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Know the West

A few scientific definitions


Note: This article is a sidebar to this issue's feature story, "Is It or Isn’t It (Just Another Mouse)?"

When people think about the creatures protected by the Endangered Species Act, they tend to picture gray wolves, grizzlies or spotted owls. But the act draws finer distinctions than that, providing protection for subspecies and even for members of a species that live in a particular geographic region.

About 75 percent of all listed animals, fish and plants are full species, while another 20 percent are subspecies. Around 5 percent are "distinct population segments" of a vertebrate species. The definitions of "species" and "subspecies" given here are general working definitions, since scientists have not pinned down exact meanings for the terms. Even the Fish and Wildlife Service’s criteria for "distinct population segments" are open to interpretation.


A taxonomic rank below a genus or subgenus but above a subspecies, composed of organisms that can successfully reproduce with each other and that share a set of distinguishing characteristics.

Examples include the Utah prairie dog, which is threatened, and the whooping crane and the jaguar, both endangered.


A taxonomic rank below a species, recognizing individuals that have certain heritable characteristics distinct from other subspecies of a species.

For instance, the spotted owl has three subspecies, two of which are threatened: the Mexican and the northern.

distinct population segment

A population of vertebrates that is discrete from other populations of the species, that has significance for the species as a whole, and that meets the ESA’s criteria for threatened or endangered status.

For example, grizzlies in the Lower 48 states are threatened, except for the Yellowstone population; the gray wolf is threatened in Minnesota but endangered in the rest of the Lower 48 (both species also have experimental populations that are not protected).