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The Endangered Species Act was created in response to a rapid decline in species biodiversity. Although Congress chose direct protection of individual species as its tool, protecting ecosystems (a necessary component of biodiversity) was clearly one of the goals for which that tool was to be used. A species can be abundant in some areas and declining in others, such that protecting the entire species does not make sense. Congress dealt with this issue by amending the Endangered Species Act in 1978 to allow for protection of “distinct population segments,” thereby allowing the population in decline to be protected in spite of the abundance of the species elsewhere. This, of course, raises the question as to why we should bother to do so. Why, if a species is thriving elsewhere, should we save its struggling members in a given population? The Fish and Wildlife Service and National Marine Fisheries Service answered this question by requiring -- in their policy statement for listing distinct population segments -- that the population be significant. While this is a very reasonable limitation, the definition of significant is far too narrow, focusing entirely on the population’s significance to its own taxon. The scientific literature on biodiversity, discussed at length in this Article, suggests that species populations vary in importance to their own ecosystems. Indeed, certain species populations in certain ecosystems can be extremely valuable to the protection of biodiversity, even if not valuable to the survival of that species. In light of clear Congressional intent to preserve biodiversity, including expressly listing the conservation of ecosystems as a purpose of the Act, it is important that we also consider the value of a distinct population segment to the ecosystem in which it lives.