Hydrobiologia 241 : 1 73-184, 1992 .© 1992 Kluwer Academic Publishers . Printed in Belgium .

Spatial and temporal trends in stream macroinvertebrate communities :the influence of catchment disturbance

Carl Richards & G. Wayne MinshallNatural Resources Research Institute, University of Minnesota-Duluth, 5013 Miller Trunk Highway,Duluth, MN 55811, USA ; Department of Biological Sciences, Idaho State University, Pocatello, ID 83209,USA

Received 15 January 1991 ; in revised form 2 January 1992 ; accepted 14 January 1992

Key words: disturbance, recovery, fire, streams, stability, macroinvertebrates, watershed

Abstract

Macroinvertebrate communities of five headwater streams in catchments disturbed by wildfire werecompared with five similar streams with no catchment disturbance . Over the five years of observation,communities in disturbed streams were more similar to one another than they were to reference streams .Communities in disturbed streams exhibited more year-to-year variation than reference streams, althoughsome indication of decreasing variation was evident through time, and species richness was greater inreference streams than disturbed streams . No increasing trend in richness over time was observed indisturbed streams . Stability of the relative abundance structure and persistence of dominant taxa throughtime may be characteristic of temperate streams over moderate time intervals . Local effects of catchment-wide disturbance have persistent effects that alter these trends .

173

Introduction

Despite the importance and widespread use ofstream macroinvertebrate communities for envi-ronmental monitoring and impact assessment,few studies have reported on long-term variationin community parameters (Townsend et al., 1987 ;1989; Weatherley & Ormerod, 1990) . Definitionof the impacts of various anthropogenic or natu-ral disturbances that occur within catchments onlotic communities ideally requires an understand-ing of both the spatial and temporal variance inbehavior of these communities in both disturbedand undisturbed catchments . Few attempts havebeen made to examine both spatial and temporalvariation in macroinvertebrate communities, par-

ticularly when contrasting disturbed and undis-turbed streams (Resh & Rosenberg, 1989) . Thetemporal scale of most analyses of macroinver-tebrate communities in response to disturbance isconfined to a few years or less (Neimi et al., 1990 ;Resh & Rosenberg, 1989) . Since the coupling ofcatchments and associated streams can be influ-enced by events that progress over relatively longtime scales (Likens & Bilby, 1982 ; Molles, 1982),several years may be required to adequately de-scribe the response of macroinvertebrate commu-nities to catchment-scale disturbances .

The scale of disturbance is an important vari-able in ascertaining effects (Minshall, 1988; Sedellet al., 1990) . The spatial scale of interest, in thecontext of this discussion, is the catchment and

174

its influence on macroinvertebrate communitieswithin respective streams . Although several in-vestigations have examined the effects of smallscale (< 1 m2) physical disturbances to streammacroinvertebrate communities (Reice, 1984 ;1985; Robinson & Minshall, 1986) and numerousstudies have examined the effects of chemical,hydrologic, and physical disturbances to streamlarger sections of stream channels (see Neimiet al., 1990), relatively few studies have examineddisturbances that occur over entire catchmentsand their consequence to macroinvertebrate com-munity parameters. Past studies of catchmentdisturbances have largely been limited to the ef-fects of logging (Gurtz & Wallace, 1984; Hawkinset al., 1982; Molles, 1982 ; Newbold et al., 1982 ;Wallace & Gurtz, 1986) and indicate significantimpacts on macroinvertebrate communities .

The effects of disturbance on macroinverte-brate communities in streams are dependant ona variety of factors. The nature and magnitude ofthe disturbance event, geomorphic and hydrologiccharacteristics of the stream and catchments(Resh et al., 1988), and organizational structureof the community (Minshall et al., 1985 ;Townsend, 1989) all play important roles in de-termining outcomes of disturbance. Ideally, com-parisons of disturbed and undisturbed streamsshould include streams with as many similaritiesamong these features as possible . Furthermore, toassure that faunal comparisons are not undulyinfluenced by recolonization mechanisms (Wal-

Table 1 . Characteristics of the study streams ;*

lace, 1990), streams should be located as closelyas possible to one another so that immigrationrates and the total species pool available to allstreams is similar .

