Disease decreases variation in host community structure in an old-field grassland

Disease may modulate variation in host community structure by modifying the interplay of deterministic and stochastic processes. For instance, deterministic processes like ecological selection can benefit species less impacted by disease. When disease consistently selects for certain host species, this can reduce variation in host community composition. On the other hand, when host communities are less impacted by disease and selection is weaker, stochastic processes (e.g., drift, dispersal) may play a bigger role in host community structure, which can increase variation in structure among communities. While effects of disease on host community structure have been quantified in field experiments, few have addressed the role of disease in modulating variation in structure among host communities. To address this, we conducted a field experiment spanning three years, using a tractable system: foliar fungal pathogens in an old-field grassland community dominated by the grass Lolium arundinaceum, tall fescue. We reduced foliar fungal disease burden in replicate host communities (experimental plots in intact vegetation) in three fungicide regimens that varied in the duration of fungicide exposure and included a fungicide-free control. We measured host diversity, biomass, and variation in community structure among replicate communities. Disease reduction generally decreased plant richness and increased aboveground biomass relative to communities experiencing ambient levels of disease. Despite changes in structure of the plant communities over the experiments three years, the effects of disease reduction on plant richness and biomass were consistent across years. However, disease reduction did not reduce variation in host community structure, providing little evidence for ecological selection by competition or other deterministic processes. Instead, disease reduction tended to amplify variation in host community structure among replicate communities (i.e., within fungicide treatment groups), suggesting that disease diminished the degree to which host communities were structured by stochastic processes. These results of experimental disease reduction both highlight the potential importance of stochastic processes in plant communities and reveal the potential for disease to regulate variation in host community structure.