Explaining the scale-dependent differences in the relationship between biodiversity and invasibility remains a challenging question at the heart of invasion biology. While at local scales more diverse communities tend to be less invaded, at larger scales native and exotic diversity tend to increase together. A number of mechanisms have been proposed to explain different aspects of this pattern, including diverse communities being more resistant to invaders and greater environmental variability at larger scales providing more opportunities for both native and exotic species to establish. It is likely that diversity-invasibility patterns emerge from multiple processes acting in concert, but these different processes are rarely evaluated together. We used aquatic plant survey data from shallow lakes in Minnesota (150,000+ plant identifications in 1,102 lakes over 13 years) to evaluate four mechanisms that could influence the richness of native or exotic species at both local (neighborhood/point) and regional (lake) scales. Using data from lakes with repeated sampling over time, we quantified both biotic resistance (more diverse sites being less likely to become invaded) and competitive exclusion (declines in native diversity following invasion). Additionally, we used environmental data to evaluate the role of environmental filtering and heterogeneity on native and exotic species richness.
Results/Conclusions
Looking across the entire dataset, we observed the classic scale-dependent patterns, with a negative relationship between native and exotic species richness at the local scale but a positive relationship at the lake scale. There was varying support for different mechanisms to explain these patterns. At the local scale, initial species richness of areas that did or did not become invaded were not significantly different, indicating that biotic resistance was not an important mechanism. In contrast, locations with invaders did lose more species over time than uninvaded locations, consistent with competitive exclusion. At the regional scale, lakes with greater diversity were more likely to gain invaders over time, and invaded lakes gained more species than uninvaded lakes. Native species richness, but not invader richness, increased with environmental heterogeneity. Native and invasive species richness were both correlated with environmental conditions, but responded to different environmental factors, potentially indicating environmental filtering along different axes. Additionally, invasive species tended to show broader environmental tolerances. These patterns suggest that biodiversity-invasibility patterns are complex and the result of multiple processes. Unlike many other (particularly terrestrial) systems, biotic resistance was not a strong mechanism, while environmental selection and niche breadth appeared to be important drivers.