The anatomy of the interspecific abundance–range size relationship for the British avifauna: I. Spatial patterns

The anatomy of the interspecific abundance±range

size relationship for the British avifauna:

I. Spatial patterns

AbstractData from the British Trust for Ornithology Common Birds Census and two atlasesof breeding birds were used to examine the form of the interspecific abundance±rangesize relationship for the British avifauna. The relationship is positive for bothfarmland and woodland habitats and over two different periods, with some evidenceof curvilinearity, using either proportion of occupied sites or numbers of occupied106 10 km squares as measures of range size, and mean density at occupied sites as ameasure of abundance. A log-linear plot gives the highest correlation. Therelationship is stronger if based on maximum local densities than if based on averagedensities, but there is no relationship using minimum local densities. Relationshipsbased on abundances at individual sites are uniformly positive for all sites, althoughthe relationships for many sites also show evidence of curvilinearity, especially whenrange size is measured as the proportion of occupied sites. Species show significantconcordance in their rank abundances across sites. We discuss some implications ofthese results.

KeywordsAbundance, birds, community structure, geographical range size, macroecology,population dynamics.

Ecology Letters (1998) 1 : 38±46

I N T R O D U C T I O N

The local abundances and the regional distributions ofspecies are not independent. Within a taxonomic assem-blage, those species that are locally common tend onaverage to be more widespread than those that are locallyrare. That is, there is a positive interspecific abundance±range size relationship (JaÈ rvinen & Sammalisto 1976;Hanski 1982; Brown 1984; Gaston & Lawton 1988, 1990;Hanski et al. 1993; Lawton 1993; Gaston 1994). Thispattern appears quite general, being observed for a varietyof taxa across a spectrum of spatial scales, and usingvarious measures of abundance and range size (for acollation of published studies see Gaston 1996).

Although there is considerable unexplained varianceabout positive interspecific abundance±range size relation-ships, the existence of the pattern has motivated a searchfor a general explanation that transcends the idiosyncrasiesof particular assemblages. At least eight mechanisms havebeen proposed, invoking sampling artefacts, phylogeneticnon-independence, range position, resource breadth,resource availability, density-dependent habitat selection,

metapopulation dynamics, and vital rates (for a review,see Gaston et al. 1997a). These postulated mechanisms arenot all mutually exclusive. Indeed, some of them may becomplementary or closely related (Collins & Glenn 1991;Hanski 1991; Gaston 1994; Brown 1995; Holt et al. 1997),making discrimination of their importance difficult. Witha few exceptions (Burgman 1989; Gaston & Lawton 1990;Hanski et al. 1993; Gaston 1994), however, we still lackexplicit tests of the validity of any of the mechanisms. Partof the problem is that current knowledge of the form ofthe relationship is very general. An unexplored avenue forimproving knowledge of the processes involved is anunderstanding of the detailed form of these relationships(Leitner & Rosenzweig 1997). For example, it remainsunclear whether greater average local abundances of wide-spread species are produced by greater minimum localabundances, greater maximum local abundances, or greaterabundances at all sites. Likewise, surprisingly little attentionhas been paid to the rate of change in mean abundance withincreasing range size, and the variation in this rate.

In this paper we examine the anatomy of aninterspecific abundance±range size relationship for the

#1998 Blackwell Science Ltd/CNRS

Kevin J. Gaston,1 Tim M.

Blackburn,2 Richard D. Gregory,3

and Jeremy J.D. Greenwood3

1Department of Animal and

Plant Sciences, University of

Sheffield, Sheffield S10 2TN, U.K.

E-mail:

[email protected] Centre for Population

Biology, Imperial College at

Silwood Park, Ascot, Berkshire

SL5 7PY, U.K.3British Trust for Ornithology,

Nunnery Place, Thetford,

Norfolk IP24 2PU, U.K.

