July 30, 2019 | By:
Mexican Wolf © Robin Silver

Biological and Sociopolitical Sources of Uncertainty in Population Viability Analysis for Endangered Species Recovery Planning

Featured image: Mexican Gray Wolf © Robin Silver

By Carlos Carroll, Robert C. Lacy, Richard J. Fredrickson, Daniel J. Rohlf, Sarah A. Hendricks, & Michael K. Phillips


Although population viability analysis (PVA) can be an important tool for strengthening endangered species recovery efforts, the extent to which such analyses remain embedded in the social process of recovery planning is often unrecognized. We analyzed two recovery plans for the Mexican wolf that were developed using similar data and methods but arrived at contrasting conclusions as to appropriate recovery goals or criteria. We found that approximately half of the contrast arose from uncertainty regarding biological data, with the remainder divided between policy-related decisions and mixed biological-policy factors. Contrasts arose from both differences in input parameter values and how parameter uncertainty informed the level of precaution embodied in resulting criteria. Policy-related uncertainty originated from contrasts in thresholds for acceptable risk and disagreement as to how to define endangered species recovery. Rather than turning to PVA to produce politically acceptable definitions of recovery that appear science-based, agencies should clarify the nexus between science and policy elements in their decision processes. The limitations we identify in endangered-species policy and how PVAs are conducted as part of recovery planning must be addressed if PVAs are to fulfill their potential to increase the odds of successful conservation outcomes.


Recovery plans act as blueprints guiding actions that determine whether endangered species survive or perish. Recovery criteria in turn are the goalposts established within each recovery plan which, if met, indicate that a species no longer requires the protection afforded by listing as endangered or threatened1. The US Endangered Species Act (ESA; 16 U.S.C. §1531–1540) requires that agencies base their decisions to add or remove taxa from the list of endangered species (and by implication, the recovery criteria on which these decisions depend) solely on biological data rather than economic costs2. The “solely” biologically-based requirement was added to the ESA a decade after its passage due in part to concerns that considering non-biological (e.g., socioeconomic) factors might prompt agencies to declare species recovered at a stage when population size and other factors were still biologically insufficient to ensure their persistence3. Lawmakers sought to ensure transparent and evidence-based recovery planning, with a clear distinction between decisions based on biological data and those based on policy preferences, in order to increase the likelihood of successful conservation outcomes and ensure government accountability to the public2. However, this mandate for science?based criteria is often challenged during real?world recovery planning processes when potential conservation actions negatively affect influential stakeholders.

Recovery of the gray wolf (Canis lupus) and other large mammalian carnivores remains controversial in many nations due to potential impacts on livestock production and the wild ungulate populations hunted by humans4. The Mexican wolf (C. l. baileyi), which historically occurred in northern Mexico and the southwestern US, was extirpated from the wild by the 1980s due to such conflicts5. Descendants of the seven founders of the captive population were reintroduced onto public lands in the southwestern US beginning in 19986. Whereas earlier gray wolf and Mexican wolf recovery plans used expert judgement to develop recovery criteria, the US Fish and Wildlife Service (henceforth “Service”) has based recent Mexican wolf recovery planning on population viability analysis (PVA), a quantitative tool for systematically eliciting and synthesizing information on factors affecting the demographic and genetic status of threatened species, and determining the influence of these factors on population viability and endangerment7.

In 2013, a team of scientists convened by the Service employed PVA to develop draft Mexican wolf recovery criteria8. These criteria, which were ultimately shelved after generating opposition from prominent politicians in southwestern US states9, proposed that a metapopulation totaling 750 wolves within the US would be necessary for recovery of the subspecies8,10. In 2017, a new set of recovery criteria were developed via a PVA conducted with greater involvement by state representatives. (Several of this study’s authors participated in one or both of the recovery teams). When compared to the 2013 effort, these new criteria called for less than half as many wolves (320) inhabiting a smaller portion of the southwestern states6. Although the 2013 and 2017 plans propose establishment of additional smaller populations in Mexico, both assume little or no connectivity between US and Mexican populations. Additional criteria addressing genetic inbreeding and other threats also differed substantially between the two plans (SI Table S1).

Because they were developed using similar data and methods but arrived at contrasting conclusions, the 2013 and 2017 recovery criteria provide a unique opportunity to examine how tensions between socioeconomic concerns and the implications of biological data can result in challenges to evidence?based recovery planning. If rigorous PVA processes for a well?studied species can produce strongly contrasting criteria, PVA may be limited in its ability to inform this aspect of recovery planning. Alternatively, if contrasting recovery criteria resulted from participants inappropriately distorting conclusions as to how many wolves were necessary for biological recovery, limitations in existing policy and planning processes may need to be addressed. We recognize that recovery planning science is embedded within a sociopolitical process, and that lawmakers have set a high bar by requiring that certain elements of this process be solely science-based. As we detail below, recovery criteria are inevitably informed by values-based decisions, including what level of extinction risk is “acceptable”. Our goal is to establish best practices for conducting PVAs that clearly distinguish science and policy elements and transparently represent available information that can be used to identify the criteria for success in species recovery.

To identify the origins of the contrasts between the two PVAs, we conducted a sensitivity analysis to categorize what proportion of the contrasts in PVA results originated from uncertainty in demographic parameters as opposed to differing interpretations of policies regarding ESA implementation. Based on our results, we propose methods to address identified sources of uncertainty (both biological and sociopolitical) and strengthen the utility of PVA in recovery planning. The lessons from this comparison resonate beyond the US context, for example in the controversy over the appropriate size of the Swedish wolf population11,12, because they highlight key aspects of the conceptual framework underpinning endangered species recovery that remain contested and present pitfalls to successful recovery planning in many nations.

Click here to continue reading the Results and Discussion, Conclusion, Methods, Data Availability, and References, and to see the endnotes.

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