Jos M M Van Damme1, Wilke Van Delden2. 1. Department of Dune Research, Institute for Ecological Research, "Weevers 'Duin," Duinzoom 20a, 3233 EG, Oostvoorne, The Netherlands. 2. Department of Genetics, University of Groningen, Kerklaan 30, 9751 NN, Haren, The Netherlands.