Historically, plant community ecology has been dominated by the study of antagonistic species interactions, mainly competition and herbivory. However, mutualistic and positive species interactions may modify antagonist interactions and can have similar, or even stronger, effects on communities than antagonisms. Mutualism effects on community diversity can be negative or positive depending on the breadth of the mutualists association with members of the community and their host’s competitive effects. Understanding how mutualism affects community-scale patterns requires manipulating mutualism and competition in combination to determine the degree to which mutualism influences the outcome of competitive interactions, and indirectly alters species composition. We tested the interactive effects of an epichloae mutualism and interspecific competition on plant community composition and ecosystem processes within a Great Lakes dune plant community. During summer 2012, we established a field experiment using a randomized 2×2 factorial design manipulating epichloid endophyte (Epichloë amarillans) presence in Ammophila breviligulata (present or absent) and intra/interspecific plant interactions (Monoculture = only intraspecific interactions or Mixture = interspecific + intraspecific interactions). Each plot is 3m×3m and contains 36 A. breviligulata plants and 9 individuals each of five common dune plant species. Species-specific allometric equations were used to estimate aboveground biomass and calculate Pielou’s evenness.
Results/Conclusions
The Epichloë mutualism shifted the effects of interspecific interactions on both its host and the competing plant community leading to shifts in the community evenness. By 2015, interspecific interactions increased community evenness by 31% while Epichloë presence decreased community evenness by 14%. However, the effects of plant-plant interactions and endophyte presence were driven by the response of A. breviligulata, which produced 40% less biomass under interspecific competition and 48% more biomass when the endophyte was present. Interestingly, the magnitude of interspecific competition were stronger when Epichloë was present than when Epichloë was absent (biomass was 42% and 37% less, respectively, when interspecific interactions were present). Since the host A. breviligulata was sowed at 4x the initial density of the other plant species, treatment effects on A. breviligulata may mask endophyte effects on the subordinate community (i.e. excluding A. breviligulata). Epichloë presence increased subordinate community evenness by 23% but had no effect on community productivity. The primary driver of the increased community evenness was the reduced biomass of little bluestem, second most dominant species, when the endophyte was present. Mutualists play an important role in mediating interspecific interactions and should play a larger role in ecological theory.