The Endangered Species Act - The Endangered Species Act Of 1973 (esa)—a Landmark Protection

Passed by the U.S. Congress in 1973, the Endangered Species Act was substantially amended in 1978, 1982, and 1987. The ESA is administered by the U.S. Department of the Interior through the U.S. Fish and Wildlife Service (FWS). The U.S. Department of Commerce, through the National Marine Fisheries Service (NMFS), is responsible for most marine (ocean-based) species and those that are anadromous (migrate between fresh waters and marine waters). The Biological Resources Division of the U.S. Geological Survey (USGS) conducts research on species for which the Fish and Wildlife Service has management authority.

It should be noted that the original Endangered Species Act defined the word "species" to include species, subspecies, or "smaller taxa." Taxa is the plural of taxon, which is a grouping on the taxonomic table. In 1978 the ESA was amended to define a smaller taxon for vertebrates (animals with a backbone) as a "distinct population segment" (DPS). A DPS is a distinct population of vertebrates capable of interbreeding with each other that live in a specific geographical area. A DPS is usually described using geographical terms, such as northern or southern, or by a given latitude or longitude. In 1991 the National Marine Fisheries Service developed a policy defining the DPS for Pacific salmon populations. Salmon are anadromous, meaning they migrate between the ocean and inland fresh waters. Most salmon migrate in groups at particular times of the year. Each of these groups is called a stock. The NMFS developed a new term, the evolutionarily significant unit (ESU), to refer to a distinct stock of Pacific salmon.

In summary, the word "species" as used in the Endangered Species Act can mean a species, a subspecies, a DPS (vertebrates only), or an ESU (Pacific salmon only).