1. Department of Experimental Plant Ecology Institute for Water and Wetland Research Radboud University Nijmegen PO Box 9100 Nijmegen 6500 GL the Netherlands
2. Department of Terrestrial Ecology Netherlands Institute of Ecology PO Box 50 Wageningen 6700 AB the Netherlands
3. Laboratory of Nematology Wageningen University PO Box 8123 6700 ES the Netherlands
4. Nature Conservation and Plant Ecology Wageningen University PO Box 47 6700 AA Wageningen the Netherlands