Paul R. Ehrlich is the emeritus Bing Professor of Population Studies in the Department of Biology and the president of the Center for Conservation Biology at Stanford University. He is the author of The Population Bomb and Human Natures: Genes, Cultures, and the Human Prospect. Gerardo Ceballos, one of the world's leading ecologists, is a professor at the Institute of Ecology at Universidad Nacional Autónoma de México (UNAM). He has established more than twenty protected areas in Mexico and is the author or coauthor of more than 55 books. Ehrlich and Ceballos are coauthors of The Annihilation of Nature: Human Extinction of Birds and Mammals. Rodolfo Dirzo, one of the world's leading conservation scientists, is the Bing Professor of Environmental Science in the Departments of Biology and Earth Systems Science at Stanford University and a senior fellow at the Woods Institute for the Environment.