Evolutionarily Significant Units and the Conservation of Biological Diversity under the Endangered' Species Act

Absrract.-The U.S. Endangered Species Act (ESA) considers "distinct" populations of vertebrates to be "species" (and hence eligible for legal protection) but does not explain how distinctness should be evaluated. A review of the legislative and legal history of the ESA indicates that in implementing the ESA with respect to vertebrate populations, the Fish and Wildlife Senice and the National Marine Fisheries Service (NMFS) should strive to conserve genetic diversity scientifically but sparingly. Based on these precepts, NMFS developed a species policy to guide ESA listing determinations for Pacific salmon Oncorhynchus spp. According to the policy, a population (or group of populations) will be considered distinct if it represents an evolutionarily significant unit (ESU) of the biological species. The unifying theme of the NMFS species policy is the desire to identify and conserve important genetic resources in nature, thus allowing the dynamic process of evolutioa to continue largely unaffected by human factors. A review of case histories in which the NMFS policy has been applied shows that it is flexible enough to provide guidance on many difficult issues for Pacific salmon, such as anadromy versus nonanadromy, variation in life history patterns, and the role of hatchery fish in regards to the ESA. Collectively, these case studies also provide insight into approaches for dealing with scientific uncertainty. Some criticisms of the ESU concept (e.g., that it is too subjective and relies too much-or not enough-on genetics) are discussed, as is its applicability to biological conservation outside the ESA.