Species abundance patterns of British birds

Abstract

Species abundances tend to follow lognormal frequency distributions, and samples often appear to be skewed to the right of the veil line. It has been suggested that the underlying distribution is likely to be skewed to the left, and that the left-hand tail would only be revealed by increasing sampling effort. Preliminary data for British birds have shown abundances to be significantly skewed to the left, and further, the degree of skew was compatible with a `sequential stick breaking' model of species abundances. Here I report an analysis of a newly published data set on British breeding birds which confirms the original findings in demonstrating a left skew to bird abundances, although the degree of skew is less than previously reported and not significantly different from zero. This result is quantitatively compatible with both the previous theoretical model and one based on independent draws from a lognormal distribution. This raises the possibility that species abundances may follow roughly symmetrical lognormal distributions; the problem in demonstrating such patterns may be the existence of extensive data sets which are free of bias.