1. Department of Ecology and Evolutionary Biology, University of Michigan, Natural Science Building, 830 North University, Ann Arbor, MI 48109, USA.
2. National Center for Ecological Analysis and Synthesis, 735 State Street, Suite 300, Santa Barbara, CA 93101, USA.
3. Community and Conservation Ecology Group, Center for Ecological and Evolutionary Studies, University of Groningen, Post Office Box 14, 9750 AA Haren, Netherlands.
4. Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA.