Defining Recovery Goals and Strategies for Endangered Species: The Wolf as a Case Study

摘要

We used a spatially explicit population model of wolves (Canis lupus) to propose a framework for defining rangewide recovery priorities and finer-scale strategies for regional reintroductions. The model predicts that Yellowstone and central Idaho, where wolves have recently been successfully reintroduced, hold the most secure core areas for wolves in the western United States, implying that future reintroductions will face greater challenges. However, these currently occupied sites, along with dispersal or reintroduction to several unoccupied but suitable core areas, could facilitate recovery of wolves to 49% of the area in the western United States that holds sufficient prey to support wolves. That percentage of the range with recovery potential could drop to 23% over the next few decades owing to landscape change, or increase to 66% owing to habitat restoration efforts such as the removal of some roads on public lands. Comprehensive habitat and viability assessments such as those presented here, by more rigorously defining the Endangered Species Act's concept of “significant portion of range,” can clarify debate over goals for recovery of large carnivores that may conflict with human land uses.nnAs human impacts on the biosphere increase, conservation biology must increasingly focus not only on preserving the current distribution of biodiversity but also on restoring species to areas from which they have been extirpated (figure 1). The success of restoration efforts depends in part on clarification of both the normative and the technical components of recovery goals (Breitenmoser et al. 2001). For example, the level of extinction risk tolerated or the extent of historic range to which recovery is desired are normative decisions guided by laws such as the US Endangered Species Act (ESA; 16 USC 1531–1540 [1988]). Once these normative aspects are resolved, conservation science can help identify which restoration strategy is most likely to ensure the desired level of recovery. Many of the species listed under the ESA are narrowly distributed endemics that can be protected by preserving a limited number of sites (Dobson et al. 1997). It is more difficult to define recovery goals for species such as the gray wolf (Canis lupus), which have large area requirements for viable populations, and whose protection may conflict with existing land uses such as livestock production. The scientific methodology used to define recovery goals and strategies for endangered species has not fully integrated recent technical advances in conservation biology, such as spatially explicit population models (SEPMs; Dunning et al. 1995). We present an example of such an analysis applied to the wolf, a high-profile endangered species whose proposed recovery goals (68 Federal Register 15804–15875) have recently been the subject of litigation (Defenders of Wildlife v. Norton, Civ. 03-1348-JO [2005]; National Wildlife Federation v. Norton, 03-CV-340 [2005]), to demonstrate how these methods can introduce key scientific knowledge into the debate over recovery goals and facilitate the decisionmaking process by illustrating the efficacy of alternate management scenarios.nnAlthough the ESA of 1973 was the third in a series of laws aimed at protecting imperiled species, it was the first to offer protection to any species in danger of extinction throughout all or a significant portion of its range. By including the phrase “significant portion of its range,” Congress signaled its intent that listed species should not simply be saved from extinction, but rather recovered so that populations inhabit relatively large areas (i.e., significant portions) of suitable habitat within historic ranges. Case law (Defenders of Wildlife v. Norton, 258 F.3d 1136 [2001], 239 F. Supp. 2d 9 [2002], Civ. 03-1348-JO [2005]; National Wildlife Federation v. Norton, 03-CV-340 [2005]) and previous delisting actions by the US Fish and Wildlife Service (USFWS) are consistent with this intent, as the 15 taxa that have been declared recovered since passage of the ESA were generally widely distributed at the time of delisting. This expectation was buttressed when Congress defined the term “species” to include “any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature” (ESA section 3[15]). The policy of recognizing distinct population segments (DPSs) allows for protective measures before the occurrence of large-scale declines that would necessitate listing a species or subspecies throughout its entire range (61 Federal Register 4722).nnIn the late 1950s, the number of gray wolves inhabiting the conterminous United States reached an all-time low, with fewer than 1000 wolves occupying less than 1% of the species' historic range in northeastern Minnesota and the adjacent Isle Royale National Park (Phillips et al. 