Love thy neighbor? reciprocal impacts between plant community structure and insect herbivory in co-occurring Asteraceae

Patterns of herbivory may vary with fine-scale plant community structure: the degree of damage plants experience may depend on their neighbors (i.e., associational resistance or susceptibility). Differential herbivory, in turn, may facilitate a shift in plant community structure. We investigated these reciprocal effects of plant community structure and insect herbivory in a field mesocosm experiment with closely related, native Asteraceae that co-occur in early-successional habitats (old fields). After one year of establishment, we excluded or augmented insect herbivores for two years in equal-density communities of three types: goldenrod-dominated (Solidago spp.) or aster-dominated (Symphyotrichum spp.) congeneric communities and mixtures of the two genera. In manipulated outbreaks, overall and species-specific patterns of herbivory by the main herbivore, the leaf beetle Trirhabda virgata, varied dramatically with community composition. In both years, the preferred goldenrods suffered 25 70% higher defoliation in mixtures with the less-preferred asters (i.e., associational susceptibility), compared to when growing with congeners; in contrast, asters experienced lower damage in mixtures (i.e., associational resistance). Insect herbivory consistently reduced overall plant productivity, and promoted colonization by other oldfield species. Importantly, herbivory also initiated a shift in the structure of the plant communities, and this effect depended on the starting community composition, implying potential reciprocal effects. For instance, only in mixtures did elevated herbivory reduce the proportional abundance of the preferred host, and the old-field dominant, Solidago altissima. Our findings underscore the importance of plant community composition for variation in and impacts of herbivory and suggest the possibility of feedbacks between herbivory and local community structure as one of the mechanisms contributing to the maintenance of vegetation heterogeneity.