abundance–range size relationships in the herbaceous flora of central england

Journal ofEcology 0887\75\ 328Ð337

Þ 0887 BritishEcological Society

AbundanceÐrange size relationships in the herbaceous

~ora of central England

KEN THOMPSON�\ JOHN G[ HODGSON and KEVIN J[ GASTON$NERC Unit of Comparative Plant Ecology and $Department of Animal and Plant Sciences\ University ofShef_eld\ Shef_eld S09 1TN\ UK

Summary

0 Using data from a survey of over 09 999 0!m1 quadrats in a 2999!km1 area\ weexamined the relationship between abundance and range for the vascular plant ~oraof central England[1 At the level of the whole landscape\ abundance was not related to local\ regional ornational range[ Local\ regional and national range were closely related to each other[2 At the level of the whole landscape\ range was signi_cantly and positively relatedto both niche breadth "expressed as the range of habitats exploited# and to habitatavailability\ although niche breadth appeared to be more important[ Abundance wasnot related to niche breadth or habitat availability[ Since specialist species are mainlycon_ned to uncommon habitats "especially wetlands#\ we conclude that the relation!ship between range and niche breadth is not an artefact of widespread species passivelysampling more habitats[3 At the level of individual habitat types\ signi_cant positive relationships betweenrange and abundance were common[ These relationships remained after controllingfor the e}ects of phylogeny[ For predominantly annual weed communities\ therelationship was linear\ but for perennial communities it was markedly {upper tri!angular|\ i[e[ all combinations of range and abundance were found except widerange:low abundance[ The evidence suggests that this di}erence can be attributed tothe greater mobility of annual weeds[

Keywords] habitat\ land cover type\ niche breadth\ phylogeny\ rarity

Journal of Ecology "0887# 75\ 328Ð337

Introduction

One of the more widely supported generalizationsemerging from ecology at the present time is thatlocally abundant species tend to be widespread andlocally rare species tend to be narrowly distributed"Gaston 0885^ Gaston et al[ 0886#[ Although thisrelationship has been reported from a variety of taxa\relatively few of the published examples concernplants "Brown 0873^ Rapoport et al[ 0875^ Gotelli +Simberlo} 0876^ So�derstro�m 0878^ Collins + Glen0889^ Hanski et al[ 0882^ Rees 0884#[ Nor has therelationship been found on all the occasions that ithas been sought for plants "Hanski et al[ 0882#[ Inthis study we used extensive published "Grime et al[0877^ Hodgson et al[ 0884# and unpublished survey

� Correspondence] Ken Thompson\ NERC Unit ofComparative Plant Ecology\ University of She.eld\She.eld S09 1TN\ UK "fax 9003 1659048^ e!mailKen[ThompsonÝshe.eld[ac[uk#[

data from central England to investigate this patternfurther[ In particular\ we attempted to answer thefollowing questions for a plant assemblage[ "i# Is apositive relationship between range and abundancefound at the whole landscape level\ or only withinmore narrowly de_ned habitats< "ii# Is the relationshipfound in all habitats\ and is the form of the relation!ship similar in all habitats< "iii# Do our results allowus to add anything useful to the debate about thecauses underlying the relationship<

Data and de_nitions

Survey methods are described in detail in Hodgson"0875# and Grime et al[ "0877#[ Brie~y\ identities androoted frequencies of all higher plant species wererecorded in over 09 999 0!m1 quadrats in a 2999!km1

area of central England[ For each species in eachquadrat\ the number of 09 × 09 cm subdivisions inwhich the species was rooted was counted\ and this

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Abundance andrange size in theEnglish ~ora

Þ 0887 BritishEcological Society\Journal of Ecology\75\ 328Ð337

number converted to percentage frequency[ Meanrooted frequency of a species is the mean of occupiedquadrats only[ The use of rooted frequency ensuresless seasonal variation than estimates of cover[ Quad!rats were deliberately placed to include all the majorhabitat types of the region\ and a signi_cant pro!portion were targeted on known localities of rareplants[ Every quadrat was allocated to one of the 21terminal categories "Grime et al[ 0877# of a detailedhierarchical habitat classi_cation system[ The surveywas designed speci_cally to record the herbaceous"and dwarf shrub# ~ora and did not adequately recordthe distribution or abundance of trees and largeshrubs[

