Exploring the transition between niche assembly and dispersal assembly in ecological communities

A central goal in ecology is to understand what maintains species diversity in local communities. Classic ecological theory posits that niches dictate the maximum number of species that can coexist in a community, and that observed species richness will be below this maximum only where immigration is very low. A novel alternative theory, developed by my research group, is that niches instead dictate the minimum number of coexisting species, and that observed species richness will usually be well above this because of ongoing immigration. I explain the novel theory and present tests of its predictions against island species–area relationships, experimental data from seawall communities, and global mangrove species–area relationships. In general the data are consistent with the novel theory and imply that (i) the number of niches in ecological communities is usually small; and (ii) natural ecological communities are primarily dispersal-assembled.