Under the Endangered Species Act ("ESA" or "Act") as originally enacted, the term species was defined to include "any subspecies of fish or wildlife or plants and any other group of fish or wildlife of the same species or smaller taxa in common spatial arrangement that interbreed when mature".

In 1978, the Act was amended and the new definition provides that a species includes “any subspecies of fish or wildlife or plants, and any distinct population segment (DPS) of any species of vertebrate fish or wildlife which interbreeds when mature” (ESA, Section 4). (emphasis added)

In 1990, the U.S. Fish and Wildlife Service and the National Marine Fisheries Service (NMFS) convened a Vertebrate Population Workshop to develop guidelines for interpreting the DPS language in the ESA. The NMFS memorandum provided that a vertebrate population (or group of populations) will be considered "distinct" for purposes of the ESA if it represents “an evolutionarily significant unit (ESU) of the biological species”. An ESU was defined as a population that (1) is substantially “reproductively isolated” from other populations of the same species and (2) represents an “important component of the evolutionary legacy of the species”.

In 1996, U.S. Fish and Wildlife and the NMFS developed a joint policy (1996 Policy) intended to clarify the meaning of Distinct Population Segment (61 Fed. Reg. 4722, Feb. 7, 1996). Three basic principles guided the development of the 1996 Policy: (1) the intent of the framers of the ESA to use it to protect genetic diversity (93rd Congress, 1st session, 1973, H.R. Report 412); (2) the 1979 directive that the government agencies involved list populations “sparingly“; and (3) the stipulation in the ESA (section 4(b)(1)(A)) that listing decisions be based “solely on the basis of the best scientific and commercial data available”.

To constitute a DPS, the policy provides a population must exhibit (i) “discreteness” in relation to the remainder of the species and (ii) “significance” to the species to which it belongs.

As to "discreteness" the 1996 Policy states that: “A population segment of a vertebrate species may be considered discrete if [either]: 1. It is markedly separated from other populations of the same taxon as a consequence of physical, physiological, ecological, or behavioral factors …. [or] 2. It is delimited by international governmental boundaries within which differences in control of explication, management of habitat, conservation status or regulatory mechanisms exist ….” (1996 Policy)

Once discreteness has been established, “the Services will consider available scientific evidence of the discrete population segment’s importance to the taxon to which it belongs.” This “significance” test may be satisfied by: “1. Persistence of the [DPS] in an ecological setting unusual or unique for the taxon. 2. Evidence that loss of the [DPS] would result in a significant gap in the range of a taxon. 3. Evidence that the [DPS] represents the only surviving natural occurrence of a taxon . . . . [and] 4. Evidence that the [DPS] differs markedly from other populations of the species in its genetic characteristics….” (1996 Policy)

Further Reading

• U.S. Fish and Wildlife Service, Endangered Species Program. 1996 Distinct Population Segment Policy (full text).
• Waples R.S. 1991. Pacific salmon, Oncorhynchus spp. and the definition of “species” under the Endangered Species Act. Marine Fisheries Review, 53:11-22