The objectives of this study were to examineyear-to-year temporal and spatial variation inmacroinvertebrate community parameters amonga series of reference and disturbed streams incatchments with a relatively homogeneous set ofhydrologic, geomorphic, and landuse character-istics. We took advantage of a naturally occurringdisturbance (fire) that strongly altered the vege-tative characteristics of a series of adjacent catch-ments. Since fire can influence physical andchemical inputs from catchments (Cushing &Olsen, 1963 ; Minshall et al., 1989; Schindleret al., 1980; Tiedemann et al ., 1978) and short-and-long term vegetative characteristics (Arnoet al., 1985; Romme, 1982), we postulated thatmacroinvertebrate community parameters in dis-turbed stream catchments should exhibit greatervariation through time than nearby referencestreams .

Study site

The study was conducted with the Middle Forkof the Salmon River basin in Idaho, USA. Mostof this basin lies within the River of No ReturnWilderness Area and has little sustained anthro-pogenic land use other than summer recreation .

* Range of channel characteristics given is for base and peak flow as measured during the period (October 1979-July 1980) .

Name Streamno .

TypeR = referenceD = disturbed

Catchment areakm'

Streamorder

Slope atstation

Catchment aspect

Widthm

Depthm

Velocitym/s

Dischargem'/s

Marble Ck 07 R 298 .8 5 1 .5 S 13 .0-18 .8 0.24-0.88 0 .30-2.07 1 .2-23 .7Indian Ck 08 R 212.7 4 2 SE 8.0-19.0 0.26-0 .63 0 .73-1 .81 1 .5-21 .8East Fk Indian Ck 11 R 17 .3 3 9 S W 0 .8-5 .0 0 .16-0 .22 0 .26-0.95 0 .10-0.94Pungo Ck 09 R 13 .8 2 11 S 1 .5-4 .5 0.11-0.26 0 .29-1 .11 0 .07-0 .68Teapot Ck 10 R 2 .9 1 18 S 1 .1-1 .7 0.05-0.13 0 .04-0.54 0 .004-0 .089Little Loon Ck 03 D 86.5 5 5 NW 3.7-5 .0 0.21-0.52 0 .45-1 .48 0 .40-3.84East Fk Little Loon Ck 01 D 84.0 4 3 NW 4.8-8 .4 0 .18-0 .59 0 .97 0 .31-3.20West Fk Little Loon Ck 02 D 20.7 4 9 NE 1 .3-2 .5 0 .16-0 .25 0 .24-1 .15 0 .09-0.64Little Ck 04 D 5 .1 2 14 NW 0.2-2 .5 0.04-0.12 0 .16-0.86 0 .004-0 .186Char Ck 05 D 3 .2 2 19 NE 0.6-1 .5 0.04-0 .13 0 .06-1 .08 0 .002-0.212

The streams in this study are accessible only bytrail . Topography of the region is rugged withelevations ranging from 1220 m to 3150 m . Thestreams have high to medium gradients with steepside slopes . Ridge tops and slide slopes are typ-ically vegetated by pine (Pinus albicaulis, Pinusponderosa) and fir (Abies lasiocarpa, Pseudotsugamenziesii) . Dominant vegetation in valley bottomsare alder (Alnus), water birch (Betula occidentalis),and willow (Salix) .

During August and September of 1979, a largefire (26 000 ha) burned within the Middle Forkcatchment . Five streams within burned areas(disturbed) and five stream in unburned areas(reference) were chosen for study based on ac-cessibility and relative similarity of stream andcatchment sizes . Disturbed streams and theircatchments were located within severely burnedareas of the fire . These catchments were entirelyburned. The catchments of reference streams werenot burned and provided a measure of undis-turbed conditions . All streams are tributaries tothe Middle Fork of the Salmon River and arelocated within 10 km of each other . Physicalcharacteristics of the ten streams and their catch-ments are listed in Table 1 .

