1. Biology Department, Grinnell College, PO Box 805, Grinnell, IA 50112, USA,
2. Department of Biology, Vanderbilt University, Nashville, TN 37235, USA,
3. Departments of Botany and Zoology, Washington State University, Pullman, WA 99164, USA,
4. Section of Ecology and Systematics, Cornell University, Ithaca, NY 14853, USA