PARENT SESSION
Oral Session # 53: Herbivory IV: Communities, Populations, and Genetics.
Presiding: JA Rudgers
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 103.

Selective herbivory as a mechanism to understand shifting plant community composition in response to simulated global changes.

Cleland, Elsa^{1, 2}, Field, Christopher², ¹ Dept. of Biological Sciences, Stanford Univerisity, Stanford, CA² Carnegie Institution of Washington, Dept. of Global Ecology, Stanford, CA

ABSTRACT- Human activities are altering carbon (C) and nitrogen (N) cycling in ecosystems worldwide. Plant communites respond to these global changes both physiology and with shifting species composition, potentially producing ecosystem-level feedbacks. In California annual grasslands, slug herbivores selectively remove species with high tissue %N early in the growing season. We investigated the hypothesis that experimentally induced global changes would alter the extent of selective herbivory via shifts in plant tissue chemistry, leading to functional shifts in species composition. In the field, we planted six species in known locations both inside and outside of herbivore exclosures, and exposed these plots to either ambient or elevated levels of two simulated global changes: elevated atmospheric CO² and N deposition. Species were chosen to represent a functional axis of tissue %N: two legume species with high tissue %N, two annual forbs with intermediate tissue %N, and two annual grasses with low tissue %N. Seedlings were monitored for germination and herbivory, and harvested to determine tissue % C and N. Under control conditions, species with high tissue %N (legumes) were selectively removed by herbivory. In plots with N deposition, all species responded with increased tisse %N, and herbivores were less selective, allowing legumes to persist in the species composition. Under elevated CO², seedlings had lower tissue %N (n.s. trend), and herbivores were more selective, removing a greater proportion of legumes and forbs than under control conditions. Thus, with elevated CO²

Key words: community composition, global change, selective herbivory, nitrogen