FACTORS AFFECTING SPECIES' RISK OF EXTINCTION: AN EMPIRICAL ANALYSIS OF ESA AND NATURESERVE LISTINGS

I. INTRODUCTION There are two principal sources of data on ecologically imperiled species in the United States: (1) listings of threatened and endangered species under the Endangered Species Act (ESA, 1973), and (2) listings of species considered by NatureServe to be at risk of extinction. The language of the ESA admits no influence on the determination of listings other than scientific necessity: The Secretary shall make determinations required by subsection (a)(1) of this section solely on the basis of the best scientific and commercial data available to him after conducting a review of the status of the species and after taking into account those efforts, if any, being made by a State or foreign nation, to protect such species, whether by predator control, protection of habitat and food supply, or other conservation practices, within any area under its jurisdiction; or on the high seas. (U.S. Code, Title 16, Chapter 35, Section 1533(b)(1)(A), emphasis added) Listings of species at-risk of extinction by NatureServe (2002) are based on: ... a consistent and rigorous methodology for assessing extinction risk that is based on evaluation of multiple factors. Evaluation criteria include: the number and condition of populations and individuals; the area or range occupied by the species; population trends (that is, whether numbers are increasing, stable or declining); and known threats. Biologists assess each species against these multiple risk factors based on the best available scientific information and assign the appropriate conservation status rank. In theory, then, listings under the ESA and NatureServe should be consistent. In fact, there is substantial discrepancy between the two indices, with the NatureServe listings containing many more species considered ecologically imperiled than have obtained recognition and protection under the ESA. In part, this discrepancy may reflect financial constraints imposed on the U.S. Fish and Wildlife Service, which is charged with implementing the ESA. In addition, it has been argued that implementation of the ESA has been influenced by economic considerations, as translated through the political process. For example, Bean (1991) and Mehmood and Zhang (2001) have argued that economic factors played an important role in determining congressional votes on amendments to the ESA. Moreover, there is evidence that how fast species proposed for listing actually get listed (Bean, 1991; Ando, 1999, 2001, 2003), the types of species that get listed (Metrick and Weitzman, 1996; Weitzman and Metrick, 1998; Ando, 2003), and the geographic distribution of listings (Rawls and Laband, 2004) are subject to the influence of economic interests through the political process. Complicating matters is the fact that implementation of the ESA is also subject to judicial intervention (Associated Press, 2004). These criticisms of how the ESA has been implemented may create doubts about whether ESA listings are based significantly on scientific criteria. In this article, the authors provide empirical evidence in support of scientific foundations for ESA listings. They do so by analyzing factors that influence species' ecological imperilment across states, using two different measures of species imperilment: the fraction of all species in a state identified by NatureServe as being "at-risk" of extinction, and the fraction of species in a state listed under the U.S. Fish & Wildlife Service's Endangered Species Act (ESA). At issue is whether the factors that influence listings compiled by NatureServe also influence ESA listings. One of the authors' principal findings is that both measures of species' ecological imperilment are influenced strongly by the fraction of species found only in each state. These endemic species exist in relatively small, ecologically distinct niches and are characterized by small populations that are consistent with being ecologically vulnerable. …