ECOGRAPHY 20: 390-399. Copenhagen 1997

Interspecific abundance-range size relationships: range positionand phytogeny

Kevin J. Gaston, Tim M. Blackburn and Richard D. Gregory

Gaston, K J , Blackburn. T M and Gregory, R D 1997 Interspecific abundance-range size relationships range position and phylogeny - Eeography 20 390-399

A number of mechanisms have been proposed to explain the widely observed positiveinterspecific relationship between local abundance and extent of geographic distnbu-tion in animals Here, we use data on Bntish birds to assess two of these hypothesesthat the relationship results from the relative position of a study area with respect tothe geographic ranges of the species which occur there, and that the relationshipresults from a simple difference between taxonomic groups, rather than any generaltendency for more abundant species to have larger range sizes We find support forneither hypothesis Phylogenetically controlled comparative analyses reveal that thepositive abundance-range size relationship is consistently found within taxa, evenwhen abundance and range size are calculated at a variety of spatial and temporalscales Analyses both across species and within taxa show that bird speaes for whichBntain is near to the centre of their distnbution in Europe tend to have larger Bntishrange sizes and higher abundances than do species where Bntain is close to the edgeof their range in Europe However, these relationships do not cause that betweenabundance and range size, because this latter relationship persists within difTerentrange position categones Whether a species is near the centre or edge of itsgeographic range in Bntain may afTect its position on the abundance-range sizerelationship, but does not produce the relationship Range position m Bntain does,however, seem to be related to the magnitude of temporal changes in the range sizesof Bntish birds There is some evidence to suggest that species for which Bntain isnearer to their European range centre have shown smaller changes in distnbutionover the penod 1970-1990 than have speaes for which Bntain is close to theirEuropean range edge

K J Gaslon (k J gaston@sheffield ac uk), Dept oJ Animal and Plant Sciences, Umv ofSheffield, Sheffield, UK S10 2TN - T M Blackburn, NERC Centre for PopulationBiology, Imperial College at Silwood Park, Ascot, Berkshire, UK SL5 7PY - R DGregorv, British Trust f^or Ornithology, Nationai Centre for Ornithology, The Nunnery,Thetford, Norfolk, UK IP24 2PU

Locally abundant species tend to be widely distnbuted, of taxa, for assemblages of varying taxonomic andand locally rare spwcies tend to be restncted m distnbu- ecological breadth, in different biogeographic regionstion That is, for an assemblage of species withm one and habitats, at different spatial scales, and usmg ataxon, there is a positive mtersjjecific relationship be- vanety of measures of abundance and range size. It cantween local abundance and range size (Hanski 1982, be regarded as a very general signature of naturalBrown 1984, 1995, Gaston and Lawton 1988, 1990, assemblagesHanski et al 1993, Lawton 1993, Gaston 1994, 1996) Several hypotheses have been proffered to explain posi-The pattern has been doctmiented for a wide diversity tive abimdance-range size relationships (for a detailed

Accepted 4 December 1996

Copynght © ECOGRAPHY 1997ISSN 0906-7590Pnnted in Ireland - all nghts reserved

390 ECOGRAPHY 20 4 (1997)

review see Gaston et al 1997) Three, in a generalsense, involve artefacts First, these relationships mayresult from inadequate sampling If locally rare speciesare less likely to be recorded at sites at which theyactually occur than are locally abtindant species, then apositive abundance-range size relationship may resultThis possibility was apparently first explicitly notedby Brown (1984), and has subsequently been discussedby a number of authors (Gaston and Lawton 1990,Wnght 1991, Hanski et al 1993, Gaston 1994, see alsoMcArdle 1990)

Second, the relationships could result from the rela-tive position of a study area with respect to the geo-graphic ranges of the species which occur there If, onaverage, abtmdances decline towards the edges of thegeographic ranges of spwcies (Gnnnell 1922), and sj>e-cies occupy a smaller proportion of a study area whenthey are closer to the edges of their ranges, then posi-tive interspecific abundance-range size relationshipscould anse as a result (Bock and Ricklefs 1983, Bock1984) Species closer to the edges of their ranges mighthave smaller range sizes in the study area in two waysa) they might only penetrate a relatively small way intothe study area (e g they might be constrained to itssouthern parts), and b) if not only do abundancesdecline towards range limits but occurrence also be-comes more patchy, then a species closer to the edge ofIts range might be widely dispersed through a studyarea but occupy a relatively small proportion of itWhichever situation applied, the species for which thecentres of their geographic ranges overlapped the studyarea would occur at relatively high abundance and bewidely distnbuted, whilst those for which only theirrange edges overlapped the study area would occur atrelatively low abundances and would be restncted inoccurrence

