Origin matters: diversity affects the performance of alien invasive species but not of native species

At local scales, it has often been found that invasibility declines with increasing resident plant diversity. However, whether resident community diversity similarly resists invasion by alien versus native species is seldom compared. We examined this issue by invading constructed native plant assemblages that varied in species and functional richness with invasive alien or native Asteraceae species. Assemblages were also invaded with spotted knapweed, Centaurea stoebe, a native European Aster that has been previously used in diversity-invasibility experiments in North America. We also conducted a field survey to explore the generality of the patterns generated from our experimental study. Both experimental and observational work revealed that increasing diversity reduced the performance of alien but not native invaders. Centaurea stoebe invading its native community performed poorly regardless of resident diversity whereas in a parallel previously published study in North America, C. stoebe easily invaded low but not high diversity assemblages. Our results suggest that diversity is an attribute of resident communities that makes them more or less susceptible to invasion by novel invasive alien but not native plant species. 19 pages, 2 tables, 3 figures; Appendix