It is commonly believed that species-rich habitats are less invasible due to niche occupation. Yet, most recent evidence shows that invasibility is much more complex and is determined by multiple factors.  As both a determinant and measure of invasibility, species richness itself can be either an absolute value or a proportion and the diversity effect may be through the diversity-biomass relations, not the number of species per se.  Empirical studies continue to generate inconsistent results including positive, negative, or no relationship or correlation between diversity and invasibility.  More importantly, the perceived “relationships” (or correlations) are scale-dependent.  To better describe and understand the interactions between native and exotic species, I ask (1) whether the native-exotic richness correlations are indeed real, and if so, (2) how these correlations vary across spatial scales.  To address these questions, metadata on species diversity and invasibility are collected from diverse ecosystems and larger eco-regions in the United States and other countries or continents around the world. 

Results/Conclusions

Preliminary observations show that, at local-scales, species-rich communities are invasible but may be so to a lesser degree.  Everything else being equal, the outcome depends on the species composition of both the existing community and invaders as each existing species can resist or promote a specific invader.  When all data are combined, a unimodal relationship between species diversity and invasibility emerges, i.e., positive when species diversity is low and negative when the diversity is high (saturated).  A community’s ability to preclude species invasions may be dependent upon a threshold level of both species richness and abundance, below which the importance of species interactions is only a weak force.  On larger- or regional-scale observations and across broader ranges of habitat types with both low and high resident species diversities, the relationships tend to be unimodal.  However, all these correlations also exist among major native species groups (e.g., woody vs. herbaceous species).  Because the number of exotics in a community very much depends on many factors, including chance, the propagule pressure, time, disturbance, and what species have been introduced, studying the interactions between exotics (often a particular species) and natives and the consequences may be more important than addressing the simple “correlations” between the two species groups and can shed much light on the mechanisms of biological invasions.  Comparisons among the major community-types within and among geographic regions can provide new insights for both invasion biology and management.