Third, the relationships could result from phyloge-netic associations among the species in the assemblageClosely related spwcies share adaptations through com-mon ancestry Relationships between sjjecies traits in ataxonomically constrained assemblage may simply ansebecause the traits were shared by a common ancestor ofthe species, and that association has been preserved insubsequent evolutionary radiations, rather than throughthe repeated evolution of the association between traits(Harvey and Pagel 1991) The clearest examples pertainto morphological traits, for instance, the associationbetween laying eggs and bearing feathers has evolvednot 9000-odd times, but once, in the common ancestorof birds However, the same argument applies to traitsthat affect ecological relationships among species Forexample. Nee et al (1991) showed that the negativerelaUonship between abundance and body mass inBntish birds resulted from a difference between passer-ines, which tend to be small-bodied and common, andnon-passennes, which tend to be large-bodied and rareWithin each group (and indeed within Bntish bird taxa

in general, Gregory and Blackburn unpubl), there wasno evidence for any association between abundance andbody mass Therefore, this relationship results from asingle evolutionary difference between passennes andnon-passennes, rather than any general tendency forabundance and body mass to be negatively related Thepositive abundance-range size relationship could like-wise represent a simple difference between taxonomicgroups (e g in British birds, if passennes also had largeranges but see Gregory 1995), rather than any generaltendency for more abundant species to have larger rangesizes

In this paper, we examine the roles of geographicrange position and phylogeny in determining positiveinterspecific relationships between abundance and rangesize for Bntish birds Such patterns have been docu-mented for this avifauna on a number of occasions,although both abundances and ranges sizes of species areheavily influenced by human activities (e g Fuller 1982,Hengeveld and Haeck 1982, O'Connor and Shrubb1986, O'Connor 1987, Gaston and Lawton 1990, Suther-land and Baillie 1993 Blackburn et al in press) Thequality of the sampling used to generate our data sets issuch that we can be confident that the relationships arenot simply generated by sampling artefacts (Gaston et alunpubl) This paper is pan of a larger investigation intothe interspecific relationship between abundance andgeographic range size (Blackburn et al in press, Gastonet al 1997, Qumn et al in press), and is the first toexplore the other two main artefactual explanations indetail for a single assemblage

Methods

Abundance and distribution

Analyses were based on two sets of abundance data andthree sets of distnbution data

Atlas dataEstimates of the population sizes and geographic rangesizes of Bntish breeding birds were taken from theBntish Trust for Ornithology (BTO) New Atlas (Gib-bons et al 1993) The geographic ranges m this atlaswere obtained from four years (1988-1991) of intensivestandardised fieldwork by an extensive network of or-nithologists co-ordinated by the BTO Population sizeswere calculated using a number of different methods,which are discussed at length m Gibbons et al (1993)Population size estimates were standardised to numberof individuals by doubling estimates of numbers ofpairs, numbers of temtones, and numbers of nestsClearly, the accuracy of the assumption that temtonesand nests are equivalent to breeding pairs wiU varybetween species, but is not unreasonable Where arange of estimates was given, we took the anthmetic

ECOGRAPHY 204 (1997) 391

mean Greater than and less than signs were ignoredWe defined the geographic range size of each birdsp)ecies as the number of 10 x 10 km squares occupiedin the Bntish Isles, excluding the Channel Islands, butincluding the Isle of Man Population sizes were log-transformed for analysis.

Common Bird Census dataThe second set of data was denved from the CommonBird Census (CBC), which again is organised and ad-ministered by the BTO A comprehensive descnption ofthe history and methodology of the CBC is given byMarchant et al (1990) The most important feature ofthe CBC data for our purposes is that the censustechnique involves multiple (normally ten) visits to eachsite, and the mapping of all species temtones at thesite This IS labour intensive, but results in data ofhigher quality than many other methods A particularlyattractive feature of the methodology is that differencesbetween conspicuous and inconspicuous species are tosome extent ameliorated a temtory recorded on threeof ten visits to a plot counts the same as one recordedon all ten visits

Census sites are defined on the basis of the habitatthey encompass as either farmland, woodland or "spe-cial" plots We have treated the habitat categonesseparately in all analyses, because preliminary analyses(Gaston et al unpubl) showed that species densitiesdiffered across habitats (see also Gibbons et al 1993)We included m our analysis CBC data from thosewoodland and farmland plots censused in the f>enod1988-1991, the same pwnod over which fieldwork forthe New Atlas was earned out

For each species in each habitat, we calculated meandensity (temtories per hectare) in each year as theaverage density across all sites censused within eachyear The mean density across years in each habitat wascalculated by first taking the average density acrosssites censused withm each year, and then averagingacross years, but using only those sites censused m allyears of the period to avoid problems ansmg fromturnover among sites All zero values were excludedfrom density calculations Plots were only used to esti-mate sp>ecies densities in a given year if the abundancedata from the plot were adjudged by the BTO to besuitable for density estimation The BTO ngorouslyassesses the quality of all CBC censuses and for variousreasons not all are recommended for density estimationFor example, small or highly elongated plots may havea significant proportion of temtones overlapping theplot boundary, which could potentially mflate calcu-lated densities, such sites are judged unsuitable to con-tnbute to density estimates. Zero counts (no mdividualsof a sjjeaes at a site in a year) were excluded from allcalculations, as were abundances denved from nestcotmts The latter are recorded for a small number of(mamly colonial) species, e.g rook Corvus frugtlegus.

woodpigeon Columba palumbus, and were judged to beincompatible with density estimates for other species(see below) All densities were log,o transformed foranalysis

We calculated the geographic range size of eachspecies in each habitat and year as the proportion ofCBC sites occupied by the sjjecies For two reasons, thenumber of sites over which range size was calculatedwas generally greater than that over which density wascalculated First, some sites considered by the BTO tobe unsuitable for use in density calculations are consid-ered appropnate for studies of species composition, andhence can be used m estimates of sp)ecies occupancyacross sites Second, the presence of a species at a sitemay be indicated by a nest count, although nest countswere excluded from our calculations of density Geo-graphic range size was calculated from an average of100 5 (range 99-102) farmland and 95 5 (range 86-103) woodland sites in any single year For the analysesacross years, it was calculated from 46 woodland and53 farmland sites (the number of sites censused in allyears in the period 1988-1991) It was untransformedin all analyses

European rangesApproximate measures of the Western Palearctic breed-mg range of each sjjecies were denved from publisheddistnbution maps (Cramp and Simmons 1977-1983,Cramp 1985-1992, Cramp and Pemns 1993-1994)The distnbutions recorded on these maps were tran-scnbed onto a cylindncal equal-area (Peter's) projectionmap, overlaid by the WORLDMAP (WUhams 1992,1993) equal area gnd for the region In this case, eachgnd square has an area of ca 153 000 km- (the standardWORLDMAP squares of 611 000 km^ were quarteredfor this analysis) European range was estimated as thenumber of these squares occupied by each species (l ethose pnnted in red or similar) within the WesternPalearctic, but excluding Bntain (The latter was neces-sary to remove any possibility of an artefactual correla-tion of Bntish and European range sizes) Squares werecounted as occupied even if the range overlapped onlya small portion of a square European range size wasuntransformed for analysis

Species list and range positions

One hundred and thirteen species were observed onCBC plots Seven (starhng Sturnus vulgaris, house spar-row Passer domesticus, house martm Dehchon urbica,swallow Hirundo rustica, rook Corvus frugilegi/s, jack-daw C monedula, woodpigeon Columba palumbus)were excluded because the CBC data may not giveestimates that are comparable with those of the major-ity of speaes. The full rationale for the first set of

392 ECOGRAPHY 204 (1997)

exclusions is explained in Gaston et al (unpubl.) Inaddition, mtroduced and/or semi-domesticated spwcies(mandarm Atx galertculata, red-legged partndge Alec-torts rufa, pheasant Fhasianus colchtcus, mute swanCygnus olor, Canada goose Branta canadensts, little owlAthene nocttia) were excluded from all range positionanalyses, because of problems with sconng range posi-tion in Bntain for such species

The range positions of species with respect to Bntainwere scored using the scheme employed by Hengeveldand Haeck (1982) This distinguishes four groups, thosefor which the region is central, subcentral, submarginaland marginal to their geographic ranges As appliedhere, tn general, central species occur throughoutBntain and their geographic ranges in Europe extend toboth north and south of the region and subcentralspecies occur throughout Bntain but Britain is on theedge of their European northerly or southerly limit Ageographic range limit crosses Bntain for both submar-ginal and marginal species with the former having asmall to moderate distnbution m the region and thelatter only a very small one The categones to whichspecies belonged were determined from distributionmaps in Jonsson (1992)

Only two species (black redstart Fhoenwurusochruros, cirl bunting Emberiza cirlus) were categonsedas marginal and therefore were excluded from analysesof range position (a conservative treatment) Togetherwith the other exclusions documented above, thisgave a working list of 98 species for analyses of rangepositions

Phylogeny

The effect of phylogeny on abundance-range size rela-tionships was examined using a phylogenetic compara-tive method (Harvey and Pagel 1991) One way tocontrol for the effects of phylogenetic relatedness is toexamine relationships within each pair of taxa below anode in a bifurcating phylogeny The relationship be-tween the vanables is then unaffected by phylogeny,since the taxa in each companson are equally related toeach other This method requires that the true phy-logeny be known (Felsenstein 1985)

Here, we tise a model (Comparative Analysis byIndependent Contrasts (CAIC), Purvis and Rambaut1994, 1995) which applies Felsenstem's approach todata sets for which only approximate phylogenies areavailable. This method calculates a single value ('con-trast') for each vanable withm each taxon (le forspeaes within each genus, genera withm each tnbe, etc)representing its magnitude and direction of changeEach contrast is then scaled usmg information on thelength of the branches leading from that node (or anassumpuon about branch lengths is made if no suchlnfonnation is available, Pagel and Harvey 1989, Har-

vey and Pagel 1991) The independent contrasts calcu-lated for two vanables will show similar changes withineach taxon if the vanables are correlated The set ofwithm-taxon contrasts can be analysed using standardregression techniques (Pagel and Harvey 1989, Harveyand Pagel 1991), although regressions must be forcedthrough the ongin (Garland et al 1992)

