There has been much recent interest in the relative contributions of "top-down" and "bottom-up" influences on patterns of herbivory, but less attention to how these patterns might vary with resource levels. We examined this issue in Lindera benzoin (Lauraceae), an understory shrub that consistently experiences increased levels of lepidopteran herbivory in low resource environments (low light). Our objective was to determine how predator exclusion and nutrient addition and their interaction influenced growth and herbivory in low and high light environments. In this experiment, we randomly selected 28 plants in each of two habitat types, shaded interior and high light forest gaps and edges, at our field site in Northampton County, PA. One of three treatments randomly applied to each plant included: 1) coverage with bird exclusion netting, 2) addition of 250 g of 13-13-13 slow release fertilizer at the base of the plant at early and mid-season, and 3) both bird exclusion and fertilizer addition. One fourth of the plants were untreated and served as controls. On three randomly selected branches on each plant, leaf and branch growth were measured; and herbivory was visually estimated and leaves were categorized as having either: no herbivory, <25% consumed, between 25% and 75 % consumed, or greater than 75% consumed.
Results/Conclusions
Growth, either as leaf addition or branch extension did not vary by treatment, but was greater in the sun than in the shade. The removal of a top-down control by bird exclusion, resulted in increased herbivory in both environments, but to a lesser extent in the resource rich (sun) environment as indicated by a significant treatment-by-environment interaction. Fertilization, a bottom-up influence, did not affect herbivory at shaded sites, but increased herbivory in the sun environment. Previous studies revealed greater rates of bird predation on model caterpillars at sun sites. The results here indicate that in spite of the potentially higher levels of predation on herbivores at sun sites, herbivory at these sites is more greatly influenced by resources (light and nutrients) and presumably leaf quality than by herbivore predation, and that tri-trophic interactions can vary with resource levels.
