1. Center for Limnology, University of Wisconsin, Madison, WI 53706.
2. Department of Geography and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742.
3. Environmental Science and Policy Program, Michigan State University, East Lansing, MI 48824.
4. Department of Biological Sciences, Stanford University, Stanford, CA 94305.
5. Department of Applied Economics and Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108.
6. Woods Institute for the Environment, Stanford University, Stanford, CA 94305, USA.
7. Natural Resources and the Environment (NRE), Council for Scientific and Industrial Research, Post Office Box 395, 0001 Pretoria, South Africa.