1. E. Ranta is in the Integrative Ecology Unit, Division of Population Biology, Department of Ecology and Systematics, University of Helsinki, Post Office Box 17, Helsinki, FIN-00014, Finland.
2. V. Kaitala is in the Department of Biological and Environmental Science, University of Jyväskylä, Post Office Box 35, FIN-40351 Jyväskylä, Finland.
3. P. Lundberg is in the Department of Theoretical Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden.