this research suggest that climate change maydiminish the benefits of wetland conservation in the centraland western PPR Simulations further indicate that restora-tion of wetlands along wetter fringes of the PPR may be nec-essary to ameliorate potential impacts of climate change onwaterfowl populations While this research provides grist fordiscussion of the restoration and management of PPR wet-lands under climate change as urged by Anderson and Soren-son (2001) we also see ways that wetland modeling could beimproved to make its output more meaningful to decision-makers Namely we have developed and are testing a new wet-land landscape simulator (WETLANDSCAPE) that calculatesthe effect of climate variability simultaneously on multiple wet-lands of each water regime (ie temporary seasonal andsemipermanent types) in a landscape This next-generationmodel also will be developed to reflect regional differences thatmay exist in wetland bathymetry across the PPR Finally weplan to use WETLANDSCAPE to evaluate the possibilitythat farming practices (crops and cropping systems) canlessen the impacts of climate change by producing more fa-vorable water budgets for prairie wetlands

AcknowledgmentsThis research was supported by grants from the US Envi-ronmental Protection Agency (Habitat and Biological Di-versity Research Program) and the US Geological Survey(USGS Biological Resources Division Global Change ResearchProgram) Rosemary Carroll and John Tracy of the Desert Re-search Institute in Reno Nevada provided groundwaterequations for wetland P1 Tom Winter of USGS generouslyprovided water-level and topographic data for wetland P1Weacknowledge the pioneering work of Karen Poiani of The Na-ture Conservancy in prairie wetland modeling and GeorgeSwanson of the Northern Prairie Wildlife Research Center andTom Winter for their vision in establishing a long-term mon-itoring program at Cottonwood Lake

References citedAlbertson FW Weaver JE 1942 History of the native vegetation of western

Kansas during seven years of continuous drought Ecological Monographs12 23ndash51

Anderson MG Sorenson LG 2001 Global climate change and waterfowlAdaptation in the face of uncertainty Transactions of the North Amer-ican Wildlife and Natural Resources Conference 66 300ndash319

Austin JE 2002 Responses of dabbling ducks to wetland conditions in thePrairie Pothole Region Waterbirds 25 465ndash473

Hargreaves GH 1994 Defining and using reference evapotranspirationJournal of Irrigation and Drainage Engineering 120 1132ndash1139

Hoekman ST Mills LS Howerter DW Devries JH Ball IJ 2002 Sensitivityanalysis of the life cycle of mid-continent mallards Journal of WildlifeManagement 66 883ndash900

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Xiaosu Deds 2001 Climate Change 2001 The Scientific Basis Contribution ofWorking Group I to the Third Assessment Report of the Intergovern-

mental Panel on Climate Change Cambridge (United Kingdom) Cam-bridge University Press

Johnson WC Boettcher SE Poiani KA Guntenspergen GR 2004 Influenceof weather extremes on the water levels of glaciated prairie wetlandsWet-lands 24 385ndash398

Larson D 1995 Effects of climate on numbers of northern prairie wet-lands Climatic Change 30 169ndash180

National Assessment Synthesis Team 2000 Climate Change Impacts onthe United States The Potential Consequences of Climate Variability andChange Report for the Global Change Research Program New YorkCambridge University Press

Ojima DS Lackett JM 2002 Preparing for a Changing Climate The PotentialConsequences of Climate Variability and ChangemdashCentral Great PlainsFort Collins Central Great Plains Steering Committee and AssessmentTeam Colorado State University

Omernik JM 1995 Ecoregions A spatial framework for environmentalmanagement Pages 49ndash62 in Davis W Simon T eds Biological Assess-ment and Criteria Tools for Water Resource Planning and DecisionMaking Boca Raton (FL) Lewis

Poiani KA Johnson WC 1991 Global warming and prairie wetlands Bio-Science 41 611ndash618

Poiani KA Johnson WC Swanson GA Winter TC 1996 Climate change andnorthern prairie wetlands Simulations of long-term dynamics Limnologyand Oceanography 41 871ndash881

Shapley MD Johnson WC Engstrom DR Osterkamp WR 2005A 1000-yearrecord of late-Holocene flooding and drought from Day County SouthDakota reconstructed from tree rings lake sediments and ancientshorelines The Holocene 15 29ndash41

Sharpley AN Williams JR eds 1990 EPICmdashErosionProductivity ImpactCalculator 1 Model Documentation Washington (DC) USDA Agri-cultural Research Service USDA Technical Bulletin 1768

Sorenson LG Goldberg R Root TL Anderson MG 1998 Potential effects ofglobal warming on waterfowl populations breeding in the northernGreat Plains Climatic Change 40 343ndash369

Tiner RW 2003 Geographically isolated wetlands of the United States Wet-lands 23 494ndash516

van der Valk AG ed 1989 Northern Prairie Wetlands Ames Iowa State Uni-versity Press

van der Valk AG Davis CB 1978 The role of seed banks in the vegetationdynamics of prairie glacial marshes Ecology 59 322ndash335

Viljugrein H Stenseth NC Smith GW Steinbakk GH 2005 Density de-pendence in North American ducks Ecology 86 245ndash254

Weller MW Spatcher CE 1965 Role of Habitat in the Distribution and Abun-dance of Marsh BirdsAmes Iowa State UniversityAgricultural and HomeEconomics Experimental Station Special Report no 43

Williams JR Dyke PT Fuchs WW Benson VW Rice OW Taylor ED 1990EPICmdashErosionProductivity Impact Calculator 2 User Manual Wash-ington (DC) USDA Agricultural Research Service USDA TechnicalBulletin 1768

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologiclandscape perspective Journal of the American Water Resources Asso-ciation 36 305ndash311

mdashmdashmdash ed 2003 Hydrological Chemical and Biological Characteristics ofa Prairie Pothole Wetland Complex under Highly Variable Climate Con-ditionsmdashthe Cottonwood Lake Area East-Central North Dakota Wash-ington (DC) US Geological Survey US Department of the InteriorProfessional Paper 1675

Winter TC Rosenberry D 1998 Hydrology of prairie pothole wetlandsduring drought and deluge A 17-year study of the Cottonwood Lake wet-lands complex in North Dakota in the perspective of longer term mea-sured and proxy hydrological records Climatic Change 40 189ndash209

Woodhouse CA Overpeck JT 1998 2000 years of drought variability in thecentral United States Bulletin of the American Meteorological Society79 2693ndash2714

872 BioScience bull October 2005 Vol 55 No 10

Articles