Because invaded communities are often only surveyed after the impacts of invasion have already begun, the measurements of invasion impacts may be confounded by the fine scale conditions that influence where the invading species establishes. To evaluate this context dependency, data from a long-term, permanent plot study of vegetation dynamics (the Buell-Small Succession Study) were used to study the invasion and impacts of the exotic grass Microstegium vimineum. From these data, we examined the fine scale properties that determined local invasion (propagule pressure, understory species richness, canopy cover, etc.). For plots which became invaded, we then related local plot conditions to the success (increase in cover) of M. vimineum. Finally, we documented the impacts of invasion via changes in understory herbaceous species richness, understory cover and the abundance of associated species.
Results/Conclusions
Invasion by M. vimineum was significantly more likely in plots with high understory herbaceous species richness and in plots that were close to other invaded plots (representing propagule pressure). However, the success of M. vimineum (cover increase) was positively correlated with understory herbaceous cover but not richness. Plots closer to established M. vimineum developed greater cover and plots with greater tree cover developed less M. vimineum cover. Plots that were more heavily invaded lost significantly more species and lost more herbaceous species cover than uninvaded plots. Both of these impacts would be confounded by patterns of initial invasion success in this system. Because of these offsetting effects, single-time assessments of the invasion reveal limited impacts of M. vimineum. Together these results suggest that Microstegium vimineum preferentially invades and impacts areas with high understory species richness and cover. Failing to account for the context dependency of invasion impacts on patterns of invisibility may lead to underestimation of invader impacts.