1. Institute for Water and Wetland Research, Animal Ecology and Physiology, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
2. Plant Ecology and Nature Conservation Group, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands
3. Bodøgata 2, 0565 Oslo, Norway