Critical resource level and its role in plant species diversity and invasion

The invasion of natural communities by introduced plant species is one of the most serious threats to biodiversity. A modified hierarchical competition-colonization model is developed for a general explanation of plant species diversity and invasion at metacommunity scale. Resource level in absence of any species at a physically homogenous, spatially sub-divided habitat defines a limit to diversity at competitive equilibrium. Species invasion occurs only when the habitat is in non-equilibrium. A species-saturated habitat with increasing resource level is prone to species invasion. There exists a critical level of resource concentration in the habitat; within a small neighborhood of that level, limited number of species belong to appropriate trait can stably coexist at competitive equilibrium. If resource concentration is sufficiently higher than that critical level, the habitat losses its stability; unstable habitat and resources above the critical level would enhance invaders’ trait. As a result invaders started spreading geographically throughout the habitat in such a way that resident response poorly to the higher availability of resource and consequently resident loss their abundances monotonically. If resource concentration is sufficiently lower than the critical level but higher than the resource concentration under which no species can survive, all species with low densities belong to certain specific trait will have monotonically increasing abundances. Our result showed that it is resource quantity, not resource heterogeneity, maintaining plant diversity.