1. Population Biology, Ecology and Evolution, Graduate School of Arts and Sciences, Emory University, Atlanta, Georgia 30322, USA
2. Department of Population Health and Pathobiology, College of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina 27606, USA
3. Department of Environmental Studies, Emory University, Atlanta, Georgia 30322, USA
4. Current address: Institute of Ecology, The University of Georgia, Athens, Georgia 30602, USA