Scale dependent effects of biotic resistance to biological invasion

As nonindigenous species continue to displace native species and disrupt ecosystems, understanding the degree to which native species richness affects the vulnerability of communities to nonindigenous species invasions has grown in importance. Native and exotic species diversity are often positively correlated in large-scale observational studies, but negatively correlated in small-scale experimental studies. This discrepancy suggests that the scale of invasion studies may be an important influence on their outcomes. Using a competition-based model that exhibits a negative relationship on a small scale, we show that changes in the number of available resources across communities can cause invasion success to become positively correlated with native species diversity at larger scales. The strength of the positive correlation, however, depends on the relationship between niche breadth and species diversity in natural communities. Adding species to a community or removing resources has a similar effect-increasing the sum of interspecific interaction strengths, which decreases invasion success.