Disentangling ecologically equivalent from neutral species: the mechanisms of population regulation matter

1. The Neutral Theory of Biodiversity explored the structure of a community of ecologically equivalent species. Such species are expected to display community drift dynamics analogous to neutral alleles undergoing genetic drift. While entire communities of species are not ecologically equivalent, recent field experiments have documented the existence of guilds of such neutral species embedded in real food webs.
2. What demographic outcomes of the interactions within and between species in these guilds are expected to produce ecological drift versus coexistence remains unclear. To address this issue, and guide empirical testing, we consider models of a guild of ecologically equivalent competitors feeding on a single resource to explore when community drift should manifest.
3. We show that community drift dynamics only emerge when the density-dependent effects of each species on itself are identical to its density-dependent effects on every other guild member. In contrast, if each guild...