Self-consistent migration puts tight constraints on the spatio-temporal organization of species-rich metacommunities

Biodiversity is often attributed to a dynamic equilibrium between immigration of species and competition-driven extinction. This equilibrium forms a common basis for studying ecosystem assembly from a static reservoir of migrants—the mainland. Yet, natural ecosystems often consist of many coupled communities (i.e. metacommunities) and migration occurs between these communities. The pool of migrants then depends on what is sustained in the ecosystem, which in turn depends on the dynamic migrant pool. This chicken-and-egg problem of survival and migration is poorly understood in communities of many competing species, except for the neutral case - the "unified neutral theory of biodiversity". Employing spatio-temporal simulations and mean-field analyses, we show that self-consistent migration puts rather tight constraints on the dynamic migration-extinction equilibrium. When the number of species is large, even weak competitions push species to the edge of their global extinction, such that the overall diversity is highly sensitive to perturbations in demographic parameters, including growth and dispersal rates. When migration is short-range, the resulting spatio-temporal abundance patterns follow broad scale-free distributions that correspond to a directed percolation phase transition. The qualitative agreement of our results for short-range and long-range migration suggests that this self-organization process is a general property of species-rich metacommunities. Our study shows that self-sustaining metacommunities are highly sensitive to environmental change and provides insights into how biodiversity can be rescued and maintained.