ECOLOGY Letters, (1998) 1 : 38±46

R E P O R T

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avifauna of Great Britain, using data obtained from theCommon Birds Census (CBC; Marchant et al. 1990) andthe two British Trust for Ornithology (BTO) atlases ofbreeding birds (Sharrock 1976; Gibbons et al. 1993). Anumber of previous studies of abundance±range sizerelationships have used data derived from this assemblage(O'Connor 1981, 1987; Fuller 1982; O'Connor & Shrub1986; Gaston & Lawton 1990; Sutherland & Baillie 1993;Gregory 1995; Blackburn et al. 1997; Gaston et al. 1997a).Here we extend these analyses to examine how therelationship at individual sites contributes to the overallpositive correlation.

M E T H O D S

A comprehensive description of the history and metho-dology of the CBC is given by Marchant et al. (1990). Inbrief, the CBC is organized and administered by the BTO.It constitutes the main scheme by which the breedingpopulations of common British birds have been mon-itored, where ``common'' refers to species with breedingpopulations of more than about 100,000 pairs. Thescheme has been running since 1961, although a standardmethodology allowing strict comparability betweencensus years has only been in place since 1964. Thecensus technique involves repeated (the standard numberis now 10) visits by an observer to a defined site (or``plot'') during the breeding season (March to July). Theobserver maps the positions of all species encounteredduring visits, and these maps are analysed by BTO staff togive estimates of the number of pairs of each speciesbreeding on the plot. Territory mapping is a relativelylabour intensive census technique, but can provide higherquality data than many other methods (e.g. line or pointcount transects). One important feature is that differencesbetween conspicuous and inconspicuous species are tosome extent ameliorated: a territory recorded on three of10 visits to a plot counts the same as one recorded on all10 visits. Furthermore, differences in conspicuousness areallowed for by the BTO analysts who convert observa-tions into numbers of territories.

Census sites are defined on the basis of the habitat theyencompass as either farmland, woodland, or ``special''plots. The last category is a ``catch-all'' that includeshabitats such as suburban parks or wetland, where manyof the latter are managed as nature reserves. We haverestricted our analyses to farmland and woodland plotsand have treated them separately in all analyses, becausepreliminary analyses suggested that species densities wereconsistently different in different habitats (see alsoGibbons et al. 1993). This treatment reflects that of theBTO, who use the CBC to calculate separate populationchange indices for farmland and woodland (e.g. Marchant

et al. 1990). In addition, the mounting evidence thatpopulations of birds on British farmland are undergoingdeclines not necessarily reflected by species in otherhabitats (Fuller et al. 1995), suggested that we should treatfarmland and woodland sites separately.

We included data from woodland and farmland plotsfrom two periods: 1968±72 and 1988±91. These years werechosen because they correspond to those over whichfieldwork was undertaken for the compilation of the twoBTO atlases of breeding birds in Britain and Ireland(Sharrock 1976; Gibbons et al. 1993; see below for therationale underlying this decision). For each ``atlasperiod'' and habitat, we included only those plots thatsatisfied the following two criteria. First, the plot had tobe censused in every year in the period. Second, theabundance data from the plot had to be adjudged by theBTO to be suitable for density estimation in every year ofthe period. The BTO rigorously assesses the quality of allCBC censuses, and not all are recommended for densityestimation. For example, small or highly elongated plotsmay have a significant proportion of territories over-lapping the plot boundary, which could inflate calculateddensities; such sites are judged unsuitable to contribute todensity estimates. From 1968 to 1972 there were 38 plots(25 farmland and 13 woodland) that satisfied our criteria,and from 1988 to 1991 there were 99 (53 farmland and 46woodland). The distribution of these plots across Britainreflects that of CBC plots in general (Marchant et al. 1990),and is shown in Fig. 1. Radical changes to the abundancesof species at CBC sites within each atlas period areunlikely; in particular there were no severe winters duringthese periods.

