1. Julie Wrigley Senior Fellow, Stanford University, Stanford, CA 94305, USA.
2. Center for Environmental Science and Policy, Stanford University, Stanford, CA 94305, USA.
3. Animal Production and Health Division, FAO Headquarters, 00100 Rome, Italy.
4. Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA.
5. Department of Geography, University of Manitoba, Manitoba R3T 2N2, Canada.
6. Department of Animal Science, University of California at Davis, Davis CA 95616, USA.
7. Fisheries Centre, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
8. Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA.