Patterns and processes of plant community invasibility
The invasion of native plant communities by exotic species is a cause for much environmental concern but also provides an opportunity to observe basic ecological processes. I quantified invasion rate in a series of experimental communities in order to test the hypothesis that lowered diversity leads to increased invasibility. The experiment was designed to provide a direct test of niche complementarity, the mechanism by which diverse communities are usually hypothesized to resist invasion. I also examined modes of invasion of one particular exotic species, crown vetch (Coronilla varia L.). In the experimental communities I found that the relationship between diversity and invasibility varies depending on the attributes of community members. Lowering diversity by removing short species from big bluestem (Andropogon gerardii Vitman) dominated communities had no effect on invasibility. However, removing tall species from these communities did cause an increase in invasibility. This was contrary to expectations, as removing short species lowered the amount of height dissimilarity present in the communities, which would be expected to reduce niche complementarity and increase invasibility. While we found no support for niche complementarity based on height dissimilarity, we found some support for niche complementarity through phenology. The vast majority of the invaders in our experimental prairie communities appeared in the spring and early summer. Communities that contained species that were actively growing and competitive at this time were better able to resist invasion. The practical implication of this result for prairie managers is that management regimes favoring warm season grasses may lead to reduced resistance to invasion by cool season forbs and grasses, which make up a substantial portion of the weed flora in many prairie areas. In the second part of my research I found that crown vetch patches have little spatial-age structure, possibly due to a rapid rate of ramet turnover. I also found that crown vetch does not build up a large presence in the seed bank of sites it has invaded, but that its ability to regenerate from vegetative fragments may be the key to its persistence.