A recurrent concern of animal ecologists is thatrangeÐabundance correlations could be sampling art!efacts\ simply because species that occur at low den!sities will tend to be recorded from fewer localities atwhich they actually occur than species that occur athigh densities "Gaston + Lawton 0889^ Hanski et al[0882^ Gaston 0883#[ It is worth emphasizing at theoutset that for our data this possibility can bediscounted[ The ~oras of Britain as a whole and thesurvey area in particular are well documented\ andapparently narrowly distributed species have notsimply been overlooked[ Note that many quadratswere directly targeted on rare plants^ no realistic pro!gramme of random sampling would have encounteredmore than a tiny minority of such species[ It is possiblethat this could have arti_cially in~ated the abundanceof rare species\ if locally abundant populations weresampled in preference to locally scarce ones[ However\this certainly did not occur in the case of the rarestspecies\ for which all known populations weresampled\ and also seems unlikely to have been a prob!lem for the less rare species[

Throughout this paper\ abundance of a speciesmeans rooted frequency in percentage at the 0!m1

scale\ measured in a 2999!km1 area of centralEngland\ as described above[ We used three measuresof range at the whole landscape scale[ National rangeis the number of 09 × 09 km grid squares "hectads#occupied by a species in the British Isles[ Regionalrange is the number of hectads occupied in north!central Britain[ Local range is the number of 0!km1

grid squares occupied in the survey area[ When weconsider patterns at the habitat level\ where range ismuch more a matter of occurrence in discrete butvariably sized habitat patches\ range is the number ofquadrats occupied in the survey area[ Before sta!tistical analysis\ we transformed both range and abun!dance\ often but not always logarithmically\ in anattempt "not always wholly successful# to homogenizevariance[

Phylogeny

Theoretically\ positive interspeci_c relationshipsbetween abundance and range could arise from the

shared ancestry of the species in the assemblage underconsideration[ This is most likely to occur if rangeand:or abundance are strongly associated withphylogeny\ i[e[ if much of the variation in these attri!butes is found at a high taxonomic level[ Peat + Fitter"0883# have already shown that for the British rangethis is not the case for plants "Table 0#^ indeed Britishrange is a relatively unusual example of a trait thatvaries almost entirely at the species level[ We con!ducted a nested ANOVA of abundance and found thatjust over half of the variation occurs at the specieslevel[ The remainder is spread rather evenly betweensubclasses\ families and genera "Table 0#[ Gaston et al["0886# also reported that whenever phylogeneticallycontrolled analyses have been performed\ signi_cantrelationships between abundance and range "in birds\mammals and macrolepidoptera# have not arisenfrom the non!independence of data points[ Thereforewe are probably justi_ed in assuming that phylo!genetic constraints make little contribution to therelationships described later in this paper[ Never!theless\ we examined the possibility that theserelationships are at least partly due to an associationbetween abundance and phylogeny\ by conductingphylogenetically independent analyses[

Landscape level patterns

We regressed abundance against range at the national\regional and local scales\ which are themselves highlycorrelated "Table 1 and Fig[ 0#[ At the national andlocal scales there was no evidence of any interspeci_cabundanceÐrange size relationship[ At the regionalscale there was a weak negative relationship\ but theamount of variation accounted for was trivial[ At thelevel of the whole landscape there was therefore noevidence of any relationship between range and abun!dance[

Perhaps this _nding is not too surprising[ The land!scape of the survey area is a patchwork\ with thebroad pattern of patches arising from variation ingeology\ topography and climate\ but their detailedsize and distribution is chie~y a consequence ofhuman activity[ We might therefore expect range tobe largely determined by the available area of suitablepatches[ Wide!ranging species will occupy abundanthabitats[ Note that this idea relates to two distincthypotheses that have been proposed as explanationsfor the interspeci_c abundanceÐrange size relation!ship] the {niche breadth| hypothesis "Brown 0873# andthe {resource availability| hypothesis "Hanski et al[0882^ Gaston 0883^ Venier + Fahrig 0885#[ Theformer hypothesis assumes that widespread "and\ lessplausibly\ locally abundant# species have an ability touse a broader spectrum of resources[ The latterassumes that widespread "and locally abundant#species use widespread "and locally abundant#resources[ As Gaston et al[ "0886# observed\ these twohypotheses are often con~ated\ are not necessarily

