PARENT SESSION
Symposium #13: Trophic cascades in complex and simple terrestrial systems.

Organized by: MD Moran and WE Snyder
Tuesday, August 6. 1:00 PM to 4:30 PM. Crystal Ballroom, TCC.

The role of the middle trophic level in tritrophic cascades: Leaftiers as ecosystem engineers.

Marquis, Robert^*,1, Lill, John², ¹ University of Missouri-St. Louis, St. Louis, MO² Washington University, St. Louis, MO

ABSTRACT- The indirect effects of bird predation on plants via changes in insect herbivore density range from strong, in which both leaf damage and plant growth are influenced, to weak or non-existent. Part of this variability in effect may be due to differential susceptibility of individual insect herbivore species to avian predation. Systems in which the herbivore faunas are dominated by non-susceptible species may be less likely to show such indirect effects. Differential susceptibility is also important because some herbivore species may play a greater role in structuring the local herbivore community than others, in turn modifying the total community's impact on the plant. In our system (saplings of white oak, Quercus alba, and their herbivores and natural enemies), leaf-tying caterpillars are both less susceptible to bird predation than other guilds and have a major influence on the structure of the insect herbivore community. These leaf-tying caterpillars engineer their environment by tying leaves together with silk. Leafties are subsequently colonized by other leaf-tying caterpillars, free-feeding species, predators, parasitoids, and scavengers. One species of leaftier in particular, Pseudotelphusa sp. (Gelechiidae), positively influences the abundance of other leaf-tying caterpillars, sucking insects, and arthropod predators, and increases the amount of leaf area lost, decreasing subsequent growth parameters of the tree. These results suggest that to predict the magnitude of indirect effects by avian predators, and in turn the likelihood of a trophic cascade, we will need to know both the susceptibility of individual herbivore species to predation and their effects on other herbivores.

KEY WORDS: trophic cascade, oak herbivores, bird predation, ecosystem engineering