Native diversity, soil nutrients, and natural enemies influence invasion in experimental plant communities
Plant diversity may alter the role of soil resources and natural enemies in exotic invasions. Although invasions are more likely when resource availability is high, diverse communities may inhibit invasion by reducing resource availability. Additionally, reduced enemy pressure among exotics may enhance invasion. This advantage may decrease in diverse native plant communities where diverse enemy communities may be more likely to limit invader performance. Thus, we expected diverse communities to hinder invasion even at high resource availability or reduced enemy pressure. To test this prediction we planted 1m2 plots denuded of vegetation with seedlings of 6 native herbaceous species at two diversity levels – monocultures of 6 focal species and 5-species polycultures lacking one focal species – totaling 12 planted community types. Planted richness and density was maintained through July 2012, when we manipulated soil nutrients (ambient, 10 g m-2 yr-1 NPK) and aboveground enemies (ambient, exclusion via biocide application). For two years after these treatments began the plots were naturally invaded. To test for differential invasion, we visually quantified the cover of all species within each plot. Here we present the first experiment to examine the interactive and independent influences of resource availability, natural enemies, and diversity in invasions.
Resource availability, differential enemy pressure, and resident diversity all strongly influenced invasion. Relative exotic abundance (exotic / total cover) was nearly double in fertilized plots compared to unfertilized plots (p < 0.001), suggesting that exotics were better able to exploit available nutrients than competing natives. Furthermore, two years of enemy exclusion reduced relative exotic abundance by 30% relative to unsprayed plots (p < 0.001). Thus, differences in natural enemy pressure between exotics and natives may enhance exotic invasion. Overall, 5-species polycultures were 20% more resistant to invasion than monocultures (p = 0.004), confirming our prediction that diverse communities resist invasion. However, we found no significant interactions between planted diversity and either resource availability or enemy presence. In this system, resource availability had the largest influence on invasion, while enemy reduction and resident diversity contributed smaller but still significant amounts to invasion success. These results demonstrate that invasion success can be enhanced by multiple independent mechanisms.