1. Natural Environment Research Council Centre for Population Biology, Imperial College, Silwood Park, Ascot, Berks SL5 7PY, United Kingdom
2. Leverhulme Unit for Population Biology and Biological Control, Imperial College, Silwood Park, Ascot, Berks SL5 7PY, United Kingdom
3. Ecole Normale Supérieure, CNRS-URA 258, Laboratoire d'Ecologie, 75230 Paris Cedex 05, France