Methods

Field sampling

Five benthic Surber samples (929 cm 2) weretaken from erosional substrates in each of the 10streams during early August each year from 1980through 1984. Although large numbers of benthicsamples are required to accurately describestream macroinvertebrate communities due to thelarge number of relatively infrequent species (El-liott, 1977), common taxa can be adequately as-sessed through smaller numbers of samples (Min-shall et al., 1985; Townsend et al., 1983). Once-a-year macroinvertebrate samples are commonlyused to describe macroinvertebrate communitiesamong differing catchments (Corkum, 1989 ;Townsend et al., 1983), and between years(Weatherly & Ormerod, 1990 ; Townsend et al .,1987). Samples taken from the same month in

1 75

each year were used to minimize variation due toannual differences in phenology . Macroinverte-brate community structure among seasons exhibita high degree of concordance (Furse et al., 1984) .Although some species may be in egg or earlyinstar stages of development in August, overalltrends in species richness patterns in RockyMountain streams of the region are similar to falland are higher than spring patterns (Minshall,1981). Samples were taken from the same loca-tions within a stream in each year and preservedin a 10% formalin solution for laboratory analy-sis .

In the laboratory, macroinvertebrates were re-moved from samples with the aid of a 40 x dis-section microscope, identified to genus, and enu-merated. When possible, some groups wereidentified to species (e.g . Ephemerellidae, Rhya-cophilidae), however, other groups were not iden-tified further than family due to constraint ontime, availability of adequate keys, or absence ofmature specimens (e.g . Chironomidae, Chlorop-erlidae) .

Data analysis

To compare species assemblages, a dissimilaritymatrix was computed among streams for eachyear and among years for each stream . Dissimi-larity was computed with Sorenson's coefficientas described by Austin & Orloci (1966) and Ban-nister (1968). Species found in less than 5% ofthe total numbers of samples were eliminated fromall calculations to decrease the influence of rarespecies in the analysis .

Spatial relationships among streams were de-termined through hierarchial cluster analysisusing Ward's linkage method (Ward, 1963) . Thedissimilarity matrices were used as input to thisanalysis. Separate analyses were conducted foreach year. This method was used to identifygroupings of streams with similar communitycharacteristics . Patterns within and among yearswere used to identify trends in communitychanges. Cluster analysis was performed usingdissimilarity matrices constructed from speciesabundance data and presence/absence data .

176

These separate analyses were conducted in anattempt to identify the relative influences of fau-nal differences among streams in determiningcommunity relationships .

Spatial relationships among streams also wereexamined through ordination with uncenteredprincipal component analysis (PCA) . This tech-nique is particularly useful when comparing twoor more groups with nonidentical lists of commonspecies or when clusters of data are present withinthe data matrix (Noy-Meir, 1973 ; Pielou, 1984) .Species abundance data were log-transformedprior to analysis .

Temporal trends were examined through sev-eral techniques . To determine the behavior of spe-cies assemblages within individual streamsthrough time, we used dissimilarity coefficientsbetween consecutive years to serve as an indica-tion of the relative amount of change that oc-curred in that period . In addition, the dissimilar-ity coefficient between the first year (1980) andsubsequent years was computed to determine thedegree of change when compared with a referenceyear. These analyses allowed us to determine ifthe amount of changes in species assemblageswas consistent among years and, whether speciesassemblages were consistent through time . Theinfluence of disturbance on these parameters wasexamined by comparing disturbed and referencestreams .

The relative stability of species assemblagesover time in each stream was examined by com-paring the concordance of species abundancerankings over the entire study period (Grossman,1982; Grossman et al., 1982; Meffe & Minckley,1987). Abundance rankings were evaluated foryear-to-year concordance using Kendall's W(Conover, 1971). Since this particular test is sen-sitive to the number of items used in the analysis(Grossman et al., 1982), a comparison was con-ducted using the top ten and 15 most abundanttaxa in each of the ten streams . These taxa com-prised over 80% and 90% of the total numbersof invertebrates during the study respectively . Anadditional calculation was made excluding Chi-ronomidae and Oligochaeta, since these taxa werenot identified beyond family .

