Distributions of phytophagous larvae in mid-successional Allegheny hardwoods: impacts of bird exclusion

Background/Question/Methods Phytophagous larvae are a crucial part of forest community structure, acting as herbivores, pollinators, and prey.   Abundance on forest trees is determined by top-down forces, primarily predation by passerine birds, and bottom-up forces such as plant quality and phenology. We examined these tritrophic effects on phytophagous larvae in forest communities of the Allegheny hardwoods forest type in northwestern Pennsylvania in May-August 2011. To examine the influence of avian predators, we compared abundance between 432 netted and un-netted branch pairs. To examine effects of plant quality and phenology, we distributed these paired branches across individuals of the six forest trees that comprise 95% of the basal area of the Allegheny hardwoods from leaf-out in mid-May to late summer.

Results/Conclusions Lepidoptera comprise over 90 percent of the collected specimen with Hymenoptera and Coleoptera constituting the remaining 10 percent. Birds significantly depressed abundance (larvae per gram foliage) on all tree species except birch (Betula lenta or B. allegheniensis) in at least one month, with the relative impact of bird exclusion on larval abundance comparable across tree species.  Differences in species diversity across hosts and between treatments can be explained by larval abundance, suggesting that differences in plant quality determine invertebrate species richness and that the impact of birds on these communities can be attributed to the density of larvae on a particular host plant.  Our findings imply a direct relationship between host plant quality for insect herbivores and the value of plants as a food source for birds.