1. Imperial College and NERC Centre for Population Biology, Silwood Park, Ascot, Berks SL5 7PY, UK.
2. Smithsonian Tropical Research Institute, Center for Tropical Forest Science, Unit 0948, APO, AA 34002, USA.
3. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA.
4. Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN 55108, USA.