1. Division of Zoology, Department of Biology, University of Oslo, POBox 1050, Blindern, N–0316 Oslo, Norway
2. Department of Population Ecology, Zoological Institute, University of Copenhagen, Universitetsparken 15, DK–1200 Copenhagen, Denmark
3. Abisko Scientific Research Station, Box 62, S–981 07 Abisko, Sweden
4. Sheffield Centre for Arctic Ecology, Department of Animal and Plant Sciences, University of Sheffield, 26 Taptonville Road, Sheffield S10 5BR, UK