The tendency for individual species to show positive relationships between abundance and occupancy makes no firm predictions about the form of the same relationship across species (Blackburn et al. 1998). On the one hand, if intraspecific relationships are strong, and exhibit some commonality of form, interspecific relationships might seem likely to follow. On the other hand, if the range of temporal variation in the abundances of individual species is small relative to the spread of their mean abundances, then intraspecific relationships may have little influence in generating interspecific ones. Intraspecific relationships are highly variable, and frequently not very strong, and interspecific variation in local abundances tends to be vastly greater than intraspecific variation. However, in the limit, species will vary from those that are in the process of colonizing an area or going extinct from it, and so passing through the origin of the abundance–occupancy plot, to those that are both widespread and abundant. Although species can potentially lie along many trajectories connecting these two points, these limits alone perhaps make it likely that interspecific abundance–occupancy relationships will be positive in some broad form. This is indeed the case.
Patterns in interspecific relationships
Positive interspecific abundance–occupancy relationships have been documented for a wide variety of taxa (plants: Gotelli & Simberloff 1987; Collins & Glenn 1990, 1997; Boecken & Shachak 1998; spiders: Pettersson 1997; grasshoppers: Kemp 1992; Collins & Glenn 1997; scale insects: Kozár 1995; hoverflies: Owen & Gilbert 1989; bumblebees: Obeso 1992; Durrer & Schmid-Hempel 1995; macro-moths: Inkinen 1994; beetles: Nilsson, Elmberg & Sjöberg 1994; bracken-feeding insects: Gaston & Lawton 1988; frogs: Murray, Fonseca & Westoby 1998; birds: Gaston & Blackburn 1996b; Collins & Glenn 1997; mammals: Brown 1984; Collins & Glenn 1997; Johnson 1998a). In Britain alone, such relationships have now been documented for vascular plants (Rees 1995; Thompson, Hodgson & Gaston 1998), macro-moths (Quinn et al. 1997), butterflies (Hanski, Kouki & Halkka 1993), birds (Fuller 1982; Hengeveld & Haeck 1982; O'Connor & Shrubb 1986; O'Connor 1987; Gaston & Lawton 1990; Sutherland & Baillie 1993; Gregory 1995; Blackburn et al. 1997b, 1998; Blackburn, Gaston & Gregory 1997a; Gaston, Blackburn & Gregory 1997a, 1997b; Gaston et al. 1998) and mammals (Blackburn et al. 1997b). In a review of published abundance–occupancy relationships, Gaston (1996a) found that around 80% were significantly positive. He concluded that the positive relationship between abundance and occupancy may be one of the general patterns in the study of ecology. Relationships published since this review have almost without exception supported this view.
The positive relationship is remarkably robust (Fig. 3). It has been known for some time that it tends to remain consistent across multiple spatial scales (Gaston & Lawton 1990) and when abundance and occupancy are quantified using a variety of measures. A prime example is provided by the range of relationships reported for British birds. A strong significant positive relationship is found when the total British breeding population size of species is plotted against its geographical range size in Britain; here, an increase in density with range size follows because, on log–log axes, the relationship between population size and range size has a slope of greater than one (Blackburn et al. 1997b). The same is true also if breeding abundance and distribution are replaced by wintering figures (Gaston, Blackburn & Gregory 1997b). If breeding and wintering assemblages are split into resident and migrant species, both subsets show significant positive interspecific relationships. A significant positive relationship is found if population size and geographical range size are replaced by species density on local sites and the proportion of these sites that each species occupies (Gaston et al. 1998). This is true if density and occupancy are averaged across the same 4-year period over which population size and geographical range size were estimated, or if density and occupancy are estimated within each of these years individually (Blackburn et al. 1998). Indeed, a significant positive density–occupancy relationship is found using data from both farmland and woodland CBC sites for every year from 1968 to 1991 inclusive (Blackburn et al. 1998b). Finally, interspecific abundance–occupancy relationships can be examined using bird densities calculated from each individual CBC site. For 137 such sites, the interspecific relationship was positive every single time, and significantly so for 85%.
