Invasive species can escape pathogens when they are introduced to a new range, which may provide them with a competitive advantage over native species. However, mounting evidence suggests that invasive species accumulate pathogens over time, some of which are generalists that can emerge from native species and spillback to them. Changes in host fitness due to disease can lead to multiple outcomes of competition: coexistence, exclusion, or priority effects. The few studies that have evaluated the impacts of pathogen accumulation on invaded communities have found that it promotes coexistence or suppresses native species. However, recent evidence suggests that an emerging fungal pathogen can suppress the invasive grass, Microstegium vimineum, favoring native species. The goal of this research is to characterize the current and long-term effects of fungal pathogen accumulation on competition between M. vimineum and native species. We employ field experiments, greenhouse experiments, and modeling to assess how fungal infection alters the outcome of competition between M. vimineum and a native perennial grass, Elymus virginicus. We monitored plant performance and infection within constructed communities of M. vimineum and E. virginicus at invaded field sites where disease severity was mitigated through repeated fungicide applications. In the greenhouse, we examined the potential for senesced M. vimineum biomass, which accumulates in invaded areas, to affect disease transmission and competition. We used data from our experiments and the literature to parameterize a discrete-time plant competition model.
Results/Conclusions
Our field and greenhouse experiments indicate that competition occurs between M. vimineum and E. virginicus through live plant interactions and litter interference. For example, litter can reduce M. vimineum establishment by 28% and E. virginicus establishment by 9%. We found that foliar fungal infection accumulates on M. vimineum over the growing season regardless of the background plant community and that live M. vimineum increases the infection rate of E. virginicus by 15%, potentially through microclimatic changes. Initial model results indicate that the strength of E. virginicus intraspecific competition and adult survival can determine whether M. vimineum alone, E. virginicus alone, or both persist in the system. In addition, reductions in M. vimineum seed production due to infection can promote coexistence. Our current results suggest that foliar fungal infections could suppress M. vimineum invasions given low impacts on native species and environmental conditions that promote survival of native perennial adults.