On the Relative Abundance of Species

Abstract

1. A distinction is made between opportunistic and equilibrium species. 2. There is little ecological interest in the relative abundances of opportunistic species, but such species abundances should frequently have a log-normal distribution. 3. The relative abundances of equilibrium species are of considerable ecological interest and frequently can be deduced from the assumption that increase in one species population results in a roughly equal decrease in the populations of other species. To make the formulae well-defined it is necessary to assume that the census-taker has sampled a small area and thus achieved a certain sort of randomness. 4. For bird populations, at least, discrepancies between observations and predictions are negligible except when the censused area is compounded from different habitats. The discrepancy is then partly due to the fact that common species in one habitat are more likely to be present in adjacent habitats than are rate ones.