U.S. Department of Commerce


Date of this Version



Published in The Endangered Species Act at Thirty, Volume 2: Conserving Biodiversity in Human-Dominated Landscapes, edited by J. Michael Scott, Dale D. Goble, & Frank W. Davis (Washington: Island Press, 2006), pp. 127-149.


In reflecting on how the U.S. Endangered Species Act (ESA) has dealt with biodiversity issues over the past thirty years, it is instructive to review the collection of fifty-seven papers in the 1988 volume by Wilson entitled Biodiversity (1988b). Anchored in time midway between enactment of the ESA in 1973 and the present (2006 and counting), Wilson's volume provides a snapshot of the issues related to biodiversity that occupied conservation biologists during the first half of these three decades. The vast majority (over 80 percent) dealt with higher levels of biological organization (species or ecosystems), while only two (less than 4 percent) dealt exclusively with diversity at the population level. In sharp contrast, the last decade and a half has seen an explosive interest in conservation of intraspecific diversity (Rojas 1992; Nielsen 1995; Hughes et al. 1997; Ceballos and Ehrlich 2002). A harbinger of this interest appeared in Wilson's Biodiversity volume in the paper by Ehrlich (1988), who argued that the loss of populations within species was at least as important a problem as the loss of entire species. Much of the recent interest in intraspecific diversity has focused on the concept of evolutionarily significant units (ESUs; Ryder 1986), and a variety of approaches to defining ESUs have been proposed (Waples 1991; Dizon et al. 1992; Vogler and DeSalle 1994; Moritz 1994; Bowen 1998; Crandall et al. 2000).

This chapter considers how these ESA approaches might compare if each were applied to a common conservation problem-how to define conservation units of Pacific salmon (Oncorhynchus spp.) from northwestern North America. These species have already been the subject of a large-scale application of biological principles to a real-world problem in applied conservation biology -identifying units that can be considered "species" under the Endangered Species Act. This body of work, based on the ESU framework developed by Waples (1991, 1995), provides a context for evaluating how different the outcomes might be if any of the other most popular ESU approaches were applied to Pacific salmon (see Ford 2004 for a brief example of this type of analysis for one species). Results of this exercise provide insights that may be relevant to conservation efforts for a wide range of species, both within and outside the aegis of the Endangered Species Act. A general discussion of the strengths and weaknesses of the various ESU approaches can be found elsewhere (Fraser and Bernatchez 2001).