Biodiversity collapse in a phase transition between neutral and niche communities

The dynamics of ecological communities can be described by two contrasting models: the first assumes that the individuals of all species are identical and do not have competitive interactions. The second assumes that species are different, adapted to particular habitat conditions, and have strong interactions. These represent extremes of a continuum: the neutral and the niche models of community organization. Real communities are actually a mixture of both types of dynamics. Here we study the simplest model of neutral-niche communities where niche dynamics is represented as a competitive hierarchy. The competition intensity is defined as a parameter that modulates the transition between these extremes. We use a stochastic cellular automata to show that there is a phase transition between the neutral and niche model with a spanning patch formed by the most abundant species. We measure the diversity as the Shannon index and the richness as the number of species. The transition implies a sharp fall of species diversity but the richness shows a gentle decline with increasing competitive intensity. As this kind of multi-species critical transition have not been described previously, we suggest new early warning signals: the rate of exponential decay in the patch distribution of the non-dominant species. This rate decreases when the community approaches the critical point and increases when the community crosses it. As an example we apply the early warnings to the Barro Colorado Tropical forest, which as expected, result to be far from a critical transition. The model shows that the critical point occur at a very low value of competitive intensity. Low values of competitive intensity were also reported for different high diversity real communities suggesting the possibility that this kind of communities are located near the critical point. This transition could happen before habitat destruction or degradation affect the community in response to changes in environmental conditions like the ones produced by climate change or exotic species invasions.