The term "distinct population segment" in the Endangered Species Act has no defined scientific meaning. The U.S. Fish and Wildlife Service and the National Marine Fisheries Service have jointly promulgated a policy requiring a distinct population segment to be both: (1) discrete and (2) significant. However, the implementation of this policy has led to inconsistent listing decisions and the failure to list distinct populations of species that require protection under the Endangered Species Act. These problems are clearly illustrated in National Association of Homebuilders v. Norton, in which the plaintiffs sued the U.S. Fish and Wildlife Service in district court, alleging that the decision to list the Arizona population of the cactus ferruginous pygmy-owl as a distinct population segment was "arbitrary and capricious." The district court upheld the listing, finding that the Arizona population constituted a distinct population segment of the species. On appeal, the Ninth Circuit reversed and held that while the Arizona pygmy-owl was distinct, it was not significant to the species. In order to avoid such judicial reversals of agency decisions, a clearer and more consistent standard of designation should be implemented. This standard should include: (1) an Evolutionary Unit standard, which would include all of the current factors for discreteness and de-emphasize the significance factors; (2) a requirement that the distinct population be a minimum viable population; (3) a uniform standard of proof requirement; and (4) the precautionary principle for reviewing distinct population segment listing decisions.

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