Invasive species can both reduce the richness and abundance of native species in ecological communities and accumulate microbial mutualists and pathogens. Theory suggests that specialist pathogens should reduce the overall impact of invasive species on native species, but that generalist pathogens may have a range of effects. Many pathogens, including most of those that have naturally colonized invasive species, are generalists. It is unclear how such accumulation jointly affects native species and modifies invasive species’ impacts. We evaluated the combined effects of an invasive understory annual grass (Microstegium vimineum) and foliar fungal pathogens (Bipolaris spp.) (which have recently increased in abundance on Microstegium) on a co-occurring native perennial grass (Elymus virginicus). We implemented field experiments to assess (1) competition between the native species and the invasive species in the absence of disease, (2) host-pathogen interactions for each grass species, and (3) the combined effects of competition and infectious disease on both grass species. Our experiments shed light on how pathogens modulate plant competition and the long-term consequences of invasions on natives.
Results/Conclusions
Live plants competed for resources and the invasive species produced large amounts of litter, interfering with seedling establishment of both species. Without disease, intraspecific competition reduced seed production of both species more than interspecific competition. However, invasive species produced many more seeds than the native species, and the invasive species’ litter suppressed the establishment of the native species more than it did its own. Our results, combined with a mathematical model, suggest that the invasive will eventually supplant the native in the absence of disease. Bipolaris infection, which caused more foliar damage on the invasive species than the native, reduced seed production of both species in the absence of competition. The disease decreased intraspecific competitive effects on seed production of both species and interspecific competitive effects on seed production of the native species but increased interspecific competitive effects on seed production of the invasive species. In addition, disease reduced biomass production of the invasive species, which should reduce litter production and associated interference on seedling establishment. Our results suggest that accumulation of generalist foliar fungal pathogens on Microstegium will ameliorate the competitive impacts of this invasive species on co-occurring native grasses.