Coexistence, saturation and invasion resistance in simulated plant assemblages

A popular hypothesis is that species-rich systems are less susceptible to i nvasion. This hypothesis is based on the idea that species richness correla tes with community saturation so that establishment by a new species is mor e difficult in saturated communities, Little attention has been focussed on how changing assumptions about the processes regulating species richness w ill alter community properties such as invasion resistance. Here, we simula te plant community assembly using four models that have different underlyin g coexistence mechanisms (and so differ in the amount of available niche sp ace) and subject them to invasion. We created species richness gradients by comparing between models or by considering the output of a single model wi th different parameter values. We found that the relation between species r ichness and invasion resistance depends critically on the model considered and the cause of the species richness gradient. Overall, our results sugges t that species richness does not necessarily correlate with saturation and is likely to be a poor predictor of invasion resistance. These results prov ide a possible explanation for the variety of outcomes reported in recent e xperimental and observational studies that examine the relationship between species richness and invasion resistance. We conclude that consideration o f the processes regulating species richness is crucial for a successful und erstanding of invasion resistance along species richness gradients.