Population Viability Study

Bison Population Viability Natural Resource Report
Bison Population Viability Natural Resource Report

Long-term Viability of Department of the Interior Bison Under Current Management and Potential Metapopulation Management Strategies

Executive Summary

The North American plains bison (Bison bison bison) once numbered in the tens of millions, with a range that extended from northern Mexico to central Canada. Yet by the end of the 1800s, a combination of commercial hunting, novel diseases, and habitat destruction had driven plains bison to the brink of extinction. The establishment of a small number of protected, federally managed herds in the early 1900s saved the subspecies from extinction in the wild. As a result of those efforts, the Department of the Interior (DOI) is now the primary national conservation steward of North American plains bison supporting approximately 11,000 plains bison in 19 herds on 4.6 million acres of National Park Service (NPS), US Fish and Wildlife Service (FWS), and Bureau of Land Management (BLM) lands in 12 states.

While plains bison are no longer threatened by demographic extinction, most DOI bison continue to exist in relatively small, isolated, range-restricted herds, confined by fences and further bound by socio-political concerns that limit their ecological recovery. Small, isolated populations are vulnerable to extirpation due to random catastrophic events such as disease outbreaks or extreme weather events. Small, isolated populations also lose genetic diversity more quickly than large populations through the process of genetic drift, which in turn can decrease the viability of populations through an accumulation of inbreeding and loss of adaptive capacity.

To mitigate the loss of genetic diversity in these isolated populations, previous researchers have suggested restoring effective gene flow among herds and managing DOI bison herds as a metapopulation. Gene flow can be restored either through the restoration of natural movements between populations or through the translocation of animals (or gametes) among populations.
In this project, the NPS partnered with the Wildlife Conservation Society (WCS) and the FWS to evaluate the ability of metapopulation management strategies to ensure the long-term population viability of DOI bison. This study had three major components:

  • In collaboration with other stakeholders including the BLM, state wildlife managers, non-government organizations, and Canadian bison managers, we collected standardized, up-to-date genetic, demographic, and management data on 16 DOI and two Parks Canada herds (collectively simplified and referred to as DOI bison herein), to assess current existing genetic variation within and between herds (Chapter 2);
  • We used these genetic, demographic, and management data to develop and parameterize individual-based, genetically explicit simulation models to project the long-term viability of each bison herd under current management practices (Chapter 3);
  • We used this model to evaluate the effects of alternate metapopulation management strategies with varying levels of genetic exchange (translocations) between herds to ensure the long-term viability of DOI bison (Chapter 4).

To evaluate and compare the performance of the alternate bison management strategies modeled in this study, project partners established the following quantitative criteria for successful management:

  • All DOI bison herds must have a 99% probability of surviving and maintaining currently established abundance objectives for 200 years;
  • The existing genetic diversity within each individual bison herd must be maintained or improved;
  • Existing genetic diversity within the DOI metapopulation as a whole must be maintained; and
  • Genetic redundancy should be retained within the DOI metapopulation, such that the loss of any one DOI bison herd does not substantially reduce the genetic diversity of DOI herds as a whole.

Results from our genetic analyses (Chapter 2) identify considerable variation in the level of genetic diversity within, and significant genetic differentiation between, existing bison herds. Current levels of genetic diversity within herds generally correspond with known herd foundation and augmentation histories, combined with the expected effects of genetic drift. Results indicate that three bison herds currently have observed heterozygosity levels (Ho) close to 0.50, a value identified with an increased risk of inbreeding depression and as a threshold for triggering genetic augmentation.

