Distinct population segment

The DPS concept was introduced in 1978 amendments to the ESA, allowing listing of "any distinct population segment of any species of vertebrate fish or wildlife."¹ In 1996, the U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service (NMFS) issued a joint policy defining DPS as a population that is both discrete and significant.² Discreteness is based on physical, ecological, or genetic separation from other populations; significance is evaluated using factors such as persistence in a unique ecological setting or loss of the segment would create a gap in the species' range.² The policy applies only to vertebrates, excluding invertebrates and plants.

DPS listings have been used to protect geographically isolated populations while avoiding listing the entire species. Notable examples include the Southern Resident killer whale (Orcinus orca) DPS, listed in 2005, and the Mexican gray wolf (Canis lupus baileyi) DPS. The DPS designation allows tailored recovery plans and regulatory protections for populations that are genetically or behaviorally distinct, such as the distinct runs of Pacific salmon. However, the policy has been controversial; some argue it can fragment conservation efforts, while others see it as a flexible tool to prevent extinction of unique populations.

Courts have often scrutinized DPS determinations. In National Association of Home Builders v. Defenders of Wildlife (2007), the Supreme Court addressed the scope of consultation but not DPS criteria directly.³ Scientific debates center on the threshold for "significance"—whether it should rely on genetic distinctiveness, ecological role, or loss of adaptive potential. The policy has been criticized for lacking quantitative standards, leading to inconsistent decisions. For example, the Preble's meadow jumping mouse (Zapus hudsonius preblei) DPS was delisted in 2005 after new genetic data showed it was not distinct, illustrating the importance of updated science.⁴

The DPS concept has been applied to anadromous fish, such as steelhead trout, where each river run can be a separate DPS. It also applies to marine mammals, like the Hawaiian insular false killer whale (Pseudorca crassidens) DPS, listed in 2012. A lesser-known fact is that the DPS policy was influenced by the need to protect Pacific salmon without listing all salmon, which would have had massive economic impacts. Additionally, the policy has been used to delist populations that have recovered, such as the gray wolf in the Northern Rocky Mountains DPS, which was delisted in 2011.⁵ The DPS designation is unique to U.S. law; other countries use different units, such as "evolutionarily significant units" in Canada.

Discreteness

The separation of a population from others based on physical, ecological, or genetic factors.

Significance

The importance of a population to the species as a whole, based on ecological, genetic, or other criteria.

Evolutionarily significant unit

A concept used in Canada and elsewhere to define populations for conservation, similar to DPS.