We classified Bntish bird spwcies using the phylogenyof Sibley and Ahlquist (1990), based on DNA-DNAhybridization data, with classification below the level oftnbes based on Sibley and Monroe (1990, 1993) Weused this phylogeny while aware of the cnticisms (e gHoude 1987. Sanch et al 1989, Harshman 1994, but seeMooers and Cotgreave 1994), despite the potentialbiases and subjectivity, it is currently probably the best,and certainly the most extensive, avian phylogeny avail-able and IS relatively uncontentious for the species weconsider We assumed that the relative ages of nodes inthis phylogeny are approximated by the AT50H valuesfor each branch Where values were not reported, weassumed the age of the node to be the mean for allother nodes of the same taxonomic level across thewhole avian phylogeny We estimated the length ofeach branch m the phylogeny as the difference in theages of the nodes connected by each branch, and usedthese estimates to scale the contrasts (see above)

We examined the relationship between abundanceand range size within taxa for each habitat in each yearin the penod 1988-1991, as well as the average rela-tionship across those years (the data descnbed in the'Abundance and distnbution' section above) In addi-tion, we examined the within-taxon relationship for allmdividual CBC woodland and farmland sites whichwere censused every year in the penod 1988-1991 (46woodland and 53 farmland sites) Abundance for eachsjjecies for each site was calculated as the mean densityon the site over the penod (excluding years m which aspecies was not recorded), while range size was theproportion of woodland (or farmland) sites from whichthe species was recorded in the pwnod 1988-1991. Allregressions were calculated using the method of ordi-nary least squares (OLS, or Model I)

Results

Range position

As previously reported, positive interspecific abun-dance-range size relationships were found across allspecies in the atlas data set and m the CBC data setsfor farmland and woodland (Table 1, Fig 1) Withinthe three range posiUon categones for which there weresufficient data, positive relationships were also recov-ered for central and subcentral species (Fig 1), but notfor submarginal species (Table 1) In several mstancescorrelation coefficients across species withm the central

ECOGRAPHy 204 (1997) 393

Table 1 The relationship between log abundance and range sizefor all species, and separately within each of the range edgecategones Atlas data range size = number of 10 x 10 kmsquares occupied, abundance = log,o number of individuals mthe Bntish breeding population, data from Gibbons et al 1993)CBC woodland range size = proportion of CBC woodlandsites censused in each year between 1988-1991 (inclusive) onwhich a species was recorded, abundance = log,o mean densityon those sites (averaged first across sues within each year, andthen across years, with zero values excluded) CBC farmlandrange size = proportion of CBC farmland sites censused in eachyear between 1988-1991 (inclusive) on which a species wasrecorded, abundance = log^ mean density on those sites (calcu-lated as for CBC woodland) slope = OLS regression slope,r = coefficient of determination, n = number of species, C =central species, SC = subcentral species, SM = submarginal spe-cies, all = all species in the above three categones

Atlas dataAllCSCSM

CBC woodlandAllCSCSM

CBC farmlandAllCSCSM

slope

0001000011000130 0003

0 560 68044

- 0 39

0 720 820 62

- 0 13

r'

0 540 570 540 07

0 300440 290 13

0 460 530 580 01

n

98531926

71421217

91511723

P

<0 0001<0 0001

000040 2

< 0 0001<0 0001

0 0720 15

<0 0001<0 0001

0 00040 68

and subcentral categones were stronger than thoseacross all species

Analyses of vanance of the different abundance andrange size measures amongst the three range position

75

7JD

§ 55

I -& «g* 4J0

35

3J0

25

•'Cent 'a l speaeso Sub-oenral speaes+ Sub-nnargnal speaes

500 1000 1500 2000 25D0 3000

Geographic range sizeFig 1 The relationship between logio population size (numberof individuals) and geographic range size (number of 10 x 10km squares occupied) in those Bntish bird speaes found onCBC census plots n = 98 Statisucs for the overall relation-ship, and for central, subcentral and submarginal speaesseparately, are given in Table 1

categones were all significant, with the exception of thatfor European range size (Table 2) Central speciesconsistently had the highest abundances and largestrange sizes, and these were generally significantlygreater than those of subcentral and submarginal spe-cies Submarginal and subcentral speaes never showedany significant differences in either their abundances ortheir distnbutions (Table 2)

