Insect Herbivores Increase The Spatial Aggregation Of A Clonal Plant

Premise of research.In a plant population, the pattern of spatial dispersion from uniform to strongly aggregated can influence the intensity of ecological interactions, including those between plants and their consumers, such as herbivorous insects. Whether insect herbivores also contribute to the generation of dispersion patterns within plant populations, however, is not well understood. We investigated how insect herbivory and plant density interact to influence the degree of aggregation of stems in experimental plots of the clonal perennial herb Solanum carolinense, a noxious weed in the southeastern United States and invasive in other parts of the world.Methodology.We planted S. carolinense in 40 experimental plots at five densities and maintained each plot at one of two herbivory levels. Stems in every plot were initially planted in a uniform dispersion pattern. For 3 yr, we quantified the dispersion of clonal recruits in each plot at six spatial scales.Pivotal results.At larger spatial scales, we found significant interactions between plant density and insect herbivory. In plots with herbivores, spatial aggregation increased as plant density decreased, but this relationship was weak or nonexistent in plots where herbivores were excluded (herbivore-excluded plots exhibited random dispersion of stems across all densities). Solanum carolinense in plots exposed to herbivores were on average twice as aggregated as those that had herbivores excluded.Conclusions.These results suggest that insect herbivores can influence clonal growth or ramet survival in ways that increase population-level aggregation of S. carolinense. Our findings also imply that S. carolinense populations in its invasive range may have increased spatial spread and decreased intraspecific competition when herbivory is low. We discuss the potential for specific types of feedbacks between insect damage and plant dispersion, highlighting important targets for future research.