We calculated the geometric mean density (territoriesper hectare) of each species on each plot over each atlasperiod. Zero counts (no territories of a species at a site in ayear) were excluded from all calculations; their inclusioncan result in artefactual abundance±range size relation-ships (Lacy & Bock 1986; Wright 1991; Gaston 1994).Note that the necessary exclusion of zeros introduces aslight tendency for the densities of the rarer species onplots to be overestimated. This is because territories thatoverlap a plot edge will sometimes be counted, andsometimes excluded from the census. If such a territory isthe only one for a species on a plot, then density will beoverestimated when the territory is counted (because therecorded density is 1 territory per site area, when theactual density is 5 1 territory per site area), andunderestimated when it is excluded; however, only theoverestimates contribute to the density of the species.This overestimation may apply to some density estimatesfor all species because, as is shown below, most speciestend to be rare on at least some CBC plots. In addition tozero values, we also excluded abundances derived from


Abundance±range size relationships I 39

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nest counts, because it is not clear that these are equivalentto the territory counts by which the majority of species arecensused. For both habitats, within each atlas period, wecalculated the mean density of each species across alloccupied sites. Minimum and maximum densities werealso calculated for each species, taken as the lowest andhighest average (over all years in the period) densityrecorded for a species on an individual CBC plot. Alldensities were log10 transformed for analysis.

Interspecific comparisons of mean densities may beconfounded by variation in the size of the census areasover which the densities of different species have beenestimated (Blackburn & Gaston 1996, 1997; Gaston et al.manuscript in preparation). Many species show negativerelationships between the density recorded for them incensuses and the size of the census area. Apparentinterspecific differences in densities may therefore arisesimply as a consequence of intraspecific density±area

relationships and interspecific differences in census area.We have explored this problem elsewhere with respect tothe CBC data, where its effects proved to be trivial whengeometric mean densities were used (Gaston et al.manuscript in preparation).

One hundred and nine species were recorded from allthe sites included in the analyses over the two atlasperiods. We excluded seven of these [starling Sturnusvulgaris L., house sparrow Passer domesticus (L.), housemartin Delichon urbica (L.), swallow Hirundo rustica L.,rook Corvus frugilegus L., jackdaw C. monedula L.,woodpigeon Columba palumba L.], to leave 102 species.House sparrow and woodpigeon were excluded princi-pally because early censuses did not record numbers(Marchantet al. 1990). The other species were omitted fortwo reasons: (i) they are often colonial, so that abundanceson plots where they occur may be overly inflated (this alsoapplies to the house sparrow); and (ii) because they arecolonial, data are principally nest counts, not territory anddensity estimates. The omission of abundance estimatesfrom nest counts causes a reduction in the data availableto calculate densities for a range of species, but excludesmost data for a few species.

We used two measures of geographical range size.The first was simply the proportion of sites (out of 13, 25,46, and 53, depending on habitat and period) contributingto the mean density of a species in each habitat in eachatlas period. Although nest counts were not used incalculating density estimates, they were used to indicatethe presence of a species at a site, and so contributed tothis measure of range size. The second, independent,estimate was the number of 106 10 km squares fromwhich a species was recorded in each BTO atlas (takenfrom Gibbons et al. 1993).

For the sake of simplicity, we limit analyses here tointerspecific relationships between abundance and geo-graphical range size, while being aware of the criticisms oftreating species as independent data points in ecologicalcomparative analyses (see, for example, Harvey 1996).Abundance±range size relationships for British birds tendto remain significantly positive when accounting for thephylogenetic associations among species: these results arereported elsewhere (Blackburn et al. 1997; Gaston et al.1997b). All regression analyses were performed using themethod of ordinary least squares with geographical rangesize as the independent variable. This assignment followsconvention in this field, and reflects not an implieddirection of causality, but rather the fact that errorvariance in geographical range size is likely to be muchlower than in abundance. Ordinary least squares is an ap-propriate technique when error variance in the indepen-dent variable is low relative to that in the dependentvariable (McArdle 1988).


40 K. J. Gaston et al.

Figure 1 The distribution across Britain of the CBC plots used inanalyses. Filled circles indicate that a 106 10 km square includedat least one woodland plot, open circles at least one farmlandplot, and half-filled circles at least one farmland and onewoodland plot. The positions of four plots are not shown, threein Ireland and one for which no grid reference was provided.