In all cases, ANCOVA indicates heterogeneityamong the regression slopes of log abundance on rangesize for the three range position categones (Atlas data,Fj 92 = 5.38, p = 0 006, CBC woodland data, F2.65 =6 74, p = 0 002, CBC fartnland data, F, 35 = 4 4, p =0 015) This IS clearly due to the lack of a significantrelationship amongst submarginal sjjecies TTie regres-sion slopes for central and subcentral species do notdiffer (Atlas data, F, g = 0 25, p = 0 62, CBC wood-land data, F, 50 = 0 45, p = 0 5, CBC farmland data,F, ^ = 0 69, p = 0 41), allowing tests for differences mslope elevation between these two categones These alsodo not differ (Atlas data, F, 69 = 0 4, p = 0 53, CBCwoodland data, F, 5, = 1 66, p — 0 2, CBC farmlanddata, F, 65 = 1 48, p = 0.23) Together with Table 2,these restilts indicate that the differences observed be-tween central and subcentral species, at least, in theirrange sizes and distnbutions in Bntam anse because ofdifferences in the position they occupy on the overallabundance-range size relationship, rather than fromthem having different abundance-range size relation-ships

The range size attained by bird spwcies in BritainIS positively correlated with their range sizes acrossEurope (r = 0I7, n = 98, p < 0 0001) Differences inEuropean range size potentially confound analyses ofrange position, because species with small ranges inBntain may have low local abundances not becausethey are closer to the edges of their geographic ranges,but because they have small European ranges, and localabundance and range size are positively correlatedEurope-wide (Gaston 1994) ControUmg for Europeanrange size, however, there is still a positive relationshipbetween abundance and range size in Bntain (Atlasdata, partial correlation = 0 73, n = 98, p < 0 0001,CBC woodland data, partial correlation = 0.56, n = 71,p < 0 0001, CBC farmland data, partial correlation =0.67, n = 91,p<00001). That is, for a given Europeanrange size, more widespread species in Bntain alsohave, on average, higher poptilation sizes and localdensities

We also analysed the residuals of range size andabundance measures on European range size with re-spect to the different range position categones Analy-ses of vanance of these residuals amongst the threerange position categones (Table 3) indicated that therelationships were slightly weaker than when Europeanrange size was not controlled for (cf. Table 2), but stiil

394 ECOGRAPHY 204 (19971

Table 2 Results of ANOVA of the vanable in the first column among range edge categones C = mean across central speciesof the vanable in the first column, SC = mean across subcentral species of the vanable m the first column, and SM = mean aCTOsssubmarginal species of the vanable in the first column Range edge category means with the same letter superscnpt m a row aresignificantly different (Scheffe F-test, p < 0 05) Prop occ = proportional occupancy, or the proportion of CBC woodland (orfannland) sites censused m each year between 1988-1991 (inclusive) on which a species was recorded log density woodland =logio mean density (temtones per hectare) on woodland sites in the years 1988-1991 (averaged first across sites within each year,and then across years, with zero values excluded), and likewise for log density fannland Units are the number of gnd squaresoccupied for European range, and 10 x 10 km squares occupied for Bntish range (see Methods) *p<0 05, • * p < 0 01,•**p<0 001

European rangeBntish rangeProp occ woodlandProp occ farmlandlog population sizelog density woodlandlog density farmland

F

2023 8 '* '4 1*

10 6 * "16 4 * "3 8*5 8**

C±SE (n)

91 8 + 262 (53)2017 + 79 (53)''''0 55 + 0 06 (42)"0 54 + 005 (51)"5 57 + 0 12 (53)'"'

-086 + 006 (42)"-1 39 + 0 06 (51)"''

SC + SE (n)

82 7 + 42 (19)1506+ 110 (19)"0 27 + 007 (12)"0 34 + 0 07(17)4 87 + 0 19 (19)"

- 1 17 + 0 06 (12)"- 1 63 + 0 06 (17)"

SM ± SE (n)

84 7 + 4 4 (26)1121 + 110 (26)''0 36 + 006 (17)0 19 + 0 04(23)"4 48 + 0 14 (26)''

- 0 98+0 07(17)- 1 63 + 0 05 (23)''

significant in all cases Central species again tended tohave the larger range sizes and population sizes for agiven European range size Analysis of covanance onthe residuals once again indicated that there was homo-geneity of slopes of log abundance on range size amongrange position categories once submarginal species wereexcluded (Atlas data F, 68 = 0 089, p = 0 77, CBCwoodland data F, 50 = 0 19, p = 0 67, CBC farmlanddata F|_M = 177, p = 019) Plots of log abundanceresiduals on range size residuals for central and subcen-tral species show no differences in their elevation (Atlasdata F, 69 = 0 39, p = 0.54, CBC woodland dataF,.s, = 1 80, p = 0 19, CBC fannland data F, 65 = 1 96,p = 0 17), but abundance and range size residuals arestill positively related when range position is controlledfor (Atlas data F, 69 = 85 9, p < 0 0001, CBC wood-land data F, 5, =42 39, p < 0 0001, CBC farmlanddata F, 65 = 78 58, p<00001) The conclusions thatspecies in different range position categories have thesame abundance-range size relationship but differentpositions on the overall regression slope are upheldcontrolling for European range size