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K[ Thompson\J[G[ Hodgson +K[J[ Gaston


Table 0 Percentage variance at each taxonomic level for abundance and British range

Division Class Subclass Order Family Genus Species

Mean rooted frequency 9[6 5[0 09[5 9 07[0 02[2 40[1No[ 09!km squares "from Peat + Fitter 0883# Ð 9 1[4 1[1 9 3[7 89[4

independent\ and are sometimes di.cult todistinguish[ Strictly speaking\ we cannot test either

Table 1 Non!parametric correlation coe.cients betweenrange in the UCPE survey area "local range#\ in north!centralEngland "regional range# and in the British Isles "nationalrange#[ n � 719[ All signi_cant at P ³ 9[990

Local range Regional range

Regional range 9[77National range 9[71 9[83

Fig[ 0 Log mean rooted frequency "abundance# in occupiedquadrats plotted against "a# local\ "b# regional and "c#national range[

hypothesis\ since we do not have a signi_cant abun!danceÐrange size relationship to explain\ but never!theless we can attempt to test some of their assump!tions and predictions about range\ abundance\ nichebreadth and resource availability[

NICHE BREADTH

One measure of "realized# niche breadth is the rangeof habitats utilized by a species[ We have attemptedto quantify this by reference to the hierarchical habitatclassi_cation of the survey quadrats[ Seven broadhabitat classes can be recognized] skeletal\ arable\ pas!ture\ spoil\ wasteland\ woodland and wetland\ eachof which "except arable# is an amalgamation of anumber of the terminal habitats described earlier[Habitat de_nitions can be found in Grime et al["0877#[ For each species\ relative frequency in each ofthese broad habitats can be assigned to one of _veclasses\ where 4 is very common and characteristic ofthe particular habitat "percentage frequency ×3 timesthat in the survey as a whole# and 0 is largely absentfrom the habitat "percentage frequency ³9[14 timesthat in the survey as a whole#[ For each species\ theseseven scores can form the basis of a {specialism index|\which is the sum of the absolute di}erences of eachscore from the median value of 2[ Thus a species thatis equally frequent in all seven broad habitat typeswould have seven scores of 2 and a specialism indexof zero[ In fact\ only one species "Poa trivialis# ach!ieves this[ At the opposite extreme\ a species that islargely restricted to one habitat type will have onescore of 4 and six scores of 0\ giving a maximumspecialism index of 03[ Most species will lie some!where between these extremes[ We do not considerwhether the success of generalists in a range of habi!tats arises from ecotypic di}erentiation or fromphenotypic plasticity[ Both may be involved "Snaydon+ Bradshaw 0850^ Sultan et al[ 0887#[ The specialismindex was not calculated for species that occurred infewer than _ve quadrats[

In Fig[ 1 we plotted local range and abundanceagainst specialism index[ The relationship with rangewas highly signi_cant and explains about 24) of thevariation in the data "Fig[ 1a#\ providing strong sup!port for the hypothesis that widespread species use awide variety of habitat types[ In contrast\ there was aweak but signi_cant tendency for more specializedspecies to be locally more abundant "Fig[ 1b#[

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Fig[ 1 Local range "a# and abundance "b# plotted against aspecialism index that measures the range of major habitattypes exploited in the survey area[ A high specialismindex � a limited range of habitats exploited[

