Null models of exotic invasion and scale-dependent patterns of native and exotic species richness

Surveys of natural habitats often indicate that native and exotic species richness are positively correlated at large scales and negatively correlated at small scales. The small-scale relationship is often presented as evidence that native richness can repel invasion or conversely that exotic invasions can reduce native diversity. The larger scale pattern has been interpreted as evidence of the importance of facilitation, variable habitat quality, propagule supply, and other ecological phenomena. However, these explanations fail to consider expected native-exotic richness relationships under a null model assuming no species interactions. We show via simulation that the null expectation for a randomly assembled community is a negative relationship between native and exotic species richness at the smallest scales and, when plots vary in total richness, a positive relationship at larger scales. We outline a procedure to compare observational data to this null expectation using a permutation test of labels of species origin (native or exotic). Our use of this technique on plant community data indicates that patterns of native and exotic richness are remarkably similar to those generated by a null model. We argue that a null model approach is needed to evaluate whether observed native-exotic richness data deviate significantly from expected patterns generated by sampling and statistical artifacts.