Indirect facilitation among invasive species through suppression of shared competitors
Facilitation among non-native species may promote invasions. Recent work suggests that one invasive species may directly facilitate another by creating favorable environmental conditions, enhancing seed dispersal, or increasing resource availability. However, species may also indirectly interact. For example, species that occur in the same habitat but do not compete for the same limiting resource at the same time may mutually benefit from suppression of shared competitors. An introduced species also might alter environmental conditions – such as light or nitrogen availability, or fire regimes – in such a way that native species are inhibited and other invasive species benefit. Here we used a long-term field experiment to test for facilitative interactions between two of the most widespread and problematic invasive plant species in the eastern USA: Microstegium vimineum (stiltgrass) and Alliaria petiolata (garlic mustard). We applied three disturbance treatments and introduced Alliaria seed into replicated plots previously invaded experimentally by Microstegium. We then measured Alliaria performance and evaluated differences in resource availability caused by the initial invasion.
There was greater colonization success of Alliaria in native-dominated plots, particularly under low or moderate levels of disturbance, but little difference in the number of plants that survived to adulthood. However, adult Alliaria plants in invaded plots produced almost seven times more biomass and had nearly four times greater reproductive output than in uninvaded plots. The superior performance of Alliaria in Microstegium-invaded plots compared to native-dominated control plots far outweighed the differences in establishment success such that, overall, invaded plots contained more than three times greater Alliaria biomass and twice as much reproduction than control plots. We found strong evidence that Alliaria benefited from the effects of Microstegium on native species. The biomass of competing native plants was greatly reduced during peak growing season in invaded plots and there was significantly more light available early in the season when Alliaria is most active. Thus, our results demonstrate that the initial plant invasion resulted in suppression of resident competitor species and increased resource availability, thereby indirectly facilitating the secondary plant invasion. Additional work on the population-level effects of these interactions and examples from other systems are necessary to understand the generality of indirect facilitation in structuring invaded plant communities.