Studies of single-species invasions dominate the literature, but plant communities typically experience simultaneous invasions of multiple plant species. Research on co-occurring invasive species is therefore critical to advance understanding of invasion mechanisms and their impacts on native communities. We addressed whether a plant functional group approach provides a useful framework for predicting the nature of invasive-invasive and invasive-native interactions. In a controlled greenhouse environment with tallgrass prairie species, we manipulated the number and identity of co-occurring functional groups of invasive legumes, grasses, and forbs. To examine invasive-invasive interactions, we established microcosms with single functional groups as well as pairwise and three-way functional group combinations. We measured the impacts of co-occurring invasive functional groups on natives by growing native communities with multiple invaders from the same or different functional groups. We hypothesized that niche complementarity would reduce competition between invasive species from different functional groups and that invasive legumes would facilitate invasive forbs and grasses. Therefore, we expected the native community to be more negatively affected as the number of invasive functional groups increased.
Results/Conclusions
Counter to our prediction, in invasive-only microcosms we saw no evidence for niche complementarity between invasive species from different functional groups. Invasive legumes did tend to facilitate both grasses and forbs in the invasive-only microcosms, but this result was not significant. Legumes were unaffected by co-occurring invasive species. While invasion negatively affected the native community, invaders had statistically similar impacts regardless of the number or identity of invasive functional groups. Our results provide limited support for a functional group approach to predicting co-occurring invasive interactions in isolation. We saw no evidence that invasive functional group diversity predicts invasive impact, at least in functionally diverse native communities.