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R E S U L T S

Table 1 presents the statistics of the relationships betweenmean log10 local density of species on CBC plots andgeographical range size for woodland and farmland CBCplots over both atlas periods; two examples of theserelationships are given in Fig. 2. In all cases, theabundance±range size relationship is positive, with rangesize explaining between 10 and 50% of the variation inlocal abundance. The correlation is strongest in these dataif range size is untransformed (in contrast, in many otherstudies both variables are logarithmically transformed).Moreover, in all cases the relationship is significantlycurvilinear, with a second-order regression term increas-ing the variance explained by the model by between 6 and19% (Table 1). In all cases, the curvilinearity is manifestedas a steepening of the relationship as range size increases.These results imply that a fixed increase in range size leadsto a greater increase in local density when range size ishigh than when it is low. Range size always explains moreof the variation in local abundance in a given habitat andtime period when measured as the proportion of sitescontributing to the density measure (Table 1).

Data were available for 137 CBC sites over the two atlasperiods (59 in woodland and 78 on farmland). For every



Table 1 Summary statistics for the relationship between the mean log10 local density of species on CBC plots and geographical rangesize, for CBC plots on woodland and farmland in the periods 1968±72 and 1988±91, using two different measures of range size (seeMethods for more details)

Period Range size measure Habitat n Simple r2 Polynomial r2 p(x2)

1968±72 Atlas Woodland 72 0.11** 0.17** 0.02Farmland 94 0.30*** 0.38*** 0.001

Prop. of sites Woodland 72 0.51*** 0.59*** 0.0003Farmland 94 0.40*** 0.49*** 0.0001

1988±91 Atlas Woodland 77 0.13** 0.19** 0.02Farmland 97 0.27*** 0.40*** 0.0001

Prop. of sites Woodland 77 0.35*** 0.49*** 0.0001Farmland 97 0.44*** 0.63*** 0.0001

n, number of species; simple r2, coefficient of determination for the simple linear regression; polynomial r2, coefficient of determinationfor the regression with a second-order term added (*P 5 0.05, **P 5 0.01, ***P 5 0.0001); p(x2), probability that the second-orderterm explains no additional variance in the regression model.

Figure 2 The relationship between (a) local density (log10territories per hectare) and geographical range size (proportionof sites contributing to the mean local density, out of a maximumof 13) for bird species on woodland CBC plots in the period 1968±72, and (b) local density (log10 territories per hectare) and geo-graphical range size (number of 106 10 km squares from which aspecies was recorded in the first BTO atlas, out of a maximum of2827) for bird species on farmland CBC plots in the period 1988±91. Statistics describing the relationships are given in Table 1.

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single site, the relationship between species geographicalrange size and local abundance at the site was positive.These local relationships often clearly violate the para-metric regression assumption of homoscedasticity, withwidespread species showing a wider range of abundancesthan narrowly distributed species. Nevertheless, we havestill characterized them using parametric regression,because this allows us more simply to assess curvilinearityin the relationships; the proportions of significant linearrelationships were similar using either parametric or non-parametric techniques. When the number of 10 6 10 kmsquares in the appropriate atlas was used as the rangemeasure, 85% of these relationships had slopes that werestatistically significantly different from zero, and rangesize explained on average about 21% of the variation inlocal abundance at the site. Around one-third of sitesshowed evidence of curvilinearity, as indicated by thesignificance of a squared term added to the regressionmodel, whereas the addition of this term increased theamount of variance explained by the model by 7.1%, onaverage. When range size was measured as the proportionof sites contributing to the density measure, 88% of theserelationships had slopes that were statistically significantlydifferent from zero, and range size explained on averageabout 31% of the variation in local abundance at the site.The percentage of sites showing evidence of curvilinearityrose to 76.6, with the addition of the squared termincreasing the amount of variance explained by theregression model by 13.1%, on average.

