Recent experimental and simulation results, and competition-based ecological theory, predict a simple relationship between species richness and the invasibility of communities at small spatial scales – likelihood of invasion decreases with increasing richness. Here we show data from 42 continuous years of sampling old field succession that reveal quite different dynamics of plant invasion. Contrary to experimental studies, when richness was important in explaining invasion probability, it was typically positively associated with species invasion. Invasion of several species had a unimodal response to resident species richness, which appeared to be a mixture of compositional influences and a richness effect. Interestingly, invasions by native and exotic species did not fundamentally differ. Control of species invasion in this system is individualistic, caused by a variety of community-level mechanisms rather than a single prevailing richness effect.
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