PhylogenyThe relationship between abundance and range sizeremained consistently positive when the phylogeneticrelatedness of sp>ecies was controlled for using theCAIC program Withm taxa, highly significant positiverelationships were found on both farmland and wood-land CBC sites in every year from 1988 to 1991, whenthe average CBC data for these habitats for these yearswere used, and when analysis was performed on abun-dance and range size data from the New Atlas (Table4) In addition, we found positive local abundance-range size relationships within taxa on all 46 woodlandand 52 out of 53 farmland sites, of which 38 and 45,respectively, were statistically significant (p < 0 05)

In addition, we calculated the within-taxon relation-ships between range position and the vanous measuresof abundance and range size CAIC requires that cate-

goncal vanables be dichotomous, so we merged subcen-tral and submarginal species into a single category Inall cases, there were positive associations between rangeposition category and abundance or range size that is,where related taxa differ with respect to range positioncategory, the taxon categonsed as central tended tohave the higher abundance or wider distribution (Table5) The only exception was that the relationship be-tween range position and density on woodland was notsignificant using either statistical test

We also tested whether there was any influence ofrange position on the relationship between populationsize and geographic range size within taxa This proce-dure involves fitting a regression line to the populationsize-range size relationship within taxa, and then calcu-lating the residuals of the species involved from a slopeof this magnitude (Purvis and Rambaut 1994) Theseresiduals can then be compared between the two rangeedge categones within taxa This method reveals that,for a given range size, there is no relationship betweenpopulation size and range edge category within taxa(t = 0 61, DF = 23, p = 0 55) In addition, the within-taxon relationship between population size and rangesize was still highly significantly positive when restnctedeither to central (r^ = 0 77, n = 47, p < 0 0001) or sub-central/submargmal (r^ = 0 41, n = 40, p < 0 0001) spe-cies In sum, there is no evidence that the abundance-range size relationship within taxa is caused bywithin-taxon vanation in range position

Discussion

Neither differences in the range positions of species northeir phylogenetic relatedness determine the positiveinterspecific abundance-range size relationships repeat-edly observed for the Bntish avifauna (see earlier refer-ences) Accepting that for this assemblage the level ofsampling is adequate to avoid disproportionate under-estimation of the range sizes of locally rare species

ECOGRAPHY 204 (1997) 395

Table 3 Results of ANOVA among range edge categones of the residuals of the regression of the vanable m the first columnon Etiropean range size Range edge category means with the same letter superscnpt in a row are significantly diflerent (ScheffeF-test, p < 0 05) See Table 2 for an expldndUon of abbreviations and vanables *p < 0 05, **p < 0 01, ***p < 0 001

Bntish rangeProp occ woodlandProp occ farmlandlog population sizelog density woodlandlog density farmland

F

21 2***3 1*7 1**

14 1***3 6*4 2*

C + SE (n)

285 7 -1- 66 8 (53)"''0 08 -1- 0 06 (42)0 09-^0 05 (51)'0 38 -(- 0 12 (53)-'''0 08 -1- 0 06 (42)-009-f006 (51)

SC ± SE (n)

-98 5-1-99 1 (19)''- 0 17-1-0 07(12)

0 04-f-0 06 (17)- 0 22-^0 18(19)-- 0 23-1-0 06 (12)-'- 0 I2-I-005 (17)

SM + SE (n)

-510 4-1-121 5(26)'^- 0 07-1-0 07 (17)- 0 18-1-0 04 (23)-- 0 63-1-0 15 (26)''- 0 03-HO 07 (17)- 0 12-1-0 05 (23)

(Gaston et al unpubl), then these relationships seem tobe caused by some other mechanism than the threeidentified as artefactual Other possible mechanisms arereviewed in detail by Gaston et al (1997) Thosebased on resource (habitat) availability and selection,and on spatial variation m vital rates seem most likelyto be important (O'Connor 1987, Gaston et al un-publ)

The fact that differences in the range positions ofspecies do not determine interspecific abundance-rangesize relationships is in agreement with studies whichhave documented the pattem across the entire geo-graphic ranges of sjjecies, where this particular mecha-nism cannot pertain (eg Bock and Ricklefs 1983) ThisIS not to say, however, that vanation amongst species mrange position does not affect the relationship observedfor Bntish birds In particular, the similanty in slopesof the interspecific abundance-range size relationshipsfor s{>ecies in central and subcentral categones, alliedwith the differences in local abundances and range sizes

Table 4 Results of the relationship between abundance andrange size within taxa, with regression forced through theongin n = number of within taxon contrasts (see Methods)Atlas data abundance is measured as log,o population size(number of individuals), range size as the number of 10 x 10km squares occupied (see Methods) CBC woodland andfarmland data for each year, abundance is measured as log^mean density on CBC sites censused in each habitat, and rangesize as the proportion of sites occupied in each habitat, acrossyears (1988-1991), abundance is measured as log,,, meandensity on those CBC sites which were sampled m all yearsin each habitat, and range size as the proportion of thosesites that were occupied in at least one year p < 0 0001 in allcases

Atlas dataCBC woodland19881989199019911988-1991CBC farmland19881989199019911988-1991

