Interspecific abundance-range size relationships: range position and phylogeny.

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]