Allee effects and the neutral theory of biodiversity

Abstract

Summary 1The neutral theory of biodiversity provides a simple yet elegant explanation of species diversity patterns. By specifying speciation rates, dispersal and community size, and assuming that all individuals in a community are ecologically identical, the theory generates species abundance distributions that are remarkably similar to those observed in nature. 2Here we show that incorporation of the Allee effect in a neutral community results in considerable decrease in species richness and radically different dominance–diversity curves, with an excess of both very abundant and very rare species but a shortage of intermediate species, than predicted by the current neutral model. 3We also find that even a small Allee effect can cause a considerable decline in the time of species coexistence. However, dispersal limitation within a community indeed enhances coexistence as presumed by Hubbell's theory, but its effect is limited and is probably insufficient to negate the ubiquitous small-population disadvantages caused by the Allee effect and other agents alike. 4The remarkably good fit between the predictions of neutral theory and certain observations implies that either the Allee effect is not important, which seems unlikely, or that other stabilizing mechanisms (i.e. rare species advantages, perhaps resulting from interspecific niche differentiation) oppose the Allee effect.