1. National Institute of Aquatic Resources, Technical University of Denmark, Charlottenlund Slot, Jægersborg Allé 1, DK-2920 Charlottenlund, Denmark
2. School of Biological Science, Queens University Belfast, 97 Lisburn Road, Belfast BT9 7BL, UK
3. Department of Theoretical Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden