1. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA.
2. Santa Fe Institute, Santa Fe, NM 87501, USA.
3. School of Biological Sciences, University of Bristol, Bristol BS8 1TQ, UK.
4. Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, CO 80523, USA.
5. Wildlife Conservation Society, Bronx, NY 10460, USA.
6. Center for Population Biology, University of California, Davis, CA 95616, USA.