Dispersal Limitation Favors More Fecund Species in the Presence of Fitness-Equalizing Demographic Trade-Offs

Demographic equivalence is the central assumption of the neutral theory of species diversity and has attracted much criticism, since species clearly differ from each other in many traits. Two simple answers-that is, dispersal limitation and demographic trade-offs-have been suggested to resolve this problem. Both processes are considered to be capable of making interspecific differences in fitness smaller on their own, thus potentially reconciling neutrality with reality. However, when the two mechanisms operate simultaneously, as they must do in natural communities, we are surprised to find that they interfere with each other in such a way that dispersal limitation favors more fecund species. Fitness equivalence is no longer guaranteed by a perfect trade-off, and contrary to popular belief, dispersal limitation is found to impede rather than facilitate the stochastic coexistence of species. Still, more species can coexist than allowed through demographic equivalence, providing a potentially alternative explanationfor biodiversity maintenance in nature.