Unless otherwise indicated, a probability levelof p < 0 .05 was used to determine significance inall statistical tests .

Results

Spatial trends

A total of 64 taxa was identified from the mac-roinvertebrate samples . The first four clustersformed by the clustering algorithm were used todepict relationships among streams . This numberof clusters described the most consistent relation-ships among the streams. Fewer or more clustersgave less consistent results among years .

Streams were assigned to the same groups dur-ing four of the five years when species abundancedata was used (Table 2) . During the first year(1980), two stream (02 & 03) were assigned todifferent clusters than in the other years . The fourclusters were defined by streams 01, 02, and 03(Group A); 04, 05, and 10 (Group B) ; 07 and 08(Group C); and 09 and 11 (Group D). All refer-ence streams except stream 10 clustered moreclosely to another reference stream than to a dis-turbed stream . These results indicate that spatialrelationships in community structure amongstreams remained constant through time .

Trends also were evident with the presence/absence data, however, they were not as distinct.

Table 2 . Cluster membership of first four clusters formed byhierarchical clustering technique (Ward's) with species abun-dance (SA) and presence/absence (PA) data sets .

Year Cluster

SA 1980 1 4, 5, 10 3, 7, 8 2, 9, 111981 1, 2, 3 4, 5, 10 7, 8 9, 111982 1, 2, 3 4, 5, 10 7, 8 9, 111988 1, 2, 3 4, 5, 10 7, 8 9, 111984 1, 2, 3 4, 5, 10 7, 8 9, 11

PA 1980 1 4, 10 3, 7, 8 2, 5, 9, 111981 1, 3 2, 5, 4, 10 7, 8 9, 111982 3 4, 5, 10 1, 2, 7, 8 9, 111983 1, 2, 3 4, 5, 10 7 8, 9, 111984 1, 2, 3 5, 10 7, 8 4, 11,9

In general, the membership of the four clusterswas similar to that observed with species abun-dance data (Table 2), although, more variationwas observed among years . Reference streamsmost often were closely associated with anotherreference stream. Streams 09 and 11 were theonly streams that remained closely associatedduring all five years . Streams 07 and 08 wereassociated together during four years as werestreams 04 and 10 and 05 and 10 . Streams 02 and03 were most variable in their association . Thesedata suggest that faunal similarities and differ-ences among the streams remained relatively con-stant through time although faunal changes indisturbed streams between years were more pro-nounced .

The first three axes of the principal componentanalysis extracted 68 .4% of the variance in thedata. The greatest amount of variation in the datawas extracted by axis 1(59 .5 %) . This axis (Fig . 1)explained differences in species compositionamong disturbed and reference streams. Refer-ence streams had greater species richness andthis is reflected in the large number of negativespecies correlations with PCA axis 1 (Table 3)and the location of reference streams to the leftportion of the plot and disturbed streams to the

PCA 1Fig . 1 . The first two axes of a principal component analysisof streams in species space . Axis 1 (species composition) ac-counted for 59 .5 % of the variation in species abundance data.Axis 2 (stream size) accounted for 8 .8% of the variation .Numbers refer to stream number (Table 1) .

177

Table 3 . Macroinvertebrate taxa for which principal compo-nent axes explained at least 30% of variance .

right portion of the plot . PCA axis 2 (Fig . 1)largely described the variation (8 .8%) due tostream size . Small streams (stream nos . 04, 05 &10) had higher abundances of several gatherers/shredders and large streams had larger abun-dances of filter feeders (Table 3) .

Temporal trends

Dissimilarity coefficients between consecutiveyears were compared with two-way ANOVA . Asignificant difference (F= 8 .48, p<0.01) existedbetween disturbed and reference streams (Fig . 3) .Dissimilarity coefficients were greater in disturbedstreams than reference streams in all compari-sons . This trend suggests that community com-position was less stable in streams from the dis-turbed catchments . Some indication of a decreasein dissimilarity coefficients between years withtime was apparent (F = 2.76, p < 0.10) .