RESOURCE AVAILABILITY

In principle\ testing the hypothesis that range isrelated to resource "i[e[ habitat# availability is simple[In practice there are two serious di.culties[ First\de_nition of the {habitat| of a species is di.cult\because species vary greatly in their habitat speci_city[We therefore resorted to determining the most com!mon terminal habitats\ listed in Grime et al[ "0877#[The most common terminal habitat is that in whichthe species is most frequent\ i[e[ occurs in the greatestproportion of quadrats[ Note that this habitat will bea good guide to the habitat preferences of specialistspecies\ but an increasingly poor one for species ofwide ecological amplitude[ Secondly\ determinationof the extent of habitats within the survey area isdi.cult[ The 09 999 survey quadrats themselves pro!vide one estimate of habitat abundance\ but of coursethe use of such data would generate an inevitablecircular relationship[ We therefore used data from the14!class version of the Institute of Terrestrial Ecology"ITE# Land Cover Map of Great Britain "Fuller et al[0883#\ although in fact only 08 habitat classesoccurred in the survey area[ Each terminal habitatwas then linked\ wherever possible\ to one or moreITE land cover types\ and thus to a correspondingpercentage of the survey area[ For example\ the ter!minal habitats {arable| and {unshaded mire| werelinked to the ITE classes {tilled land| and {rough:

marsh grass| plus {lowland bog|\ respectively[ It wasnot possible to link some terminal habitats\ e[g[{hedges| and {walls|\ to any ITE cover type[ For eachterminal habitat that was the most common for atleast 09 species\ a mean local range and abundancewas then calculated for those species\ using data fromthe whole survey area[ Mean range and abundancewere then plotted against percentage of the surveyarea occupied by the habitat "Fig[ 2#[ Note that thisapproach involves many approximations\ both in theestimation of species| habitats and in the area occu!pied by these habitats in the survey area\ and is there!fore very conservative[

There was a signi_cant positive relationshipbetween local range and habitat availability "Fig[ 2a#^widespread species occupy widespread habitats[Moreover\ the true relationship may be stronger thanthat shown[ Two habitats\ road verges and acidicunenclosed pasture\ stand out because although theyare very abundant\ they contain species with rathersmall ranges[ The road verge habitat is linked to theITE cover type {meadow:verge:semi!natural grass|\which was abundant in the survey area[ However\most of the species that achieve their highest fre!quency in road verges are not generally common inother types of grassland\ and therefore Fig[ 2a over!estimates the area available for these species[ No sim!ple explanation can be o}ered for the low mean rangeof species of acidic pasture[

In contrast\ there was no relationship betweenabundance and habitat availability^ locally abundantspecies do not tend to occur in widespread habitats[Indeed there was a non!signi_cant tendency for thereverse to be true "Fig[ 2b^ see also Fig[ 1b#[ In Fig[ 2cwe tested the obvious remaining question] Are plantsof narrowly distributed habitats more specialized thanplants of widespread habitats< The answer is a quali!_ed {yes|[ The overall relationship in Fig[ 2c is notstatistically signi_cant\ but two habitats stand out asbeing both rare and occupied largely by specialists[These are {lakes\ canals\ ponds and ditches| and {un!shaded mire|\ the only two wetland habitats in Fig[ 2c[Thus while no pattern is evident in the terrestrialhabitats\ there is a clear tendency for plants of wetlandhabitats "which were rare in the survey area# not tooccur outside wetlands and thus to have small rangesin the survey area[

If\ as Fig[ 2c suggests\ niche breadth and habitatavailability are not independent\ which has the largeste}ect on range< We can test this by a step!wise mul!tiple regression of both variables on range for the 390species for which both are known[ The specialismindex enters the regression _rst and accounts for15[4) of the variance in the data\ while habitat avail!ability accounts for only a further 0[4) of the vari!ation[ Thus\ to the extent that the specialism index isa measure of niche breadth\ this is a more importantdeterminant of range than habitat availability[Neither is related to abundance[

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Fig[ 2 Relationship between the proportion of the surveyarea occupied by a range of habitats and "a# mean localrange\ "b# mean abundance and "c# mean specialism indexof the species that reach their highest frequency in thosehabitats[ See text for habitat selection criteria[ Unlabelledhabitats in "c# are terrestrial[

Habitat level patterns

There was no evidence of any relationship betweenrange and abundance at the landscape level[ Does theposition change at the level of individual habitats\ andhow _nely do we have to focus in order to detect anypattern< We can begin to answer this question byregressing abundance on range for each of the sevenbroad habitat types described above[ The results "notshown# do not di}er appreciably from those at thewhole landscape scale[ The regressions are either notsigni_cant or "rarely# marginally signi_cant and theproportions of variance explained are negligible"c[ 0) in most cases#[ The lack of any relationship

suggests that the broad habitat types are still toodiverse\ and that the processes determining range "i[e[habitat patch occupancy# are di}erent from thosedetermining abundance within patches\ as describedabove for the landscape scale[

