Motivation: Plant communities can experience species invasions and infectious disease at the same time, but it is unclear what the consequences are, partly because most investigations feature only one native species. We examined the effects of disease and invasion on three native species. The invasive species is Microstegium vimineum, a forest understory grass, which hosts fungi in the genus Bipolaris that are spreading across the invaded range.
Design: In a greenhouse experiment, we manipulated, pathogen inoculation, native species identity, and M. vimineum density to measure the effects of disease, competition from an invasive plant, and both on the biomass production of the native species. We used two inoculation treatments (pathogen inoculation with a spore solution of Bipolaris gigantea and a mock inoculation control), three native species (Elymus virginicus, Eragrostis spectabilis, and Dichanthelium clandestinum), and five levels of M. vimineum density (0, 2, 10, 50, 100 seeds per pot). Crossing these three treatments in a full-factorial design led to 30 treatments with four replicate pots each.
Measurements: We quantified disease incidence by counting the number of M. vimineum leaves per pot with lesions and then counting the total number of leaves on three seedlings (or two when only two were available). For the native plants, we counted the number of leaves with lesions and the total number of leaves per plant when plants had lesions. At the end of the experiment, we harvested aboveground biomass, oven-dried it to a constant weight, and weighed it.
Analysis: We used the brms package to fit Bayesian models to each response variable. We fit a binomial linear regression to disease incidence data for M. vimineum with native species identity, M. vimineum density, and their interaction as the explanatory variables. We could not fit a model to the disease incidence data for the native species because there were too few data points. We fit a linear regression to M. vimineum biomass with pathogen inoculation treatment, native species identity, M. vimineum density (represented with a linear and quadratic term), and their interactions as the explanatory variables. We fit a Beverton-Holt function to native plant biomass across the range of M. vimineum density values with different parameters for each native species-pathogen inoculation treatment combination.