When each year was examined against 1980 asa reference year (Fig . 4), the only significant trendwas that streams in disturbed catchments exhib-ited greater dissimilarity coefficients over timethan did reference streams (F= 15 .06, p<0.01) .

Taxa r r'

PCA 1Chloroperlidae -0.67 0 .45Cinygmula sp . -0.75 0 .56Zapada sp . -0.76 0 .57Rhyacophila vepulsa -0.65 0 .42Setvena sp . -0.59 0 .35Turbellaria -0.71 0 .51Capniidae -0.62 0 .39Rhyacophila acropedes -0.57 0 .33

PCA 2Neothremma sp . 0 .70 0 .49Yoroperla sp . 0 .82 0 .68Zapada sp . 0 .70 0 .49Brachycentrus sp. -0.78 0.61Hydropsyche sp . -0 .65 0 .42Turbellaria 0 .78 0 .61Capniidae 0.64 0 .41

N<0

-1 -

-2 -

-3 -

4I

3 -

1 -

0 --1

9

i---9-~1 -

1010 10 104

105

2

4 1

455

1t y ----9g

3 32

8

7 8

2

1 8

8

23 37

3 11

7

-4-8

-6

-5 -4 -3 0

1 78

a)0CCU

35

30

cn 25Nco 20rr

A 15

$. .U) 10

5

01

Fig . 2 . Species richness of disturbed and reference streamsduring the five years of observation . Error bars indicate 95confidence intervals . n = 5 for each treatment and year .

Neither disturbed nor reference streams displayedan increase or decrease in dissimilarity coeffi-cients over time after the first year .

PCA scores for each stream among years wereanalyzed to determine if control or disturbedstreams exhibited significant directions of changethrough time along either PCA axis . Ranks oflocations on the PCA axes for disturbed and ref-

1-2 2-3

3-4

Time (years)Fig. 3 . Mean distance (Sorenson's dissimilarity) betweenmacroinvertebrate communities during consecutive years indisturbed and reference streams . Error bars indicate 95 %confidence intervals . n = 5 for each year and treatment .

2

3

4

Time (years)5

4-5

erence streams were compared with Friedman'stest (Zar, 1974) . No significant trends were ob-served .

Trends in species richness over the five years indisturbed and reference streams were comparedwith two-way ANOVA . A significant difference(F=23.53, p<0.01) existed between disturbedand reference streams. Reference streams hadhigher species richness (Fig . 2). A significant dif-ference was also noted among years (F= 2.66,p < 0.05). A multiple comparison test (Least Sig-nificant Difference) indicated that year three hadlower species richness. Although we expected aan increase in species richness in disturbedstreams following the disturbance event, no in-creasing trend was noted .

Baetid mayflies and Chironomidae were amongthe top three most abundant groups in disturbedstreams during the study (Table 4) . Other taxaexhibited more variation among the streams .Within individual streams, several taxa were notcollected in all years of the study . In referencestreams, more variation in rankings for individualtaxa was observed among streams, although, Chi-ronomidae and Oligochaeta often were mostabundant (Table 4) . Of the four methods exam-ined for determining the taxa to be included in theKendall test for concordance, use of both 10 and15 taxa with Chironomidae and Oligochaeta gave

UCCZCn

n

Time (years)Fig. 4 . Mean distance (Sorenson's dissimilarity) betweenmacroinvertebrate communities references against the firstyear of the study . Error bars indicate 95% confidence inter-vals . n = 5 for each year and treatment .

Tabl

e4.

Aver

age

rank

abu

ndan

ce a

nd r

ange

of

dens

itie

s (n

umbe

r/m

2)for the then most abundant taxa of each stream

;

Taxa

Stre

ams

SOl

S02

S03

S04

S05

S07

S08

S09

SlO

S11

Anto

cha

6,0-419.

6Ba

etis

bic

auda

tus

1,2.

2-647.

82, 103

.3-520

.8

2, 6.5-413

.22,

36.

