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Summary 1. Positive relationships between the density and distribution of species in taxonomic assemblages are well documented, but the underlying mechanisms remain poorly understood. Two factors that are expected to be important in explaining variation in these relationships are the spatial scale of analysis and the relative mobility of the study species. 2. We examined density–distribution relationships in British butterflies at a variety of spatial scales. Distributions were proportions of grid squares occupied: 50 m grid within 0·25 km2 areas (local), 500 m grid in 35 km2 (regional), 10 km grid across England, Wales and Scotland (national), 153 000 km2 grid squares across Europe (European), and also seven categories of international distribution (Global; 1 = European endemic to 7 = in 5 + continents). Densities were measured using transect counts at local, regional and national scales. 3. Different relationships between density and distribution occurred at different scales of analysis. When we controlled for the effects of mobility and/or phylogenetic association, a positive relationship between density and distribution was apparent at local, regional and national scales. Species’ national densities in Britain were positively correlated with their European distribution sizes, but significantly negatively correlated with their global range sizes. 4. Butterfly mobility had a positive effect on distribution and a negative effect on density at all spatial scales. For a given total abundance, more mobile species had lower densities but wider distributions, i.e. they were less aggregated than more sedentary species. 5. The decreasing strength of the density–distribution correlation, and the eventual reversal of the pattern, with the increasing magnitude of difference between the scale at which density was measured relative to distribution, suggests that some element of niche may be important in determining densities and distributions. However, the measure of niche breadth analysed did not explain significant variation in density, distribution, or in the density–distribution relationship. [ABSTRACT FROM AUTHOR]

The detailed forms of species-range size distributions in Britain are determined and contrasted for ten taxonomic assemblages (liverworts, vascular plants, molluscs [aquatic and terrestrial], dragonflies, macro-moths, butterflies, birds [breeding and wintering], mammals). All are strongly right-skewed when range sizes are untransformed. A logarithmic transformation fails to normalise the distribution for all but one group, and the distributions for several groups are not readily normalised at all. Taxa with larger median range sizes have species-range size distributions that are less strongly right-skewed. The median observed range sizes of species in each of the taxonomic groups fall, in terms of decreasing range size, in the sequence wintering birds > breeding birds > mammals > butterflies > terrestrial molluscs >dragonflies > aquatic molluscs > vascular plants > moths > liverworts. Despite the difficulties in deriving a simple and sensible mechanistic model for range size distributions, this is likely to be the most important next step towards understanding their forms. [ABSTRACT FROM AUTHOR]

A general positive interspecific relationship between local abundance and geographic range size in animals has prompted speculation that a similar relationship might exist intraspecifically, such that a species is widespread at times when it is locally abundant, and more restricted in distribution when it is locally rare. Current evidence suggests that intraspecific relationships often are positive, but that there is considerable variation in the pattern exhibited by species. Here, we use data on British birds to test the hypotheses that speciesshowing a high mean or wide spread of local densities or range sizeswill be more likely to show strong intraspecific relationships between abundance and geographic range size. These data show only inconsistent support for an effect of the range of densities or of occupancies on intraspecific abundance-range size relationships. However, the strength of an intraspecific relationship does seem to be related to the mean occupancy of species, and whether or not a species exhibits temporal trends in density, with the strongest relationships found in species with simultaneous trends in both density and occupancy. We suggest that these results are explained by time lags in the loss or gain of species at occupied sites in response to reductions or increases in density. [ABSTRACT FROM AUTHOR]

A number of mechanisms have been proposed to explain the widely observed positive interspecific relationship between local abundance and extent of geographic distribution in animals. Here, we use data on British birds to assess two of these hypotheses: that the relationship results from the relative position of a study area with respect to the geographic ranges of the species which occur there, and that the relationship results from a simple difference between taxonomic groups,rather than any general tendency for more abundant species to have larger range sizes. We find support for neither hypothesis. Phylogenetically controlled comparative analyses reveal that the positive abundance-range size relationship is consistently found within taxa, even when abundance and range size are calculated at a variety of spatial and temporal scales. Analyses both across species and within taxa show that bird species for which Britain is near to the centre of their distribution in Europe tend to have larger British range sizes and higher abundances than do species where Britain is close to the edge oftheir range in Europe. However, these relationships do not cause that between abundance and range size, because this latter relationship persists within different range position categories. Whether a species is near the centre or edge of its geographic range in Britain may affect its position on the. abundance-range size relationship, but does not produce the relationship. Range position in Britain does, however, seem to be related to the magnitude of temporal changes in the range sizes of British birds. There is some evidence to suggest that species for which Britain is nearer to their European range centre haveshown smaller changes in distribution over the period 1970--1990 than have species for which Britain is close to their European range edge. [ABSTRACT FROM AUTHOR]