If we focus on the terminal habitats of the hier!archical habitat classi_cation\ a di}erent pictureemerges[ Signi_cant positive relationships are foundin nearly every case\ although the proportion of vari!ance explained varies considerably[ Figure 3 showssome typical examples[ In plant communities thatconsist entirely or largely of perennials "Fig[ 3aÐe#\there is a distinctly {upper triangular| pattern\ inwhich all combinations of range and abundance seempossible except high range:low abundance[ In manycases the regression explains about 19) of the vari!ance\ but in some cases the proportion is much lower[In communities of annual plants "Fig[ 3fÐg# there is amore straightforward linear relationship[ In fallowarable communities\ which consist largely of annualweeds but have a perennial component\ the pattern isintermediate\ but closer to that in annual communities"Fig[ 3h#[

We also conducted separate phylogenetically inde!pendent analyses of the data for each community inFig[ 3\ using a modi_ed version of the CAIC "Com!parative Analysis by Independent Contrasts# packagefor the Apple Macintosh computer "Purvis + Ram!baut 0884#[ This new package runs under the Micro!soft Windows v2[0 Graphical User Interface for IBM!compatible personal computers and performs thesame calculations as the original program[ Stan!dardized linear contrasts were calculated using theCrunch procedure designed for continuous variables[Further details of the program and of the phylogenyemployed can be found in Hodkinson et al[ "in press#[We then regressed the standardized linear contrastsof abundance against the contrasts for range\ forcingthe regression through the origin "Purvis + Rambaut0884#[ The results of these analyses are summarizedin Table 2[ Strictly speaking\ one cannot compare r1

values from regressions forced through the origin withthose from unconstrained regressions[ Nevertheless\the correspondence between the regressions in Fig[ 3and Table 2 is striking\ suggesting that the relation!ship between range and abundance at the habitat levelowes little or nothing to phylogeny[

Discussion

Previous reports of a positive interspeci_c abun!danceÐrange size relationship in plants have tendedto concentrate on quite narrowly de_ned habitattypes^ for example prairies "Gotelli + Simberlo} 0876^Collins + Glen 0889#\ bryophytes on logs "So�derstro�m0878#\ sand dune annuals "Rees 0884# and coastalmeadows "Hanski et al[ 0882#[ Previously\ only Rapo!port et al[ "0875# had reported a positive relationshipfrom a diverse landscape "Berkshire#\ but their study

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Fig[ 3 Log mean rooted frequency "abundance# in occupied quadrats plotted against local range in eight habitat types occupiedmainly by perennials "aÐe# or annuals "fÐh#[ Spearman correlation coe.cients] "a# r � 9[33 "P ³ 9[990#\ "b# r � 9[14 "P � 9[990#\"c# r � 9[37 "P ³ 9[990#\ "d# r � 9[35 "P ³ 9[990#\ "e# r � 9[12 "P � 9[996#\ "f# r � 9[33 "P ³ 9[990#\ "g# r � 9[34 "P ³ 9[990#\"h# r � 9[35 "P ³ 9[990#[

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Table 2 Regression analysis of standardized linear phylogenetically independent contrasts between range and abundance ineight contrasted plant communities[ In each case regression was forced through the origin[ n � number of contrasts

Community n r1 P

Unenclosed limestone pasture 012 9[19 ³9[990Woodland on limestone 71 9[99 9[78Lawns 51 9[16 ³9[990Unenclosed pasture on acidic substrata 56 9[18 ³9[990Riverbanks on calcareous substrata 67 9[01 9[990Brassica _elds 38 9[08 9[990Wheat _elds 47 9[15 ³9[990Fallow arable 090 9[08 ³9[990

employed an unknown\ {random| sample of only 79species and measured abundance at a very coarsescale[ The results presented here are therefore the _rstwell!documented study of the entire ~ora of a diverselandscape at all scales from individual habitats up tothe whole landscape[ They allow a new perspectiveon some of the hypotheses proposed to account forpositive abundanceÐrange size relationships[

