1. Department of Plant Biology; University of Georgia; Athens GA 30602 USA
2. Department of Ecology; Evolution & Behavior; University of Minnesota; Saint Paul MN 55108 USA
3. Bren School of Environmental Science and Management; University of California; Santa Barbara CA 93106 USA
4. Departments of Applied Economics and of Ecology; Evolution & Behavior; University of Minnesota; Saint Paul MN 55108 USA
5. Centre for Biodiversity Theory and Modelling; Station d'Ecologie Expérimentale du CNRS; Moulis 09200 France