6-454.

1

3, 21.

5-370.

1

6,0-120.

55,

32.

3-279.

8

1, 1

22.0-841

.4

9,0-

139.

9B. tricaudatus

6,0-

99.0

4,0-71

.0

6,0-193.

7

5, 21

.5-189

.44,

40.

4-234.

6

7, 4

3.0-180.

8

5, 37.

7-129.

1Br

achy

cent

rus

5,0-112.

0

5,0-56

.0

2, 1

2.9-393 .

8

1, 17.

2-1125

.5Chloroperiidae

10,0

-120

.5

10,0

-88.

2

10, 8.

6-75

.3

9, 2

6.9-122.

7

7, 8.1-144

.2Ch

iron

omid

ae

2, 5.53-432.

61,

137.7-536

.51, 37.

4-984.

51, 89.

5-72

5.2

1, 1

07.6

-230

6

1, 1

13.0-619

.8 3, 21

.5-2

75.5

2, 4

0.3-671.

4

7, 45.

7-163.

6

3, 1

2.9-359.

4Cinygmula

7,0-146 .

3

8,0-159.

3

9,0-120.

5

10,0

-178

.6

3, 81.

8-30

7.7

5, 58.

1-360.

5

2, 91.

5-322.

8Epeorus deceptivus

8,0-

36.6

9,2.

2-77

.5

9,0-17

.2

9, 2.2-206

.6

4, 1

7.2-378.

8

10,0

-47 .

3E. flavina

9,0-21

.5

5, 2.

2-157.

1

7,0-

21.5

7, 10.

8-156.

0

7, 4.3-150

.6

10,

21.5-127

.0E. longimanus

3,0-467

.0

4, 8.6-228

.1

8,0-30

.1E

. ti

bial

is

6,0-49

.5

9,0-

228.

1

10,

10.8-60.

3Heterlimnius

8, 21.

5-139 .

9

8, 13.

5-127.

0Hydracarina

3, 2.2-234

.6

7,0-

157.

1Lepidostoma

8,0-120.

5Lu

mbri

culu

s

4,0-161.

4Ne

othr

emma

3, 37.

7-573.

0Rh

yaco

phil

a va

grit

a

10,0

-30.

1

6,0-

86.1

10,0-1

5.1

8,0-

131

.3R

. vepulsa

5,0-

260.

4Si

mull

um

4, 4.3-301

.3

3,0-682 .

2

3,0-66

.7

7,0-200.

1

8,0-

189.

3

9,5.

4-99

.5Tubificidae

7,0-

36.6

8,0-157.

1

6,0-150.

6

9,0-

133

.4

4, 2.7

-230

.3

2,0-

927.

5

1, 43

.0-8

35.5

6, 8.1-363

.7

1, 32.3-

992.

6Turbellaria

3, 9

9.5-247.

5

2, 2.2

-263

.6

6, 1

0.8-236.

72,

148

.0-411

.0

4, 37.

7-230.

3Zapada oregonensis

10,0-53.

8

4,0-

413

.2

5, 4

0.4-

307.

7 4,

122

.7-242

.1

6, 5.

4-241

.0Yo

rope

ria

brev

is

5, 8.6

-275

.5

8, 21.

5-152.

8

1 80

Table 5 . Analysis of benthic invertebrate assemblage stability using Kendall's Coefficient of Concordance . For each stream, fiveconsecutive years of data were used . Separate analyses were conducted with 10 and 15 dominant taxa, and with (10, 15) andwithout (10', 15') Chironomidae and annelids .

essentially the same results (Table 5) . Only stream08 did not exhibit concordance over time . WhenChironomidae and Oligochaeta were removedfrom the analysis, results were different . Four ofthe five disturbed streams exhibited no concor-dance over time versus two of five of the referencestreams when 10 taxa were used . Three of thereference streams (streams 09, 10, and 11) hadsignificant concordance over time no matter howthe data were treated (Table 5) . These results sug-gest that the dominance relationships of majortaxa in these streams is relatively stable, but ismore unstable in disturbed drainages .