As Gaston et al[ "0886# observed\ neither the nichebreadth nor the resource availability hypothesis hasmuch di.culty in explaining why species that utilizewidespread resources are themselves widespread[They _nd it much harder to explain why such speciesshould also be locally abundant[ Essentially bothhypotheses require that widespread resources shouldalso be locally abundant[ The present study showsthat\ at the landscape scale\ this is clearly not the case[Resources "habitats# suitable for\ say\ wetland plantsmay well be locally abundant where they occur\ yetscarce in the landscape as a whole[ It is therefore notat all surprising that no rangeÐabundance relationshipcan be detected in a diverse\ patchy landscape[ Ourdata indicate that the distribution of plants can beexplained\ at least partly\ by the distribution of habi!tats[ Abundance cannot be explained in such terms\and we have deliberately avoided the di.cult topic ofexactly what does determine abundance in plants[Why some plants routinely attain high abundance\but others rarely if ever do so\ is a complex subjectwith its own large literature "for example\ Grime0876#[

Breadth of resource usage appears to explain quitea lot of variation in range "Fig[ 1a#[ However\ as Burg!man "0878# has elegantly demonstrated\ such appar!ently convincing patterns can easily arise fromsampling bias[ Even if species sample the landscapeat random\ widely distributed species will occur in awider range of habitats "and thus appear to be moregeneralist# than narrowly distributed species[ For!tunately\ however\ sampling bias makes some quitespeci_c predictions about the observed pattern ofspecialism\ and these predictions are easily tested[ Ifthe pattern in Fig[ 1a is a sampling artefact\ then nar!rowly distributed species will be more frequent in

common habitats[ Thus most apparent specialists willbe plants of arable _elds or various types of grassland\which overwhelmingly dominate the survey area[ Incontrast\ if the pattern has a real biological explan!ation\ we would not expect specialist species to becon_ned to common habitats[ It is less clear what wewould expect^ perhaps the most realistic expectationis that specialists would be most frequent in the mostdistinctive habitats\ i[e[ those habitats that di}er mostfrom the majority of the survey area[ Figure 1b isentirely consistent with this latter prediction[ In awide variety of terrestrial habitats\ there is no relationbetween habitat area and the mean specialism indexof the species present^ specialists are not con_ned tocommon habitats[ Habitats that are rare but distinc!tive\ i[e[ mires and "especially# water\ are characterizedby a highly specialized ~ora[ This interpretation ofthe data is con_rmed by an analysis of unique species\i[e[ those that occur only in one narrowly de_nedhabitat type[ For the habitats analysed in Fig[ 2\ thisnew analysis con_rms that wetlands\ despite theirlimited extent\ contain many unique species\ whilethe number of unique species in terrestrial habitats isunrelated to habitat area "Fig[ 4#[ We therefore thinkit is unlikely that the pattern in Fig[ 1a is an artefact[

Fig[ 4 Relationship between the proportion of the surveyarea occupied by a range of habitats and the number ofspecies unique to those habitats[ Unlabelled habitats areterrestrial[

