1. Department of Ecology; University of Oulu; PO Box 3000 FI-90014 Oulu Finland
2. Department of Biology; University of Eastern Finland; PO Box 111 FI-80101 Joensuu Finland
3. School of Forest Sciences; University of Eastern Finland; PO Box 111 FI-80101 Joensuu Finland
4. Department of Ecology; Swedish University of Agricultural Sciences; Grimsö Wildlife Research Station SE-730 91 Riddarhyttan Sweden