The role of interspecific interactions in structuring the folivore community on American beech (Fagus grandifolia)

The factors structuring the diverse insect herbivore communities associated with woody plants are poorly understood. In particular, the relative importance of interspecific interactions such as competition and facilitation have been contentious. In this study, the factors determining the abundance, species richness, composition, and turnover of leaf-chewing herbivores on permanently marked understory American beech (Fagus grandifolia L.) saplings were examined over a series of summers from 2003-2006. Visual censuses of focal trees were conducted each summer, recording the abundance and species of each caterpillar encountered. Abundance and richness estimates for each tree were standardized by leaf area sampled to enable comparisons among trees varying in size and to gauge growth  responses (leaf biomass increments) of individual trees. In addition, in the late summer of 2004, half of the trees in the study received a brood X Magicicada inoculation treatment in which 50 cicada egg nests were placed at the base of each tree in an attempt to increase root herbivory by cicadas over controls. A total of 67 species of caterpillars representing 19 families of Lepidoptera were recorded using beech as a host during the summer months. Annual increase in leaf biomass production in the year following the cicada emergence increased 50-80% over the previous year's increase, presumably in response to the cicada-mediated nutrient pulse; however, densities of leaf-chewing insects remained relatively constant and showed no response to treatment. No negative covariances among feeding guilds were detected suggesting that competition (direct or indirect) is not playing a large role. However, within a season, the density of shelter-building caterpillars was a positive predictor of both total tree-level herbivore richness and abundance in later censuses. The long-term effects of Magicicada addition on above-ground herbivore populations and tree growth will continue to be monitored as part of this ongoing study.