Symbiosis and the Regulation of Communities

Ecologists have long been interested in factors that controlthe structure of communities and the relative importance oftop-down effects of predators versus bottom-up effects of resources.There is a growing body of evidence that microbial symbiosesare important determinants of plant community structure andindirectly affect herbivore and predator trophic levels. Studieswith mycorrhizal fungi, N-fixing bacteria and endophytes ofgrasses have demonstrated that they can affect competition,coexistence, soil nutrient dynamics and plant-herbivore interactions.Long-term field experiments with one grass/endophyte interactionsuggest that grassland community structure is determined bythe fungus. While total plant productivity of experimental plotswas similar, the composition of the vegetation was altered byendophyte symbiosis. The host grass tall fescue (Festuca arundinacea)dominated plots when infected while other grasses greatly increasedin uninfected plots. Indirect evidence suggests that changesin prairie vole (Microtus ochrogaster) grazing patterns andreproductive physiology may be in part responsible for vegetationalchanges. These results provide evidence that, in addition tobottom-up and top-down forces, microbial symbionts of plantsare important determinants of community structure.