Increased disturbance-mediated competition between invasive and native plants: An unintended consequence of managing for increased species diversity

Background/Question/Methods

Most biotic resistance studies have focused on how species losses and/or disturbances increase invasion by reducing competition, whereas relatively few studies have considered how management activities designed to use disturbance to increase diversity might increase the competitive impacts of invasive species. Using a simple model, I investigated the potential for ecological restoration of historical fire regimes to simultaneously increase both native groundcover plant species diversity and the competitive effects of invasive plant. Predictions were then evaluated by examining changes in native species composition and increases in the abundance of a non-native grass, Microstegium vimineum, within a long-term ecological restoration experiment designed to increase native groundcover plant diversity in an upland oak forest in north Mississippi.

Results/Conclusions

A simple model illustrated how disturbances (e.g., increased light availability associated with thinning the forest over- and midstory and surface fires) could increase native plant diversity by increasing the abundance of sparse disturbance-dependent natives suppressed by the lack of disturbance. The model also showed, however, how such disturbances could increase the competitive impact of a non-native species that otherwise would have been kept at low densities by the lack of disturbance. Examination of compositional changes in response to ecological restoration treatments (i.e., thinning and prescribed burning) revealed significant increases in native plant diversity in areas in which Microstegium vimineum was absent but modest decreases in native diversity in areas in which Microstegium vimineum was initially sparse but then increased in response to the treatments. Results reveal how management actions aimed increasing native plant diversity could have the unintended consequence of increasing the competitive effect of established invasive plants on native plants. In some cases, the effects of disturbances on competition between invasives and natives may be more important than effects on invasion.