2004). Three decades after passage of the ESA, owing to the expansion of populations in Minnesota and Canada and to reintroduction efforts in the northern Rocky Mountains (USFWS 1994) and the southwestern United States (USFWS 1996), about 4500 wolves occupy about 5% of the species' historic range in the conterminous United States (figure 2). In response to this improved conservation status, in April 2003 the USFWS published a reclassification rule that divided the lower 48 states into three DPSs (figure 2), retaining the experimental–nonessential population areas in the northern Rocky Mountains (USFWS 1994), but elsewhere downlisting the eastern and western gray wolf DPSs from endangered to threatened and indicating that recovery objectives for both had been met (68 Federal Register 15804–15875). However, in 2005, two federal court rulings vacated and enjoined the rule on the basis, in part, that it lacked comprehensive consideration of the phrase “significant portion of range” and misapplied the DPS policy (Defenders of Wildlife v. Norton, Civ. 03-1348-JO [2005]; National Wildlife Federation v. Norton, 03-CV-340 [2005]). When considered with the two earlier rulings cited above, this indicates that future recovery plans for wolves and other listed species should be guided by a rangewide determination of habitat suitability and relevant principles of conservation planning. The three principles of representation (establishing populations across the full array of potential habitats), resiliency (protecting populations large enough to remain viable), and redundancy (saving enough different populations that some can be lost without a loss of the species) are widely invoked guidelines for ensuring conservation of threatened species, even in the face of geographically widespread threats such as climate change (Shaffer and Stein 2000). By broadening recovery criteria to encompass representation, these principles recognize that a single population may not represent species recovery, even if it is large enough to be significantly resilient to extinction. For wide-ranging species such as the wolf, the importance of connectivity (protecting linkage areas, especially those that enhance viability by connecting larger with smaller populations) may justify its addition as a fourth principle for defining recovery goals (Soulé and Terborgh 1999).nnIn the 2003 proposed rule, the USFWS conflated the concepts of population viability and recovery. The claim that the ESA mandates only maintaining a species' viability (preventing extinction) rather than effecting recovery was first made in a 1986 revision to the regulations governing ESA enforcement (50 CFR 402), but has been repeatedly rejected by the courts (Suckling and Taylor 2005). This distinction is especially important for species such as the wolf or grizzly bear (Ursus arctos) that currently occupy a small portion of their historic range, because ESA mechanisms for maintaining viability restrict only “take” of individuals or occupied habitat, whereas ESA mechanisms for effecting recovery may restrict the destruction of unoccupied but suitable habitat and call for proactive measures to promote population reestablishment (Suckling and Taylor 2005). Although the bulk of the ESA's language addresses recovering individual species, Congress also included language that mandates the conservation of ecosystems on which listed species depend. Because of this, some researchers have proposed an additional guideline for recovery planning, the principle of ecological effectiveness (Soulé et al. 2005). An ecologically effective population contains enough individuals with a wide enough geographic distribution to reestablish the species' role in ecosystems. The argument for reestablishing ecologically effective populations is most persuasive in the case of the wolf and other “keystone” species that strongly influence ecosystem function through interspecific interactions such as predation (figure 3). For example, the return of wolves to Yellowstone has triggered a cascade of top-down effects on that ecosystem (Smith et al. 2003). Wolf predation has reduced the ability of elk to concentrate browsing on preferred species such as aspen (Populus tremuloides), leading to the recovery of riparian vegetation and associated species (Ripple and Beschta 2004). Because the wolf is a keystone species that was historically widespread throughout the western United States, yet whose recovery may conflict with current land-use practices such as livestock grazing on public lands, it provides an ideal case study of the role of conservation science in clarifying species recovery goals. We first present an example of a rangewide analysis for the wolf in the western contiguous United States, and then describe the use of an SEPM to help define recovery goals and strategies at a finer scale for the southwestern DPS (SWDPS) for the gray wolf (figure 2).