1. Univ. of Fribourg, Unit of Ecology and Evolution; Ch. du Musée 10 CH-1700 Fribourg Switzerland
2. Research station Agroscope ACW Changins-Wädenswil; route de Duillier 50 CH-1260 Nyon Switzerland
3. Dept of Ecology and Evolution; UNIL Sorge, Le Biophore, Univ. of Lausanne; CH-1015 Lausanne Switzerland
4. Ecology and Evolutionary Biology; Univ. of California; Irvine CA 92697 USA