Positive effect of diversity on a natural invasion in small-scale arid communities

The increased movement of species throughout the world has prompted investigations of factors that influence the invasibility of communities. The biotic resistance hypothesis posits that community diversity should affect invasion: invasion is predicted to be less likely as diversity of a community increases. Empirical evaluations of this hypothesis have been mixed. At large scales, invasion is seen more often in high diversity communities, whereas at small scales, low diversity communities are often more susceptible to invasion as the hypothesis predicts. We investigated factors affecting the natural invasion of Pectocarya recurvata into small scale (0.25 m²) annual plant communities at an experimental site near Portal, Arizona, U.S.A. Counter to the biotic resistance hypothesis, we found that invaded communities had higher species richness than non-invaded communities. Invaded communities also had higher productivity than non-invaded communities. Furthermore, P. recurvata invaded plots subjected to rodent disturbance significantly more often than plots not subjected to rodent disturbance. Our results suggest that disturbance and productivity more strongly affected the invasion of P. recurvata than did biotic resistance. Based on our findings, we suggest that biotic resistance may not always strongly affect invasion success in small-scale communities.