Interactive effects of natural and anthropogenic disturbances differentially impact native and exotic plants
Recent research has suggested that both natural and anthropogenic disturbances can differentially impact native and exotic species. However, disentangling the effects of multiple types of disturbances in the same ecological community can be a significant challenge. Here, I conducted a factorial experiment to examine the effect of a natural disturbance, the deposition and removal of wrack on oceanic beaches, and an anthropogenic disturbance, the addition of nitrogen fertilizer, on native and exotic plants. To determine the individual and interactive effects of these two types of disturbance, I experimentally manipulated strandline plant communities in Narragansett Bay and assessed changes in native and exotic richness and cover over the course of the growing season. These communities are an ideal system for examining the differential response of native and exotic species because they have been heavily invaded for over 200 years and are known to have large numbers of native and exotic plants coexisting together.
I found that native and exotic richness and cover were not significantly influenced by either the addition of fertilizers or wrack removal. In contrast, I found that the addition of wrack significantly lowered native and exotic richness, but had opposite effects on native and exotic cover as native cover decreased while exotic cover increased. In addition, I found that there were significant interactive effects between these disturbances. For natives, both the addition and removal of wrack coupled with nitrogen addition led to significant decreases in richness and cover. For exotics, I observed that the addition of wrack and nitrogen decreased richness, but led to an increase in cover. Similarly, in the case of wrack removal and nitrogen addition, I found that exotic richness stayed the same while exotic cover went up. Collectively, these findings indicate that while native and exotic species may have similar responses to individual disturbances, responses to multiple disturbances at the same time may lead to differential outcomes. Finally, these results also suggest that alterations in disturbance regimes may have significant effects on native and exotic plant coexistence over time.