The Endangered Species Act (ESA) protects wildlife and plants in danger of extinction or those likely to become endangered in part or all of their range. On occasion, the Act also protects specific populations of vertebrate species that meet the legal definition of a “distinct population segment,” or DPS.

For example, in 2005, the federal government determined that the Southern Resident killer whales could be considered a DPS as they are genetically and culturally distinct from all other killer whale populations in the Pacific Northwest. They do not interbreed or mix with other populations. Essentially, the DPS designation treats the Southern Resident killer whale as a different species, ensuring specific protections under the ESA, such as identifying and protecting critical habitats, together with a federal management recovery plan.

Similarly, within Pacific salmon, populations of one species, such as Chinook, might be recognized under the ESA as an “evolutionarily significant unit,” or ESU; that is, they are genetically isolated from other Chinook populations. NOAA Fisheries considers eight populations of Chinook to be ESUs, and sadly all these populations are vital prey to the endangered Southern Resident killer whales.