Discussion

Even in the relatively small geographic area de-fined in this study, reference and disturbedstreams were distinct from one another for theentire 5-year period . These differences were duelargely to contrasts in species richness and faunaldissimilarities between the two types of streams :disturbed streams had fewer species than refer-ence streams . Furthermore, spatial relationshipsamong undisturbed streams remained consistentthroughout the period. These trends were attrib-utable to stability in the relative abundance struc-ture of the communities and persistence of thefauna through time .

The relative abundance of the most commonspecies in undisturbed streams was remarkablystable through the five years of study . This trendhas been observed in other studies that reportedannual variability in relative abundances of mac-roinvertebrates among several years . McElravyet al. (1989) found only small changes in the late-summer benthic fauna of a California coastaldrainage with high species richness over a sevenyear period. Meffe & Minckley (1987) found highstability in a desert stream with a limited speciespool repeatedly subjected to flood events over aperiod of four years, and Weatherley & Ormerod(1990) reported high stability in a series of streamsobserved over five years in the United Kingdom .All of these studies indicate that major structuraltrends of stream macroinvertebrate communitiesin temperate areas exhibit stability when observedover moderate periods of time (0-7 years) .

We found no strong evidence for increasingdissimilarities among faunas at the same locationthrough time in undisturbed streams, althoughour ability to detect changes in uncommon spe-cies was limited due to small sample sizes . Otherstudies in lotic systems indicate that some long-term faunal changes may occur . Both Townsendet al. (1987) and Weatherly & Ormerod (1990)reported some changes in species groups overmoderate time periods, however, as with thepresent study, relationships among sites remained

Streamno .

10 10' 15 15'

w p w P w Pw P

1 0 .476 0.011 0 .344 0.078 0 .427 0.008 0.297 0.1082 0 .452 0.016 0 .318 0.112 0 .474 0.003 0.397 0.0153 0 .477 0.011 0 .262 0 .25 0 .408 0.012 0.266 0.1794 0.499 0.008 0 .428 0.023 0 .509 0.001 0.441 0.0055 0.397 0.037 0 .195 0.457 0.345 0.044 0.513 0.0017 0 .511 0.006 0 .293 0.154 0 .440 0.006 0.326 0.0638 0.245 0.273 0.263 0.223 0 .303 0.099 0.373 0.0259 0 .639 0.001 0 .463 0.012 0 .619 0.001 0.513 0.001

10 0.628 0.001 0.748 0.001 0.729 0.001 0.723 0.00111 0 .563 0.003 0 .520 0.005 0 .659 0.001 0.645 0.001

the same. Although some faunal changes might beexpected due to chance extinction and immigra-tion events over time, most changes may be at-tributable to environmental and biological phe-nomena that occur on large temporal and spatialscales. For example, long-term climate changesor atmospheric deposition patterns may influencefaunal composition over large geographic areasand consequently, such changes would be ob-served simultaneously in streams of adjacentcatchments and spatial relationships amongstream communities would remain similar . Alter-natively, disturbances that occur on a subset ofcatchments within the region such as fire couldcause some streams to deviate from patterns ob-served on the regional scale with undisturbedstreams . Long-term successional changes in ter-restrial vegetation are an example of a biologicchange that could influence stream fauna (Min-shall et al ., 1989; Molles, 1982) .

Spatial relationships among streams in thisstudy were modified by influences of stream andcatchment size that are typical of streams in thearea. Other studies within the Salmon Riverdrainage also found changes in macroinvertebratefunctional groups and species richness of majortaxonomic groups along a gradient from small tolarge streams (Bruns & Minshall, 1985 ; Brunset al ., 1982; 1987). The largest portion of thesechanges occurred over headwater and smallstream sizes similar to those sampled in this study .As with the present study, the number of filterersand overall species increased with increasingstream size .

