Department of Ecology, Evolution and Environmental Biology, Columbia University, 1200 Amsterdam Avenue MC 5557, New York, NY 10027, USA (AGS) Instituto de Ecologı́a, Universidad Nacional Autónoma de México, Apartado Postal 70-275, México, Distrito Federal, México (OG, RAM) Present address of AGS: Unit of Biology and Physical Geography, Irving K. Barber School of Arts and Sciences, University of ...