Phase transitions in the coexistence of competitive species
One of the most important questions in ecology concerns the role that species-specific characteristics play in determining the structure of ecosystems. It has long been recognized that specialization of organisms into niches can provide a mechanism for species coexistence and the maintenance of biodiversity. In recent years, however, neutral models that ignore species-specific characteristics within trophic levels have been remarkably successful at explaining some macroscopic features of ecosystems. The assumptions underlying neutral models are not believed to be true - species' do have differences, and these differences are often important - and neutral models do not describe all ecosystem characteristics. Nevertheless, the success of neutral models raises the question: when is a neutral approximation accurate enough for describing the large-scale features of an ecosystem?
Here, we present numerical and theoretical evidence that niche and neutral regimes correspond to distinct ecological phases, separated by a phase transition analogous to those observed in magnetic spin systems. The niche-neutral phase transition occurs at a critical environmental carrying capacity. We argue that there is a possibility of an abrupt loss of biodiversity in a stressed ecosystem if the carrying capacity decreases to the critical value.