Environmental Gradients and Variation in the Strength of Bird Predation on Oak Herbivores

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Most ecologists now agree that predators and resources interact with the abiotic environment to influence the ecology of most herbivores. Recent interest lies in determining the relative effects of these two factors on herbivores, and how these factors vary in space and time. This study compares three competing theories to explain why predation pressure on herbivore communities varies in space. We propose that A) foliage quality, not plant productivity, determines the productivity of herbivores on plants, and B) herbivores vary in their vulnerability to predators. Consequently, C) predation pressure on herbivores increases with increasing foliar quality, and prey that are less vulnerable to predation will accumulate on high-quality plants. We plan to test these predictions by exploring variation in the strength of bird predation on arthropods of red oak along a gradient in elevation at the Coweeta Long-term Ecological Research site. Recent work has shown that some herbivores increase in density with elevation, while others decline. At the same time, foliar quality increases with elevation, while plant productivity does not. Together with results from bird predation experiments, these results suggest that the impact of birds on free-living herbivores increases with elevation at Coweeta, leading to the accumulation of less vulnerable prey (leaf miners) on high-quality plants. The goals of this study are: 1) to validate apparent changes in herbivore with elevation that have been observed at Coweeta; 2) to assess the relative impact of bird predation on arthropod communities along the gradient in elevation; and 3) to assess the effects of changes in foliar chemistry on oak herbivores over this elevational gradient. These goals will be addressed using a combination of field sampling, exclusion experiments, and laboratory feeding trials. Our study will provide fundamental information on how changes in predation pressure at the landscape level influence populations and communities of herbivores. The study will contribute to the training of undergraduate and graduate students. Outreach activities at Coweeta will ensure that research activities are available to K-16 students and to the general public. All data collected will be available to the public and the scientific community via the Coweeta web page. Arthropods collected during the study will form part of the Coweeta collections, hosted by the State Museum of Georgia, and open to the general public. In addition, the data collected will contribute to an understanding of the role of insectivorous birds in forests and the potential impacts of avian declines on ecological processes in forests.