Native-exotic richness relationships: a biogeographic approach using turnover in island plant populations.

Spatial variation in exotic species richness is often correlated with native species richness, for reasons that are poorly understood. To better understand the mechanisms underpinning native-exotic richness relationships, I quantified the colonization and extinction of 18 exotic and 16 native plant species on 39 small islands located off the coast of New Zealand for 8 consecutive yr. Results revealed a positive native-exotic richness relationship, which could be explained by similar demographic responses of native and exotic species to island area. However, native and exotic species showed subtle differences in their response to other island attributes. Turnover in native species declined with island isolation, whereas turnover in exotic species increased with the exposure of islands to ocean-borne disturbances. Overall results illustrate how long-term observations of species turnover can be used to better understand the mechanisms underpinning native-exotic richness relationships, and demonstrate that large, exposed islands can be especially susceptible to invasions by exotic species.