OF MICE AND WRENS - THE RELATION BETWEEN ABUNDANCE AND GEOGRAPHIC RANGE SIZE IN BRITISH MAMMALS AND BIRDS

We examine the relation between population size and geographic range s ize for British breeding birds and mammals. As for most other assembla ges studied, a strong positive interspecific correlation is found in b oth taxa. The relation is also recovered once the phylogenetic related ness of species has been controlled for using an evolutionary comparat ive method. The slope of the relation is steeper for birds than for ma mmals, but this is due in large part to two species of mammals that ha ve much higher population sizes than expected from their small geograp hic ranges. These outlying mammal species are the only ones in Britain to be found only on small offshore islands, and so may be exhibiting density compensation effects. With them excluded, the slope of the abu ndance-range size relation for mammals is not significantly different to that for birds. However, the elevation of the relation is higher fo r mammals than for birds, indicating that mammals are approximately 30 times more abundant than birds of equivalent geographic range size. A n earlier study of these assemblages showed that, for a given body mas s, bats had abundances more similar to birds than to non-volant mammal s, suggesting that the difference in abundance between mammals and bir ds might be due to constraints of flight. Our analyses show that the a bundance-range size relation for bats is not different from that for o ther mammals, and that the anomalously low abundance of bats for their body mass may result because they have smaller than expected geograph ic extents for their size. Other reasons why birds and mammals might h ave different elevations for the relation between population size and geographic range size are discussed, together with possible reasons fo r why the slopes of these relations might be similar.