Species extinction and the relationship between distribution and abundance

Within taxonomic groups, there is almost always a positive relationship between the size of geographic range and the local abundance of species(1-4). This pattern has attracted much interest, and several ecological mechanisms have been proposed as causes of it(5). However, these hypotheses do not consider the effect of the extinction of rare species on range-abundance relationships. If both range size and local abundance influence the risk of extinction, species with small ranges might avoid extinction if they have high local abundance, whereas species with low local abundance might avoid extinction if they are widespread; species with both small range and low local abundance should be at high risk. This interaction between range, abundance and extinction should produce negative correlations between range and abundance in groups that have experienced many extinctions. Here I test this idea using Australian marsupials, and I show that although the relationship between range size and local abundance is positive for recently evolved species, it is negative for ancient species. This indicates that positive relationships between range size and abundance may be generated during adaptive radiation, but are then gradually reversed as a result of differential extinction.