Weak vs. strong invaders of natural plant communities: Assessing invasibility and impact

In response to the profound threat of exotic species to natural systems, much attention has been focused on the biotic resistance hypothesis, which predicts that diverse, communities should better resist invasions. While studies of natural communities generally refute this hypothesis, reporting positive relationships between native species diversity and invasibility, some local-scale studies have instead obtained negative relationships. Most treatments of the topic have failed to recognize that all exotic invaders do not behave alike: while "weak" invaders become minor components of communities, "strong" invaders become community dominants at the expense of native species. At the same time, the specific impacts of strong invaders on communities are poorly documented yet critical to understanding implications of diversity loss. With these shortfalls in mind, we examined local-scale relationships between native and exotic plant taxa in buncligrass communities of western Montana, USA. We found that measures of native species diversity and invasibility were positively correlated in communities with low levels of invasion where both weak and strong invaders occurred at low densities, but negatively correlated in communities with high levels of invasion where the strong invader Centaurea maculosa dominated. Furthermore, at both low and high levels of invasion, weak invaders tended to vary positively with native species richness while strong invaders varied negatively. Weak invaders determined positive overall relationships between native and exotic species in low invasion communities and appeared to coexist with native taxa. Strong invaders prevailed in high invasion communities, driving negative overall relationships therein. That negative relationships primarily reflected strong invader impacts was supported by the fact that negative correlations of C. maculosa with native species diversity and abundance held when we statistically controlled for levels of native species representing preinvasion conditions. In addition, negative associations of C. maculosa with native taxa varied in strength by functional group and species, with large effects on numerically dominant species, suggesting a dramatic shift in community composition and structure. The distinction between weak and strong invaders is important for reconciling conflicting results from previous studies of natural communities, and moving attention beyond the current debate surrounding the biotic resistance hypothesis.