PARENT SESSION
Oral Session #83: Insect Population and Community Ecology.
Presiding: C. Rodriguez-Saona
Thursday, August 8. 9:00 AM to 11:30 AM. Palo Verde Room, Radisson.
The effects of predators and nutrients on insect herbivores at Mount St. Helens.
Bishop, John*,1, Fagan, William2,3, Shade, John2, 1 Washington State University Vancouver, Vancouver, WA2 Arizona State University, Tempe, AZ3 University of Maryland, College Park, MD
ABSTRACT- Previous work demonstrated that lepidopteran herbivores decrease population growth rate of lupines (Lupinus lepidus) in primary succession at Mount St. Helens. Herbivory is concentrated at low-density (edge) patches or at margins of high-density (core) patches, creating remarkable, persistent negative density dependence. Here we evaluate whether predators or plant tissue quality are responsible for this pattern. In 1999 we placed 60 pitfall traps at a range of plant densities. Adult moths were trapped primarily in low-density areas, suggesting a lack of ovipositioning females in the high-density center of core patches. In contrast, carabid predators and tachinid parasitoids were predominantly in core patches. Within core patches they concentrated at low-density margins and tracked prey abundance. This suggests that these predators are not responsible for long-term suppression of herbivory in the core center. In 2000 we determined elemental ratios of C, N, and P in lupine and herbivore tissue along density/herbivory gradients in core patches. Plants at low-density margins were ~25% richer in P per unit N compared to plants at high density. Herbivore N:P matched that of edge plants but was P rich compared to core plant tissue. Feeding trials indicate that Gelechiid leaf miners grow more rapidly on a diet of edge plant leaves. Thus, density-related gradients in nutrient or defensive chemistry are a more likely explanation for the pattern of herbivory than are top-down effects of predators.
KEY WORDS: herbivory, lupinus lepidus, predator, nutrient stoichiometry