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At the level of individual habitats\ a positive abun!danceÐrange size relationship seems universal in thisstudy[ Can this pattern be accounted for by a cor!respondence between the large scale and local abun!dance of habitats< Here we need to digress brie~y intowhat is meant by {habitat availability|[ Plants\ say inlimestone grassland\ have unique tolerances along thevarious axes that make up the limestone grasslandhabitat[ These axes include "inter alia# grazing inten!sity\ altitude\ aspect\ soil moisture and depth\ pH andother mineral nutrients and incidence of _re[ Manyof these variables will vary both within and betweenhabitat patches\ but some\ particularly past and pre!sent management practices\ will be relatively uniformwithin patches[ Some species\ either because their par!ticular requirements are widespread or because theyhave wide tolerances\ will _nd their requirements metin more limestone grassland patches than other species[However\ since most environmental variables varywithin patches as well as between them\ plants that_nd their requirements met in many patches are verylikely to _nd them met over a large part of individualpatches[ This satisfactorily explains why\ within asu.ciently narrowly de_ned habitat type\ widespreadplants are also locally abundant "Fig[ 3#[ Pursuing theexample of limestone grassland\ it also explains whysome scarce species "at least in perennial plant com!munities# are locally abundant[ Some species\ eitherbecause their particular requirements are scarce orbecause they have narrow tolerances\ will _nd theirrequirements met in relatively few limestone grasslandpatches[ The factors that make these few patches suit!able\ which may involve any of the variables listedabove\ may also be scarce within patches[ Alter!natively\ these factors may be scarce at the landscapescale but common within patches[ This latter situationis particularly likely to apply to site factors that arisefrom an unusual site history^ there is a good deal ofevidence that the occurrence of uncommon plants isoften related to peculiarities of site history rather thanany obvious modern features "Pigott + Walters 0843^Rackham 0879#[ Thus narrowly distributed speciesmay be either locally abundant or not[ The only for!bidden combination of range and abundance is widerange and low abundance\ which would require habi!tat features that are widespread in the landscape butalways rare within individual patches[ It is hard toimagine what such features would be\ and it is there!fore not surprising that we do not observe widespreadplants with low abundance[ Rabinowitz "0870#described plants of this sort as exhibiting {the mostcurious form of rarity|\ and suggested that such plantswere frequent in North America\ for example sparseprairie grasses[ It is not clear whether Rabinowitz|sopinion\ which seems to be based on anecdotalevidence\ would be supported by objective evidence[Certainly Gotelli + Simberlo}|s "0876# data\ fromtallgrass prairie in Kansas\ seem to show a completeabsence of widespread but locally abundant species[

An alternative explanation for the existence ofnumerous locally abundant but narrowly distributedperennials is that the extant populations of thesespecies are the remnants of formerly widespread dis!tributions[ We do not consider this possibility furtherhere\ but the fact that many British plants havesu}ered major recent reductions in range suggests thatthis may be a contributory factor in at least somespecies "Thompson + Hodgson 0885#[ It is also worthnoting that the whole subject of abundance in{patches| is very scale!dependent^ see\ for example\Pearman|s "0886# discussion of the dependence of rar!ity and rates of decline on whether one measuresdistribution at the level of hectads\ tetrads\ 0!km1

squares or individual populations[The existence of many scarce but locally abundant

species "Fig[ 3# also makes the various meta!population hypotheses unlikely\ at least in com!munities of perennial plants[ All variants of thesehypotheses "Hanski et al[ 0882^ Gaston et al[ 0886#require that locally more abundant species will alwaysoccupy more patches at equilibrium\ although clearlythis will depend to a large extent on relative dispersalcapacities[ We explicitly consider the role of dispersalin a subsequent publication\ but at present meta!population hypotheses do not appear to be consistentwith most of the data from this study[ Meta!population processes do\ however\ seem more impor!tant for annual plant communities "see below#[

The annual plant communities of arable _elds andrelated habitats do not conform to the above pattern\in that they do not contain narrowly distributedspecies with high abundance "Fig[ 3#[ Two explan!ations for this di}erence seem likely[ First\ it maybe that arable _elds\ as a result of massive humanintervention\ are much less heterogeneous "bothwithin and between patches# than semi!natural habi!tats[ Much e}ort has been expended\ after all\ inattempting to make them optimal for the growth ofone or other of a handful of fast!growing cultivars[Arable weeds are therefore likely to be either welladapted to the arable habitat\ and therefore bothwidespread and abundant\ or less well adapted\ andtherefore neither widespread nor abundant[ A speciesthat is successful in one wheat _eld will probably _ndits requirements met by most wheat _elds[ A secondexplanation depends on the life histories of arableweeds[ Successful annual weeds must possess a num!ber of traits\ including a phenology that _ts with thetiming of the prevailing crop sowing and harvestingregime and\ increasingly in the modern landscape\herbicide resistance[ However\ an annual weed that isabundant in one or a few arable _elds must also byde_nition possess a high seed output[ Given the sub!stantial transfer of seed between _elds in soil andon vehicles "McCanny + Cavers 0877^ Bakker 0878^Milberg 0880^ Prach et al[ 0884^ Hodkinson +Thompson\ 0886#\ it is inevitable that such species willspread rapidly to other _elds and therefore become

