The application of coexistence theory to restoration
Invasive species are a continual threat to the biodiversity and functioning of ecosystems and a regular focus of any land management program. Potential synergistic interactions between an exotic’s competitive ability and environmental change effects on resources requires that we refine our ability to assess how changes in resource competition influence invasion pathways – when an exotic may either coexist with native residents or dominate to the detriment of native diversity. We propose that coexistence theory can help land managers understand when environmental change factors may interact with competitive hierarchies to facilitate invasion and can reveal potential management strategies to mitigate these interactions. In monocultures of either an exotic annual grass (Avena fatua) or native perennial grass (Stipa pulchra), we manipulated nitrogen (N) inputs (low, ambient, high) to simulate atmospheric N deposition. We conducted reciprocal invasions within each plot, where each species was added to its own community type as well as the other. We measured survivorship for the perennial and spikelet production for the annual to assess the fitness of an individual when rare (invading heterospecific) or dominant (invading conspecific). This invasibility criterion – ability to increase when rare – can indicate whether the invasion would result in coexistence, resistance, or complete transformation of the community.
The exotic annual had greater fitness when rare (invading the perennial community), particularly under elevated N conditions (where its fitness more than tripled). This result suggests that the exotic should invade and coexist with the native at all N levels, but increased N inputs will increase the probability of invasion resulting in exotic dominance. The native perennial’s fitness was higher when common (establishing in its own community type) across all N levels, and negative heterospecific effects were strongest at high N. This result suggests that the native would have trouble re-establishing in invaded exotic-dominated communities, particularly at high N. Lowering N levels would facilitate the re-establishment and restoration of natives. Despite high native survivorship in conspecific communities under all N levels, invasion by exotic annuals when rare make coexistence rather than complete resistance a more probable scenario even under low N conditions. In summary, our results illustrate the potential for coexistence theory to inform land managers of high-risk invasion areas and also suggest potential management strategies to improve the efficacy of restoration efforts.