Biodiversity collapse and early warning indicators in a spatial phase transition between neutral and niche communities

The dynamics of ecological communities can be described by two contrasting models: the first assumes that individuals of all species are identical and do not have interactions. The second assumes that species are different, adapted to particular habitat conditions, and have strong interactions. It is known that a critical transition exist between these two states, but the spatial aspect of this transition has not been studied. Here we study the simplest model of neutral-niche communities in a spatially explicit way using a stochastic cellular automata. The neutral model follows the Hubbell's original formulation and 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 the theory of percolation to study the geometric properties of species patches. The transition is signed by the apparition of a large patch that connects the whole area, the properties of this patch can be used as an early warning signals: the proportion of the largest patch with respect to the total area covered by the species, variance of the size fluctuations and the skewness of the fluctuations distribution. These three indices can be combined to show that a critical transition is approaching. The model shows that at the critical point there is a sudden fall of species diversity but the richness shows a gentle decline with increasing competitive intensity. The critical point occurs at a very low value of competitive intensity and the rate of migration from the metacommunity greatly influences the position of the critical point. As an example we apply the early warnings to the Barro Colorado Tropical forest, which as expected, appear to be far from a critical transition. Low values of competitive intensity were also reported for different high diversity real communities suggesting the possibility that these kind of communities are located before the critical point. A small increase of competitive interactions could bring them to the other side where diversity is much lower. 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.