In the past decade a great deal of attention has been focused on the problem of protecting endangered species. Indeed, for historical reasons, biodiversity has largely been discussed by biologists in terms of the diversity of species, leading economists and others to reasonably conclude that conserving Earth's species diversity is the crucial task before us. But the loss of species is only one aspect of the extinction crisis, and in many parts of the world may not be the most important facet of the decay of biological diversity (biodiversity). This paper examines the complementary issue of the extinction of populations, offers a preliminary assessment of its importance relative to the extinction of species, and examines relevant policy implications.