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| A popular hypothesis is that species-rich systems are less susceptible to invasion. This
hypothesis is based on the idea that species richness correlates with community
saturation so that establishment by a new species is more difficult in saturated
communities. Little attention has been focussed on how changing assumptions about
the processes regulating species richness will alter community properties such as
invasion resistance. Here, we simulate plant community assembly using four models
that have different underlying coexistence mechanisms (and so differ in the amount
of available niche space) and subject them to invasion. We created species richness
gradients by comparing between models or by considering the output of a single
model with different parameter values. We found that the relation between species
richness and invasion resistance depends critically on the model considered and the
cause of the species richness gradient. Overall, our results suggest that species
richness does not necessarily correlate with saturation and is likely to be a poor
predictor of invasion resistance. These results provide a possible explanation for the
variety of outcomes reported in recent experimental and observational studies that
examine the relationship between species richness and invasion resistance. We conclude
that consideration of the processes regulating species richness is crucial for a
successful understanding of invasion resistance along species richness gradients. | |
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