The positive relationships between average localdensity across CBC plots and geographical range size(Table 1) arise because widespread species attain highermaximum local densities than do narrowly distributedspecies, although minimum densities are more similar forall species. The relationship between the geographicalrange size of a species and the maximum density it attains

on CBC plots is always positive and highly statisticallysignificant, in both atlas periods, in both habitats, andusing both measures of range size (Table 2). In only onecase (across sites in woodland habitat in the first atlasperiod, when proportion of sites occupied is used as themeasure of range), however, is there a significant positiverelationship if minimum density is substituted for themaximum (Table 2). This implies that the density±rangesize relationships in any given habitat and period wouldbe triangular if all the sites that contribute to the meandepicted in Fig. 2 were plotted together on the same axes.They are. Figure 3 shows an example.

D I S C U S S I O N

The results of analyses for the British avifauna substanti-ate the broad generality of the positive interspecificabundance±range size relationship for birds (e.g. Fuller1982; Bock & Ricklefs 1983; Brown 1984; Brown &Maurer 1987; Gaston & Lawton 1990; Gaston 1994;Mehlman 1994; Blackburn et al. 1997). Widespread speciestend, on average, to be locally more abundant thanrestricted species, whether range size is expressed asproportion of occupied sites or of occupied 106 10 kmsquares (Fig. 2, Table 1). Moreover, we can be confidentthat the positive abundance±range size relationshipsobserved in the British avifauna do not result from asampling artefact. An artefactual positive relationship canarise when the same set of sites is used to calculate rangesize and local density if locally rare species are notrecorded from sites at which they actually occur, becausetheir low density means that they are overlooked (Brown1984; Wright 1991; Hanski et al. 1993; Gaston 1994; seealso McArdle 1990). Because the CBC methodologyinvolves repeated visits to individual census sites in eachyear, the likelihood that species are not recorded from



Table 2 Summary statistics for the relationship between either the minimum or the maximum log10 local density of species on CBC plotsand geographical range size, for CBC plots on woodland and farmland in the periods 1968±72 and 1988±91, using two different measuresof range size

Minimum density Maximum densityÐÐÐÐÐÐÐÐÐ ÐÐÐÐÐÐÐÐÐÐ

Period Range size measure Habitat n r2 p r2 p

1968±72 Atlas Woodland 72 0.010 0.39 0.126 0.0023Farmland 94 0.007 0.42 0.456 5 0.0001

Prop. of sites Woodland 72 0.086 0.01 0.697 5 0.0001Farmland 94 0.015 0.25 0.593 5 0.0001

1988±91 Atlas Woodland 77 0.002 0.73 0.140 0.0008Farmland 97 0.030 0.09 0.405 5 0.0001

Prop. of sites Woodland 77 0.007 0.47 0.498 5 0.0001Farmland 97 0.034 0.07 0.599 5 0.0001

n, number of species; r2, coefficient of determination; p, probability that the regression slope does not differ from zero. All significantrelationships are positive.

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sites at which they actually occur because they are locallyrare is greatly reduced. Also, the relationship is observedusing a measure of range size that is derived indepen-dently of the abundances of species at CBC sites.

A feature of our results is that the abundance±range sizerelationships presented often show evidence of curvili-nearity, and this is most marked when range size ismeasured as the proportion of sites occupied by a species.The latter observation suggests that curvilinearity ismainly the result of the relatively limited span of rangesizes possible when range sizes are measured as propor-tional occupancy, because species with a range ofabundances may occupy all possible sites; a larger sampleof sites would be needed to distinguish betweenmoderately and widely distributed species. If abun-dance±range size relationships were truly linear, thisinadequacy would lead to the compression of the right-hand side of the range axis, and so to curvilinearity as therelationship steepens at high occupancies. This effect,however, seems inadequate to explain all of the curvili-nearity we observed, as one-third of sites still showedevidence of it when range size was measured as thenumber of 10 6 10 km squares occupied, yet no birdspecies occupies every 10 6 10 km square in Britain.Nevertheless, abundance±range size relationships fromthe majority of CBC sites are better characterized by linearthan by curvilinear regressions when range sizes aremeasured with greater resolution.