Figure 3. Examples of the interspecific abundance–occupancy relationship for birds in Britain. (a) The relationship between log10 breeding population size (number of individuals) and log10 breeding geographical range size (number of 10 × 10-km squares from which a species was recorded in Gibbons, Reid & Chapman 1993); r2 = 0·83, n = 193, P < 0·0001 (from Blackburn et al. 1997). The slope of this relationship is significantly greater than 1, indicating that the population size of widespread species is higher than expected simply on the basis of their larger geographical distributions. (b) The relationship between local density (log10 territories per hectare) and geographical range size (number of 10 × 10-km squares from which a species was recorded in Sharrock 1976) for bird species on farmland CBC plots in the period 1988–91; r2 = 0·27, n = 97, P < 0·0001 (from Gaston et al. 1998). (c) The relationship between local density (log10 territories per hectare) and geographical range size (proportion of all sites that the species occupies) for bird species on farmland CBC plots in 1979; r2 = 0·35, n = 98, P < 0·0001 (for data sources and methods see Blackburn et al. 1998). (d) The relationship between local density (mean log10 territories per hectare) and geographical range size (number of occupied sites) for bird species on a single farmland CBC site in the period 1988–91; r2 = 0·60, n = 51, P < 0·0001 (for data sources and methods see Gaston et al. 1998).
Download figure to PowerPoint
Such relationships imply that the same species will lie in the tails of the species abundance and species range size distributions for an assemblage. This is largely the case. Seventy per cent of the 5% of bird species with the lowest British population sizes are also among the 5% of species with the smallest British geographical range sizes, while 50% of the 5% of species with the highest British population sizes are also among the 5% of species with the largest British geographical range sizes.
Exceptions to the positive interspecific abundance–occupancy relationship do exist, but are scarce. Gaston (1996a) found that about 5% of the relationships in the literature were significantly negative. Often, the exceptions concern unusual sets of circumstances (Gaston & Lawton 1990; Gaston 1996a; Johnson 1998b). For example, negative relationships can arise when abundance is measured in an area that is highly atypical of the region over which distribution is assessed.
Curiously, while generally reporting positive correlations, studies of interspecific abundance–occupancy relationships that have been conducted for plants have tended to concentrate on quite narrowly defined habitat types. Thompson, Hodgson & Gaston (1998) found that for the vascular plant flora of central England, there was no overall relationship between abundance and occupancy, but that at the level of individual habitat types significant positive relationships were quite common.
Anatomy of interspecific abundance–occupancy relationships
A number of facets of the anatomy of interspecific abundance–occupancy relationships have become apparent, particularly with reference to birds in Britain.
(i) For the one data set for which this issue has been carefully dissected, British birds on farmland and woodland CBC sites, the relationship appears to be driven primarily by increases in the maximum abundance that a species can attain at occupied sites. Minimum abundances are, not surprisingly, not related to range sizes across all but perhaps the most widespread species (Gaston et al. 1998). Thus most species are rare at some sites at which they occur, but only the more widespread species attain high densities at others. This pattern results in a roughly triangular relationship between abundance and occupancy when the abundance at every site occupied by each species is plotted on the same axes (Gaston et al. 1998). Thus mean density varies with occupancy (Fig. 3) primarily because maximum density does so.
(ii) The relationship tends to become poorer with cruder measures of range size (Table 1). For British birds, the finest scale at which distributions across the whole country are mapped is that of the 10 × 10-km square on the British National Grid. These fine scale data can be used to derive cruder measures of species occurrence, based on latitudinal or longitudinal extents, or minimum convex polygons (Quinn, Gaston & Arnold 1996). The strengths of the resultant abundance–occupancy relationships plotted using different range size measures are essentially inversely related to the crudity of this measure (Table 1), as one might expect if, as the measurement scale becomes coarser, the error variance in the estimation of occupancy increases.
