ability of the earth to support human societies. This appreciation has given rise to conservation biology, which makes it clear that people depend on ecosystem services. These services include food, construction materials, medicinal plants, wild genes for domestic plants and animals, crop and plant pollination, absorption and detoxification of pollutants, generation and maintenance of soils, and the regulation of air and water quality as well as climate (Ehrlich and Ehrlich, 1991). In addition, biodiversity is the foundation of biotechnology.

Recognizing the value of its biological resources, the United States has enacted laws and policies to protect plants and animals from extinction. The United States has also demonstrated a strong commitment to the wise, responsible, scientifically based stewardship of its biological resources through regulatory programs, acquisition of public lands, and various preservation efforts (NRC, 1993b). Despite these efforts, the nation's biological diversity is in decline and there are questions about how it should be sustainably managed.

The leading cause of species extinction is habitat destruction (Pimm and Raven, 2000). In assessing the condition of approximately 20,500 species of U.S. plants and animals, the Nature Conservancy found that about one-third were of conservation concern (Stein et al., 2000). Animals that depend on freshwater habitats and flowering plants are in the worst condition. More than 500 U.S. species may have already disappeared. These losses have affected virtually every state, but Hawaii, Alabama, and California have been especially hard hit.

Surveys of many groups of plants and animals indicate global extinction rates at least several hundred times the rate expected based on the geologic record (Pimm and Brooks, 2000). Ten percent of the world 's 10,000 bird species are threatened with extinction (Collar et al., 1994). About 500 of these birds are likely to go extinct in the next 50 years, producing an extinction rate of 1,000 extinctions per million species per year (Pimm and Brooks, 2000).

With the loss of biological diversity and the alteration of ecosystems that support it, many social and economic consequences follow. The decimation of pollinating insects decreases crop yields (Nabhan and Buchanann, 1997). Degradation of wetlands exposes communities to increased flood damage. Land use changes in watersheds impoverish water purification processes at substantial cost to urban communities (e.g., the cost of installing and maintaining water treatment plants) (Chichilnisky and Heal, 1998).