0 74

0 220 380400 29040

0410 430.51054051

79

5860605959

7071727174

(Table 2), suggests that the overall abundance-rangesize relationship can be viewed as an amalgam of suchrelationships for sjjecies differing in their range posi-tion The higher correlations for relationships withinrange position categories also suggests that some of thevanation about overall relationships can be explainedby differences in the range positions of species

Species closer to the edge of their ranges have smallergeographic ranges and lower local abundances inBntain (Table 2) Differences in range sizes betweencategories are in part an inevitable result of the way inwhich the categones were defined (see Methods) How-ever, the larger ranges in Bntain of central sp>ecies thansubcentral (Table 2) does not inevitably follow, becauseboth categones embrace species whose distnbutions inthe main broadly extend throughout Bntain

The analyses reported would seem to provide addi-tional evidence for a decline m the local abundances ofindividual spiecies towards the edges of their geographicranges A growing number of studies have demon-strated dechnes in average abundances or occupancytowards range limits (eg McClure and Price 1976,Bock et al 1977, Hengeveld and Haeck 1981, 1982,Brown 1984, Ban and Klosiewski 1989, Roberts et al

Table 5 Results of t-tests and binomial tests of the hypothesisthat there is no association between the vanable in the firstcolumn and range position category (either central or subcen-tral/submarginal) withm taxa If Bntish range, say, is dis-tnbuted at random withm taxa with respect to range position,we would expect equal numbers of positive and negativewithin-taxon contrasts, with the mean value of contrasts notsignificantly different from zero (Purvis and Rambaut 1995) tIS the t-value (with n — 1 degrees of freedom) from a one-sam-ple t-test of the hypothesis that the contrasts are sampled froma population with a mean of zero, and z is the result of abinomial test of the hypothesis that the number of positivecontrasts (out of n) does not differ from random *p < 0 05,**p<OOI, ***p<OOOOI

European rangeBntish rangeProp occ woodlandProp occ farmlandlog population sizelog density woodlandlog density farmland

t

2 9*6 6*3 1*5 7*6 0*1 52 5*

n

• 25»* 25• 1 8'* 24•* 2 5

1824

Numberpositive

19241321251319

z

2 4*4 4 * "1 653 5**4 8"*1652 65**

396 ECXX3RAPHV 20 4 (1997)

Table 6 Results of ANOVA of the vanable in the first column among range edge categones C = mean across central speciesof the vanable in the first column, SC = mean across subcentral species of the vanable m the first column, and SM = mean acrosssubmarginal species of the vanable in the first column Range edge category means with the same letter superscnpt in a row aresignificantly different (SchefTe F-test, p < 0 05) 'Range change' is the change in the number of 10 x 10 km squares occupied byspecies between the first and second atlases calculated as (R2 - Rl), where Rl and R2 are the ranges in the first and secondatlases 'Change index' is (R2 - R1)/([R2 + Rl]/2), this index scales the range change for each species by its range size, and cantake values between —2 and + 2 (Bohning-Gaese and Bauer 1996) |range change| and |chdnge index| are the absolute values ofthese two measures, normalised by square root and loganthmic transformation, respectively Increasing and decreasing speciesare, respectively, those which have undergone increases and decreases in the numbers of occupied squares between the first andsecond atlases Note that larger, i e more positive, values of log |change index| indicate a larger change in range size

all speciesrange changechange indexsqrt jrange change|log {change index{

increasing speciessqn {range change{log {change index{

decreasing speciessqrt {range change|log {change index]

F

1071493 54*9 03***

2 696 63**

1 553 12*

C ± SE (n)

- 7 0 9 + 245 (53)- 0 04 + 0 02 (53)

104 + 0 75 (53)- 1 4 ± 0 0 8 7 (53)'

8 48 + 1 60 (16)- 1 67 ± 0 21 (16)"

11 2 + 0 80 (37)- 1 28 + 0 08 (37)

SC ± SE (n)

-106 6 + 71 1 (19)- 0 07 + 0 05(19)

140+ 1 56 (19)- 1 0 3 ± 0 13 (19)

13 51 + 2 40 (6)- 0 9 8 ± 0 12 (6)

14 2 + 2 05 (13)- 1 05 + 0 18 (13)

SM ± SE (n)

- 1 2 8 + 44 2(26)0 04 + 0 07 (26)

12 79 + 0 95 (26)- 0 82 ± 0 1 (26)"

13 28+1 69 (11)-0.71 ± 0 17 (11)"

124+ 1 13 (15)- 0 9 1 ± 0 12 (15)

1992, Svensson 1992, Telleria and Santos 1993,Hengeveld 1994, Maurer 1994, Whitcomb et al 1994.Brown et al 1995, Carey et al 1995) For example,Telleria and Santos (1993) observe such a pattern forspecies of insectivorous passenne in Ibenan forestsMaps of abundance surfaces across parts or all of thegeographic ranges of panicular sjjecies, including birdsin Bntain, illustrate that these are often very complex(e g Root 1988, Gibbons et al 1993, Price et al 1995)This makes simple models of decline in average abun-dances seem inadequate A more appropnate model isperhaps one in which species near the edges of theirranges show tmiformly low abundances, whilst thosenear the centres exhibit a wide range of abundances(Brown et al 1995, Enquist et al 1995)

