1. Plant Conservation and Population Biology; Department of Biology; University of Leuven; Kasteelpark Arenberg 31 Heverlee B-3001 Belgium
2. Ecology and Evolution Section; Department of Biology; University of Copenhagen; Universitetsparken 15 København 2100 Denmark
3. Research unit of “Ecologie et Dynamique des Systèmes Anthropisés”; Jules Vernes University of Picardy; 1 Rue des Louvels Amiens F-80037 France
4. Research Institute for Nature and Forest; Gaverstraat 4 Geraardsbergen B-9500 Belgium