Interactions and ecological impacts of two forest understory invasive grasses: Cooling invasion meltdown
Very rarely are ecosystems invaded by a single exotic plant; more often, multiple invaders are present. It has been hypothesized that, through synergistic effects on native plant communities and ecosystem processes, the presence of multiple invasive species produce positive feedbacks that result in a “meltdown”. However, our understanding of the interactions between invasive species is poor, as most studies focus on the ecological impacts of single species. Our objective was to examine the interactions among invasive plants and how these interactions may influence the invaded landscape. We conducted our study in an Eastern US forest dominated by two invasive plants: Japanese stiltgrass (Microstegium vimineum) and wavyleaf basketgrass (Oplismenus undulatifolius). Our study site was located near Patapsco State Park in Maryland, close to where wavyleaf basketgrass is thought to have initially been introduced in 1996 and where Japanese stiltgrass is well established. We installed a 50 plot manipulative study to identify the community impacts of each invader. Additionally, we surveyed 162 1m2quadrats within the overlapping range of these two invasive grasses to determine how these invaders interact to influence the abundance of other native and exotic plant species.
Both wavyleaf basketgrass and Japanese stiltgrass had non-linear effects on species, total plant richness, and exotic plant richness, with highest diversity occurring at intermediate levels of either species. Our model suggests that invasive plant richness peaked at 53% total cover of the two invaders, and total plant richness peaked at 36% cover. Additionally, when manipulative plots were managed to remove either invader, invasive richness was greater (5.3±0.43) than in invaded (4.0±0.38) or uninvaded (2.4±0.32) plots. Together, these results suggest that plots invaded by these two grasses are associated with higher invasive species richness but that some exotic species become suppressed at higher cover.
At any level of invasive plant cover, greater proportions of Japanese stiltgrass relative to wavyleaf basketgrass led to reductions in plant species richness but not diversity. However, there were no synergistic or antagonistic interactions between these two invasive grasses on any plant community metrics.
Our results suggest at moderate levels of cover these two species may facilitate the establishment of other invasive species, referred to as “meltdown”, but that at high population densities facilitation is overcome by competition and/or suppression; therefore, reducing other invaders to near uninvaded levels. Moreover, there is no evidence that interactions between these two species contributed to losses in native plant diversity at these sites.