1. J. H. Lawton and R. M. May Eds. Extinction Rates (Oxford Univ. Press Oxford UK 1995)
2. E. O. Wilson The Diversity of Life (Harvard Univ. Press Cambridge MA 1992).
3. G. C. Daily Ed. Nature's Services: Societal Dependence on Natural Ecosystems (Island Press Washington DC 1997).
4. N. Myers The Primary Source (Norton New York 1984); P. R. Ehrlich and A. H. Ehrlich Ambio 21 219 (1992).
5. We necessarily assume that the number of populations per unit area and the range size of a species are independent.