1. U.S. Geological Survey, Columbia Environmental Research Center, 4200 New Haven Road, Columbia, Missouri 65201, U.S. Environmental Protection Agency, NHEERL, MED-Duluth, 6201 Condgon Boulevard, Duluth, Minnesota 55804, U.S. Fish and Wildlife Service, 3817 Luker Road, Cortland, New York 13045, and Cornell University, Division of Biological Sciences, Section of Ecology and Systematics, Ithaca, New York 14850