Equalisation of prey numbers by migratory shorebirds

Abstract

STUDIES of the effects of predators on prey diversity have usually focused on patterns of predation that do not by themselves increase diversity. For example, size-selective fish decrease diversity by driving large bodied zooplankton to local extinction¹. Size-selective predators can maintain a diversity of species only if large bodied prey hold some advantage over smaller prey, such as the difference in feeding efficiency hypothesised by Brooks and Dodson (ref. 1 and see ref. 2 for review). The diversifying effect of nonselective predation on competitors for space in the rocky intertidal zone³ depends on the presence of differences among competitors in time of settlement during the year. In contrast, selective removal of common prey tends to reverse any numerical advantage gained by one species over another. Thus it is of interest to see whether numerically selective predators increase diversity by equalising prey numbers. The results presented here show that general predators in fact repeatedly equalise the relative abundance of prey species and hence tend to stabilise prey communities.