Species relatedness and plant community structure: consequences of interspecific competition and herbivory

Background/Question/Methods  Ecological theory predicts that the degree of relatedness among species in a community is likely to shape species interactions and local community structure. Because closely related species are expected to have similar phenotypes and ecologies, the strength and outcomes of their interactions may differ compared to those among more distantly related species. For instance, interspecific competition is predicted to be stronger among close relatives than more distant relatives. Relatedness may also influence top-down effects of consumers on the community: plants are predicted to experience different patterns and impacts of herbivory depending on whether neighbours are close or more distant relatives. However, the relative importance of species relatedness in the joint effects of competition and herbivory on local community structure has not been assessed experimentally. We tested these predictions in field mesocosm communities of differing relatedness, in the presence and absence of herbivores, using seven perennial, co-occurring herbaceous Asteraceae (three goldenrods (Solidago), the related Euthamia graminifolia, and three asters (Symphyotrichum)). During three growing seasons, we compared relative species performance, community productivity, damage by herbivores, and invasibility by other plant species between congeneric mesocosms (goldenrods or asters) and mixtures of the two genera.

Results/Conclusions   Relatedness did not affect the intensity of competition: most species tended to perform equally in congeneric communities and in mixtures, which also showed similar patterns of productivity. On the other hand, the patterns of damage by insect herbivores differed depending on the relatedness of neighbours: goldenrods consistently suffered greater herbivory in mixtures than congeneric communities, whereas asters tended to escape herbivory in mixtures. Consequently, herbivory initially lead to differential performance of goldenrods and asters depending on relatedness, even though community productivity was reduced equally. However, by year 3, tolerance to herbivory in goldenrods resulted in their equal productivity in the presence and absence of herbivores, and similar performance in congeneric communities and mixtures, at the expense of asters which no longer benefited from herbivory-mediated competitive gain in mixtures. Finally, richness of plant species that have invaded the mesocosms was lowest in the mixtures, and unaffected by herbivory, but consistently reduced in the presence of herbivory in both types of congeneric communities. Community relatedness thus influenced species performance and dynamics in plant community structure, but its effects were mediated via direct and indirect effects of herbivory, rather than via direct effects of competition.