Effects on Species’ Conservation of Reinterpreting the Phrase “Significant Portion of its Range” in the U.S. Endangered Species Act

The U.S. Endangered Species Act (ESA) requires listing of a species as threatened or endangered not only when it is at risk of worldwide extinction, but also when it is at risk in any “significant portion of its range” (SPOIR) (16 U.S.C. Section 1532(6)). Many species have been listed as endangered under the ESA in part because they were threatened in a SPOIR (e.g., FWS 1990, 1995, 2000). On 16 March 2007, the solicitor of the U.S. Department of Interior issued a memorandum substantially redefining the agency’s position on SPOIR (U.S. Department of the Interior 2007). The solicitor argued that only a species’ current range is relevant to the analysis of endangerment, and that once a species is determined to be endangered in a SPOIR, it is entitled to be listed in only the portion identified as significant. The solicitor does not completely preclude consideration of historic range. Rather, the solicitor acknowledges that “data about the historical range and how the species came to be extinct in that location may be relevant in understanding or predicting whether a species is ‘in danger of extinction’ in its current range,” but adds that “the fact that it has ceased to exist in what may have been portions of its historical range does not necessarily mean that it is ‘in danger of extinction’ in a significant portion of the range where it currently exists” (U.S. Department of the Interior 2007). These positions represent a substantial departure from past implementation of the ESA. Under the ESA, listing is limited to species, subspecies, and distinct population segments of vertebrate species (herein referred to as species) (16 U.S.C. § 1532[16]). Before the memorandum, species were listed throughout their historic and current ranges without exception. For example, the U.S.