Some abundance±range size relationships have beenfound to be approximately triangular (polygonal); abun-dant species have large geographical ranges, but rare

species can be restricted or widespread in distribution(e.g. Brown & Maurer 1987). Such a pattern appears morefrequent at very large (e.g. continental) scales, and fortaxonomically and ecologically diverse assemblages. It isevident to some extent in the overall abundance±rangesize relationships documented for British woodland andfarmland birds (e.g. Fig. 2), but is much more apparentfor these assemblages in abundance±range size relation-ships for which the abundances of species were derivedfrom only a single site. In one sense, the triangularityobserved in abundance±range size relationships for singlesites is the trivial consequence of the minimum density ofspecies of all range sizes being constrained to 1/A, where Ais the area of the site. What is important, however, is notthe arbitrary value of the minimum density at any givensite, but the finding that widespread and common speciesare no more abundant than more restricted and rarespecies at some sites. If abundance±range size relation-ships based on abundances derived from single sites areapproximately triangular, then the overall positiveabundance±range size relationship (Fig. 2) results notfrom the most widespread species being relativelyabundant at all localities, but from the most widespreadspecies attaining a higher average abundance.

This interpretation is confirmed by four other results.First, there is normally no significant relationship betweenthe minimum density attained by a species and its rangesize (Table 2); all species are rare somewhere. Second,there is a consistent significant positive relationshipbetween the maximum density attained by a species andits range size (Table 2). Coefficients of determination fromlinear analyses of maximum abundance±range size rela-tionships are always higher than those from equivalentanalyses using mean abundance, and are usually higherthan the coefficients of determination from curvilinearanalyses using mean abundance. Third, there is a strongpositive, albeit curvilinear, interspecific correlation be-tween maximum and mean density; one example is shownin Fig. 4. Fourth, there is an approximately triangularabundance±range size relationship generated when theabundances of species at different sites are plotted separ-ately, with widespread species on average exhibiting awider range of abundances than restricted species (Fig. 3).

The broad variation in the abundances of the mostwidespread species appears at odds with studies demon-strating a degree of spatial concordance in the abundancesof species in taxonomic assemblages, with commonspecies remaining common and rare remaining rare (Fager& McGowan 1963; Grubb et al. 1982; Mitchley & Grubb1986; McGowan & Walker 1993; Gaston 1994, 1997;Watkins & Wilson 1994). What is critical for theconcordance observed, however, is the spatial variancein the densities of individual species relative to how



Figure 3 The relationship between local density (log10 territoriesper hectare) and geographical range size (number of 106 10 kmsquares from which a species was recorded in the second BTOatlas) for bird species on all farmland CBC plots analysed here inthe period 1968±72. Each data point represents one species onone site. A total of 25 sites are plotted.

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widely separated the mean abundances of different speciesare, and not simply the intraspecific range of abundancevalues. Despite the breadth of local abundances ofwidespread species, for both habitat types and periods,the densities of species do show significant spatialconcordance (Table 3); this is strengthened by exclusionof the rarest species.

The generality of the positive abundance±range sizerelationship observed for the British avifauna reinforcesthe idea that rare species potentially face a double jeopardythrough having both a low average local abundance and asmall range size (Lawton 1993, 1995). In this case, thereare few rare species that face only a single jeopardy,through having a low average local abundance and a largerange size or through having a high average localabundance and a small range size (e.g. Fig. 2), and specieswith low local abundances are rare at more of the sites atwhich they occur (Fig. 5). Although Britain constitutesonly part of the entire geographical range of all speciesrecorded on CBC plots, and although most speciescensused by the CBC are relatively common, these resultsshould not be dismissed lightly as parochial. The rangesizes of birds in Britain are positively correlated with their



Figure 4 The relationship between the mean and maximum localdensity (log10 territories per hectare) attained by species onfarmland CBC plots in the period 1988±91. Each point is a singlespecies. The relationship is significantly curvilinear, and isquantitatively similar if woodland plots, or plots from the period1968±72, are used.

