Fig. 1.

A schematic illustrating the intuition underlying our hypothesis for a phase transition between the neutral and niche regimes in ecology. (A) The important ingredients of our model are a large pool of diverse species, implying a diversity of species interactions, subject to stochastic population dynamics. (B) Stochastic ecological drift will dominate the dynamics of communities with small population sizes and/or fluctuating environments. (C) By contrast, stabilizing selective forces will cause a community with a large population size and a constant environment to freeze into a unique, optimal configuration. (D) We predict that there is a transition between a drift-dominated (neutral) phase and a selection-dominated (niche) phase. That is, the community behaves exactly neutral when the inverse stochasticity is less than a critical threshold, and the deviation from neutrality rises quickly once the inverse stochasticity is larger than the critical threshold. The red line represents an order parameter based on the distance from neutrality, the dashed blue line represents an order parameter based on the niche phase, and the dashed black line denotes the critical stochasticity.