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widespread[ In other words\ the life!history traits thatconfer success in a particular patch would inevitablylead to spread into other patches[ In this sense\ arableweeds may be behaving more like animals\ in tendingto disperse away from high!density patches[

These two hypotheses lead to readily testable pre!dictions[ If the absence of narrowly distributed specieswith high abundance is a consequence of habitat uni!formity\ the same pattern should be found in otheruniform habitats\ even those occupied by perennials[If\ on the other hand\ this absence is due to the highseed output and mobility of annual weeds\ uniformhabitats occupied by perennials should show the{upper triangular| pattern[ We tested these ideas byanalysing data from lowland\ improved ryegrass pas!tures[ These communities\ as a consequence of drain!age\ reseeding\ fertilizer and herbicides\ have beenrendered relatively uniform\ but contain an almostexclusively perennial vegetation[ The results "Fig[ 5#conform to the {upper triangular| pattern\ suggestingthat the mobility of annual weeds is mainly respon!sible for the observed patterns[ It therefore seemspossible that metapopulation hypotheses may bemore important in communities of short!lived\ mobilespecies than in communities of perennial plants[

We conclude that the prevalence in the literatureof positive abundanceÐrange size relationships forplants is a consequence of a concentration of previouswork in relatively uniform plant communities[ Whena diverse\ patchy landscape is investigated\ no suchrelationship is evident at the landscape scale[ At thisscale\ range is strongly linked to habitat availability\which itself appears to be largely a consequence ofrange of habitat tolerance[ This _nding does not\ ofcourse\ preclude the possibility "indeed probability#that in other landscapes highly specialized speciesmight be widespread\ if their preferred habitat hap!pened to be widespread[ At the scale of the individualhabitat type\ positive abundanceÐrange size relation!ships are common[ We provisionally explain the{upper triangular| shape of most of these relationships

Fig[ 5 Log mean rooted frequency "abundance# in occupiedquadrats plotted against local range for species of lowland\improved ryegrass pasture[

as a consequence of the usual\ but not universal\ {self!similarity| of key habitat variables\ i[e[ such variablesare common or rare at all spatial scales[ The absenceof narrowly distributed species with high abundancein arable weed communities is best explained by thegreater mobility of such species[

Acknowledgements

This research was supported by the Natural Environ!ment Research Council\ through its Large Scale Pro!cesses in Ecology and Hydrology programme\ grantnumber GST:91:0100[ We thank Sue Wallis of theInstitute of Terrestrial Ecology at Monks Wood forthe provision of land cover map data\ Henry Fordfor invaluable assistance with the Ecological FloraDatabase\ and two referees for comments on anearlier version[ K[ J[ Gaston is a Royal Society Uni!versity Research Fellow[

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Burgman\ M[A[ "0878# The habitat volumes of scarce andubiquitous plants] a test of the model of environmentalcontrol[ American Naturalist\ 022\ 117Ð128[

Collins\ S[L[ + Glenn\ S[M[ "0889# A hierarchical analysisof species| abundance patterns in grassland vegetation[American Naturalist\ 024\ 522Ð537[

Fuller\ R[M[\ Groom\ G[B[ + Jones\ A[R[ "0883# The landcover map of Great Britain] an automated classi_cationof Landsat Thematic Mapper data[ Photo`rammetricEn`ineerin` and Remote Sensin`\ 59\ 442Ð451[

Gaston\ K[J[ "0883# Rarity[ Chapman + Hall\ London[Gaston\ K[J[ "0885# The multiple forms of the interspeci_c

abundanceÐdistribution relationship[ Oikos\ 64\ 100Ð119[

Gaston\ K[J[ + Lawton\ J[H[ "0889# E}ects of scale andhabitat on the relationship between regional distributionand abundance[ Oikos\ 47\ 218Ð224[

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Received 10 April 0886

revision accepted 2 November 0886

abundance–range size relationships in the herbaceous flora of central england

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