Stability and species richness in complex communities

Using both numerical simulations and analytical methods, we investigate how the stability of ecological communities depends on the number of species t hey contain. To investigate complex communities, we construct communities f rom modular "subcommunities" that can have arbitrary community structure; e .g. subcommunities could consist of pairs of predator and prey species, tri os of prey, specialist predator and generalist predator, or any collection of interacting species. By building entire communities from subcommunities, we can change the number of species in the community without changing comm unity structure. We further suppose that species sharing the same ecologica l role in different subcommunities act additively on the per capita populat ion growth rates of other species. Under these assumptions, the interaction s between species from different subcommunities have no effect on community -level stability, measured by the variability in the combined densities of species sharing the same ecological role in different subcommunities. Furth ermore, increasing species richness (i.e. the number of subcommunities comp rising the community) increases community-level stability only when it intr oduces species that respond differently to environmental fluctuations. Ther efore, our results support the "insurance hypothesis" that species richness increases community-level stability by insuring that some species in a com munity are tolerant of different environmental fluctuations.