species, although not endangered, is likely to become so in the foreseeable future. As a practical matter, threatened species are often listed so that the ESA’s protective measures may reduce their chance of declining further into endangered status. As of April 2000, 960 species were listed as endangered and 270 were classed as threatened in the United States.
The listing action is a regulatory process that is usually initiated by the Services, although the agencies must also take into account petitions for listing that may be submitted by private parties. The Services maintain a priority list of species that have enough supporting information to warrant a listing decision, and they are required to review the list of protected species every 5 years to determine if any changes in status should be made. Status changes (delisting or reclassification) also are regulatory actions. The listing process includes provisions for public review and comment to solicit input about the species’ status, the nature and degree of threat, and protective actions that may be needed or are in place. A species may be listed if it is considered threatened or endangered for one or more of the following reasons: habitat reduction, overharvesting, disease or predation, absence of adequate protective measures, and other unspecified factors that may contribute to its imperiled condition.
Listing decisions are to be made solely on the basis of the best scientific and commercial information available at the time, without regard to political or economic interests. The Services must also consider protective measures that are in place by government agencies. For some time, there has been an extensive backlog of species whose listing action is pending because of funding constraints (Doremus 2000). Recent legal decisions that require the FWS to take action on many species that are eligible for listing have also added to the backlog. The result of this hourglass effect on the pace of listing decisions is that many species do not get listed until their populations are substantially reduced. Listing of Atlantic salmon that were designated as a DPS in eight Maine rivers is a recent example.
In response to congressional direction, the Services have adopted priority-setting guidelines to help manage the backlog. FWS guidelines consider the magnitude of threat, the imminence of the threats, and the taxonomic uniqueness of the species. Taxonomic uniqueness is applied hierarchically; first priority is given to monospecific genera, followed by full species, and then subspecies. NMFS guidelines do not include taxonomic factors, but their listing regulations invoke the concept of “evolutionary significant unit” (NRC 1995, Waples 1991b) to define eligible entities for listing below the species level.
At the time of listing, the Services are required to designate critical habitat to the extent that necessary information is available and it is prudent to do so. Critical habitat is the area within the species’ range that contains those physical or biological features essential to the conservation