1. Community and Conservation Ecology, Univ. of Groningen; PO Box 11103, NL-9700 CC Groningen the Netherlands
2. Alterra Wageningen Univ. and Research; PO Box 47, NL-6700 Wageningen the Netherlands
3. Dept of Ecology; Radboud Univ. Nijmegen; Toernooiveld 1 NL-6525 ED Nijmegen the Netherlands
4. Dept of Ecology and Genetics; Uppsala Univ.; Norbyvägen 18D SE-75236 Uppsala Sweden
5. Station Linné; Ölands Skogsby 161 SE-38693 Färjestaden Sweden