Table 1. Comparison of the abundance–range size relationship for birds in Britain using different methods to estimate range size (Quinn, Gaston & Arnold 1996) and total population size (log10 transformed). n is the number of phylogenetically independent contrasts. All relationships are significant at P < 0·00001. Range methods are ranked in ascending order according to the amount of variation in abundance which they explain
| ||Full model||After removal of outliers/leverage points |
|95% latitudinal extent||107||0·338||8||93||0·675||4|
|95% longitudinal extent||107||0·406||7||90||0·663||7|
|Latitudinal × longitudinal extent||107||0·648||4||104||0·662||8|
|Minimum convex polygon||107||0·719||3||96||0·797||3|
|100 × 100-km squares||107||0·778||2||98||0·835||2|
|10 × 10-km squares||107||0·849||1||97||0·900||1|
(iii) The relationship shows consistencies between different habitats. Thus, using data from the CBC, Blackburn et al. (1998) found that the slopes of the interspecific relationships for species censused on farmland and woodland sites were very similar, and that year-to-year variation in the slope value was positively correlated between these habitats. In other words, in years when abundant species occupy a higher proportion of woodland CBC sites, they also occupy a higher proportion of farmland sites. This is presumably a simple consequence of good years for common or rare species being the same in both habitat types, and spill-over of individuals from good to poor habitats as abundances increase. This pattern arises despite consistently lower densities being recorded on farmland than woodland sites. A British bird species recorded on a single woodland CBC site in any given year will have a density three to four times higher, on average, than a species recorded on only a single farmland site (Blackburn et al. 1998), this difference being a consequence of much of the area of most farmland CBC sites being relatively barren for birds (e.g. crop land).
(iv) The relationship remains stable from one season to another. Thus, the resident species of the breeding and wintering bird assemblages in Britain (i.e. those species they have in common) show a common interspecific population size–geographical range size relationship, such that neither the slope nor the intercept of the relationship differ significantly whether summer or winter data are used (Gaston, Blackburn & Gregory 1997b). However, resident birds do show significantly larger population sizes and geographical range sizes in Britain in winter than in summer. Coupled with the non-significant differences in the winter and summer population size–geographical range size relationships, these results imply that bird species shift up the interspecific relationship from summer to winter, and back down again from winter to summer.
Other interesting regularities arise if migrant species are considered. On average, summer migrants have smaller breeding population sizes (F1,168 = 6·50, P = 0·012) and range sizes (F1,168 = 7·96, P = 0·005) than do residents (see also Greenwood et al. 1996), yet summer migrants and residents do not differ in either the slopes (F1,166 = 1·18, P = 0·28) or the intercepts (F1,167 = 0·003, P = 0·96) of their breeding population size–geographical range size relationships. Likewise, winter migrants have smaller breeding population sizes (F1,156 = 30·3, P < 0·0001) and range sizes (F1,156 = 30·4, P < 0·0001) than do residents, yet winter migrants and residents do not differ in either the slopes (F1,154 = 0·09, P = 0·77) or the intercepts (F1,155 = 2·47, P = 0·12) of their wintering population size–geographical range size relationships. When all species are included, the slopes of the interspecific population size–geographical range size relationship differ between summer and winter (Gaston, Blackburn & Gregory 1997b), but residents and migrants lie on the same line in both seasons. As migrant populations are subject to different environmental pressures to resident populations for around half the year, yet settle on the same interspecific relationship as residents when they arrive in Britain, this implies a commonality in the forces setting this relationship for both groups. What causes this commonality is currently unclear.
(v) The relationship remains reasonably stable from one year to the next. Thus, Blackburn et al. (1998) found relatively little interannual variation in the slope, intercept and coefficient of determination for abundance–occupancy relationships for farmland and woodland birds in Britain. In major part, this occurs because abundant and widespread species remain abundant and widespread from year to year, while rare and restricted species remain rare and restricted (see discussion of concordance above). However, the assessment of the amount of interannual variation in these interspecific relationships is necessarily subjective, as there is no baseline against which to compare.
(vi) The relationship is very similar for different groups of organisms in the same area, where estimates of abundance and occupancy are broadly comparable. Thus, Blackburn et al. (1997b) found that the slopes of the relationships between overall population size and geographical range size were not significantly different for breeding birds and mammals in Britain, once two outliers were excluded from the mammal data (a similar result was found for the slopes of population size–body size relationships in these two assemblages; Greenwood et al. 1996). However, for a given range size mammals had densities that averaged 30 times higher than those of birds (at comparable masses, non-flying mammal species are about 45 times more abundant than resident bird species; Greenwood et al. 1996). The two species outlying the mammal relationship [white-toothed shrew Crocidura suaveolens (Pallas) and Orkney vole Microtus arvalis (Pallas)] are the only species of terrestrial mammal or bird confined in Britain to offshore islands. They have exceptionally high abundances for their range size, as might be expected if they were exhibiting density compensation in response to species-poor island faunas (MacArthur 1972; MacArthur, Diamond & Karr 1972; Blondel, Chessel & Frochot 1988; Adler & Levins 1994) or if their ranges in Britain could be much larger were they to escape the confines of their current insular habitats.