1. Department of Biology, Duke University, Durham, North Carolina 27708, United States
2. U.S. Geological Survey, Columbia Environmental Research Center, Columbia, Missouri 65201, United States
3. U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon 97330, United States
4. Department of Natural Resources and the Environment & Center for Environmental Sciences and Engineering, University of Connecticut, Storrs, Connecticut 06269, United States