The stability observed in this study may beattributable to the relative predictability of envi-ronmental conditions in streams in this geo-graphic area. Rocky Mountain snowmelt streamsexhibit high predictability in terms of timing andmagnitude of flow events (Poff & Ward, 1989) .Highest flows occur during snowmelt and sedi-ment yield is correlated with this pulsed input(Bjornn et al., 1977). The frequency of spates dur-ing other portions of the year is relatively low . Thepredictive nature of this environment may allowfor the development of deterministic conditions inwhich density dependent processes control the

1 8 1

relative abundances and distributions of species .These conditions manifest themselves in the formof a persistent or stable assemblage of species(Grossman et al., 1982; Connell & Sousa, 1983)as opposed to a less stable community influencedstrongly by less predictable flow and other phys-ical events .

The increased variation observed in disturbedstreams was likely a response to decreased chan-nel stability caused by large-scale loss of catch-ment and riparian vegetation . We have observedstriking changes in riparian characteristics of thedisturbed streams (G . Minshall, unpublisheddata). Such changes can be associated with in-creases in the total amount and timing of sedi-ment yield (Megahan et al., 1980) and, changes inthe amount of food resources and retentive ca-pacity of the stream that are related to riparianand terrestrial cover (Likens & Bilby, 1982 ;Molles, 1982). These conditions may take severalyears to return to their original state (Minshallet al ., 1989). Substrate instability from catchmentdisturbance can be linked to instability in mac-roinvertebrate communities (Gurtz & Wallace,1984). The frequency at which rocks shift in astream can significantly alter food abundance andmacroinvertebrate communities (Robinson &Minshall, 1986). Many macroinvertebrates re-spond to subtle changes in habitat quality (Rich-ards & Minshall, 1988 ; Robinson et al., 1990) .Because of the constant redistribution of benthicinvertebrates through drift and colonization instreams (Townsend & Hildrew, 1976), ample op-portunity for rapid development of variation inspecies assemblages among patches on small spa-tial scales (e.g . individual rocks) is provided . Inaddition, several studies have demonstrated thatdisturbance determines the outcome of competi-tion for food or space between co-occurring taxa(Hemphill & Cooper, 1983 ; McCuliffe, 1983 ;1984) .

The lower species richness of disturbed streamsduring the time of observation may be attributedpartly to the availability of colonists in the dis-turbed streams . Recolonization of denudedstreams can be a complex process (Cushing &Gaines, 1989) . However, recovery of macroinver-

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tebrate communities in streams following disturb-ance is rapid (< 2 yrs) provided habitat is suitableand sufficient sources of colonists are available(Sheldon, 1984 ; Neimi et al., 1990; Wallace,1990) . In catchment-wide disturbances, refugiathat serve as source pools of individuals for col-onization by drift or aerial colonization by adultsmay be reduced in size and number .Large dis-tances from sources of colonization have beenshown to increase the time of recovery (Gore,1982; Minshall et al., 1983) . In montane aridlandscapes such as much of the western RockyMountains, harsh environmental conditions andthe presence of mountain ridges around evensmall catchments may further impede the rate ofaerial immigration among adjacent catchments .Combined with decreased habitat stability andsuitability, considerable time could be requiredfor complete reestablishment of a pre-disturbancecommunity .

The long-term requirements for completestream recovery following catchment disturbanceindicated by this study denotes the importance ofunderstanding both the temporal and spatial po-sition of a stream reach in relation to past dis-turbance events. Persistent effects of physical andvegetative changes in a catchment can have strongimpacts on organization of stream communities .The consistent response of stream macroinverte-brates across like watersheds make these com-munities excellent long-term monitors of water-shed condition .

Acknowledgements

D . A. Andrews, J . T. Brock, and C . T. Robinsoncontributed significantly towards the completionof this project . We are grateful to many othergraduate students and colleagues that assisted infield and laboratory work . T. LaPoint provideddata for some of the streams for one of the years .The US Forest Service-Challis National Forestprovided logistical assistance . Partial support forthis project was provided by the US Forest Ser-vice and by several Faculty and University Re-search Grants from Idaho State University . Two

anonymous reviewers provided valuable com-ments on the manuscript .

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