1. Department of Animal Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Post Office Box 50, 6700AB Wageningen, Netherlands.
2. Centre for Conservation Biology, Department of Biology, Norwegian University of Science and Technology, NO-7491, Trondheim, Norway.
3. Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
4. Centre d’Etudes Biologiques de Chize, Centre National de la Recherche Scientifique, F-79360 Villiers en Bois, France.