All species Excluding rarest speciesÐÐÐÐÐÐÐÐÐÐÐ ÐÐÐÐÐÐÐÐÐÐW d.f. P W d.f. P

1968±72 Woodland 0.523 71 5 0.001 0.544 53 5 0.001Farmland 0.525 93 5 0.001 0.561 69 5 0.001

1988±91 Woodland 0.522 76 5 0.001 0.572 57 5 0.001Farmland 0.456 96 5 0.001 0.498 72 5 0.001

Table 3 The values of Kendall's coefficientof concordance (W) for the local densitiesof species across CBC plots on woodlandand farmland in the periods 1968±72 and1988±91, for all species and excluding the25% of species with the lowest meanabundance across plots (calculated onlyusing sites where the species was present)

Figure 5 The interspecific relationship between maximum localdensity (territories per hectare) and the proportion of woodlandsites at which a species attains a local density of less than 0.25territories per hectare, for (a) 1968±72 [Spearman rank correla-tion corrected for ties, rho=±0. 88, n (number of species)=72,P 5 0.0001] and (b) 1988±91 (rho=±0.90, n=77, P 5 0.0001).Spearman rank correlations were significantly negative in allcases examined (i.e. when 5 0.25 was substituted by 5 0.01, 50.05, 5 0.1, 5 0.5, or5 1), except those cases where there is novariation (i.e. proportion is always 1 or 0): rare species attain lowdensities at a higher proportion of sites than do abundantspecies. Results were similar for farmland.

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range sizes in Europe (Gaston et al. 1997b; Gregory &Blackburn, in press). Given the generality of the abun-dance±range size relationship (Gaston 1996), the situationwe describe in Britain may also apply to these speciesmore widely.

C O N C L U S I O N

A positive interspecific relationship between local densityand geographical range size is one of the most consistentlyobserved patterns in studies of the abundance anddistribution of species at large spatial scales. Althoughwidely documented, the detailed anatomy of the relation-ship had not previously been explored, a failing that hasbeen criticised elsewhere (Leitner & Rosenzweig 1997).Yet, given that most positive interspecific relationshipsare the result of averaging information from many speciesacross many sites, there are potentially a number ofdifferent ways in which they could arise. We have shownfor British birds that the average interspecific relationshipis repeated at every single local site examined. It arises inpart because of higher maximum abundances exhibited bywidespread species, and not because widespread specieshave higher minimum local abundances (Table 2). Inother words, all species are rare somewhere, but restrictedspecies are rare everywhere. Widespread species are foundat higher density at more sites at which they occur thanare restricted species (Fig. 5). Positive interspecificabundance±range size relationships apparently arise de-spite the different patterns of abundance across sitesdisplayed by different species.

In this paper, we have restricted our attention to howvariation in the form of the interspecific abundance±rangesize relationship at local sites contributes to the meanrelationship. It is clear from the summary above, how-ever, that the patterns we observe across sites are affectedby the patterns displayed by different species, as is mostobvious in the differences between those that areabundant and those that are rare. The logical next steptowards understanding the interspecific relationship, then,is to examine the contribution to it made by each species.For example, some hypotheses predict that the intraspe-cific abundance±range size relationship should also bepositive, whereas it is much harder to see why this mightbe true for others. The interaction between the inter-andintraspecific forms of the relationship between abundanceand range size in British birds is the subject of thesubsequent paper (Blackburn et al. 1998).

A C K N O W L E D G E M E N T S

This work was funded by NERC grant GST/03/1211.K.J.G. is a Royal Society University Research Fellow.

The map was kindly produced by Rachel Quinn using theDMAP program written by Alan Morton, ImperialCollege. We thank John Lawton and two anonymousreferees for comments on versions of this work, JohnMarchant for help and advice on using the CBC data, andthe thousands of volunteers who carried out the censusesand atlas fieldwork on which the data were based.

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B I O S K E T C H

Kevin J. Gaston has research interests in the fields ofbiodiversity, conservation biology, and macroecology, withparticular emphasis on the ecologies of rare organisms, thestructures of geographic ranges, and patterns in speciesrichness.

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Documents

The anatomy of the interspecific abundance–range size relationship for the British avifauna: I. Spatial patterns