Some of the evidence for declines m average abun-dances towards range limits is drawn from studies thathave observed that those species in a study area whichare closer to the edges of their geographic ranges tendto be locally less abundant (e g Hengeveld and Haeck1982, Hodgson 1986) A limitation of such studies hasbeen that it has not been possible to discriminatewhether this was because abundances declined towardsthe penphery of ranges, or because those species closerto their range limits also had small overall geographicrange sizes, and there was a pwsitive interspecific rela-tionship between local abundance and overall geo-graphic range size (Gaston 1994) The demonstration ofpositive relationships between the range sizes of speciesin small areas nested within larger ones (see Gaston1994 for a review) lends support to the latter argumentHowever, the analyses here suggest that it is the formerprocess (the declme in abundance towards the edge of aspecies range) that contnbutes most to the observedlower abundances of species m areas closer to theirrange limits The range sizes of species in Bntain and in

Europe were weakly positively correlated, but havingaccounted for this effect, differences in the local abun-dances of species in the different range position cate-gories were still significant (Tables 2 and 3)

In addition to being at lower abundance, and havingsmaller overall geographic range sizes, species whichattain their range limits in an area are often thought tohave more fluid distnbutions Range edges are knownto ebb and flow with changes in environmental condi-tions (eg Taylor and Taylor 1979, Hengeveld 1990,Gaston 1994) For the bird species considered here, it ispossible to test this hypothesis at a rather crude level,by companng differences in the numbers of 10 kmsquares recorded to be occupied by species in the first(1968-1972) and second (1988-1991) atlases of theBntish avifauna (Sharrock 1976, Gibbons et al 1993)There are no differences between species in differentrange position categories in terms of whether rangesincreased or decreased (x^ = 1 19, DF = 2, NS), of theoverall change in numbers of occupied squares, or of anindex of change in range size (Table 6) However,Ignoring the direction of change, its magnitude tends tobe least in central sf)ecies (Table 6), providing somesuppon for the idea that towards the limits the rangesof species are indeed more fluid This concords withHaeck and Hengeveld's (1980-1981) findings for Dutchplant species, where species m all range position cate-gones were found to have declined to varying extents,but the species which had gone extinct were predomi-nantly at the margins of their geographic ranges

It IS well-known that species do not provide indepen-dent information for the evaluation of hypothesesabout relationships between vanables, because theymay share combinations of traits through commonancestry (Harvey and Pagel 1991) This apphes as muchto ecological as to other life history traits of the species

ECOQRAPHY 2(M (1997) 397

(Harvey 1996) Interspecific patterns may, therefore,result from differences in traits between different taxa,rather than because of any causal relationships (see e gNee et al 1991) Nevertheless, few studies have re-poned relations between macroecological vanables con-trolling for the effects of phylogenetic relatedness (butsee Brandl et al 1994, Blackburn and Gaston 1996a,b,Blackbtim et al 1996, Gaston and Blackburn 1977)

Our analyses show that relationships between abun-dance and range size are consistently positively corre-lated across phylogenies as well as across species Notonly are widespread species more abundant, but thesub-taxon in any higher taxon which has the morewidespread members also tends to have the most abun-dant members For example, the most widespread spe-cies in a genus also tends to be the most abundant Thispattern holds over all temporal and spatial scales exam-ined Most stnkmg is that it applies not only to coun-try-wide abundances, but also to densities withinalmost every local site examined compare two closelyrelated species on a patch of British farmland and themost widespread of the pair m Bntain will likely be themost abundant on that patch

The phylogenetic analyses we present clearly indicatethat any worthwhile mechanism for the widely observedpositive abundance-range size relationship will need toexplain positive relationships not only across species,but also within taxa We have already argued that therange position mechanism cannot explain the interspe-cific patterns Phylogenetic compansons also show thatIt cannot explain the relationships within taxa There areassociations between range position and range size andabundance within taxa (Table 5). However, these do notcause the abundance-range size relationship, becausepositive relationships are found withm taxa when analy-ses are limited only to species in different range positioncategones, and range position explains none of thevanation in density within taxa beyond that explainedby range size Neither of these results supports a deter-tninmg role for range position As with the analysesacross species, taken together they imply that taxa onthe edge of their range have smaller ranges and lowerabundances, but the abundance-range size relationshipoccurs over and above, and not because of, this effect

Acknowledgements - This work was funded by NERC grantGST/03/1211 K J G is a Royal Soaety University ResearchFellow We are grateful to the thousands of volunteers whohave earned out the surveys on which the bird populationestimates were based, to Andy Purvis for providing the CAICprogram, and to John Lawton and Rachel Quinn for com-ments on the manuscnpt

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