Results of simulation models for individual bison herds (Chapter 3) project that all herds will lose genetic diversity over the next 200 years under current management conditions without additional gene flow. Herd size was the most important driver of genetic diversity loss, though the effect of herd size could be modulated by the effect of the removal strategies used to manage herd abundances. Overall, larger herds (>500 animals) lost modest amounts of genetic diversity (3-7% decrease) over time, while small herds (<100 animals) lost considerable diversity (34-81% decrease) over time, with correspondingly large increases in mean inbreeding levels. After 200 years under current management conditions eight herds were projected to have heterozygosity levels < 0.50, with mean inbreeding coefficient levels similar to those shown to impact the reproduction and survival of bison reported by other studies. Increasing the size of herds can reduce rates of genetic diversity loss due to genetic drift, as can removal strategies that maintain even sex ratios, target younger age classes instead of adults during removals, or target genetically overrepresented animals (e.g., bison most closely related to the rest of the herd).

Results of simulation models for individual bison herds indicate that 15 of the 18 herds in this study have a >99% probability of persisting for the next 200 years without additional gene flow. Three of the smallest herds (<100 individuals) are projected to be vulnerable to extinction (>10% probability of extinction) if maintained at current abundance levels without additional gene flow.
We identified logistical, biological, or political issues that could limit particular herds from acting either as a source or as a recipient for translocations, then modeled 25 metapopulation management scenarios in which we altered the criteria used to select source herds for translocations, the frequency of translocations, and the number of animals moved per translocation. We modeled four general scenarios for the selection of source herds for translocations: 1. Source herd must be the genetically least-related herd to the recipient herd; 2. Source herd must be genetically similar to the recipient herd; 3. Source herd must be systematically rotated at every translocation (in order from least-related to most-related to the recipient herd); 4. Source herd must be the geographically closest herd to the recipient herd. For each source herd scenario, we varied the number of bison transferred from two to eight bison, and frequency of transfers from five, eight, or 10-year intervals.

Results of our metapopulation management models (Chapter 4) indicate that translocation management strategies vary considerably in their efficacy. In particular, the criteria used to select potential source herds for translocations, and interactions between criteria used to select source herds and the size and initial levels of genetic diversity of recipient herds, strongly affected the efficacy of translocations to increase or maintain genetic diversity within herds. Using genetically least-related herds as sources for translocations resulted in the largest gains in genetic diversity for almost all herds. However, the less information-intensive strategy of systematically rotating source herds every translocation (in order of least- to most-related) led to almost identically large and consistent increases in the genetic diversity of recipient herds.

If a uniform translocation management strategy were to be adopted for all herds in the bison metapopulation, then a strategy of smaller, less frequent translocations (e.g., 2 bison every 10 years, 3 every 7 years) using either least-related herds as source populations or systematically rotating source herds at every translocation would be adequate for increasing the genetic diversity of most herds while also minimizing the loss of diversity at the metapopulation level. However, our results suggest that individual herds differ in their management needs, and that exploring a more tailored, herd-specific translocation strategy may be most beneficial. In particular, smaller herds benefit from more frequent translocations, larger herds require fewer and less frequent translocations, and herds with low initial levels of diversity are likely to benefit from any translocation.

In summary, this study confirms that management of DOI bison herds in isolation promotes the loss of genetic diversity within all herds. More importantly, this study demonstrates that increased herd size and targeted removal strategies can reduce rates of diversity loss, and that adopting a Departmental metapopulation strategy through facilitated periodic movement of modest numbers of bison among DOI herds (i.e., restoring effective gene flow) can substantially reduce the negative impacts of geographic isolation. Analyses of an array of scenarios for practical bison translocations indicates that the selection of appropriate source herds and numbers of animals to translocate must be considered carefully to most effectively conserve genetic diversity and ensure the long-term population persistence of bison. Long-term monitoring of genetic diversity, both at the individual herd level and across the metapopulation, will be essential to refine the implementation of an appropriate metapopulation management approach to maximize benefit to the species.

Based on these results, we recommend that the DOI Bison Working Group, as chartered under the DOI Bison Conservation Initiative (2008), initiate and oversee a technical task force to develop a comprehensive bison metapopulation management strategy for use by DOI agencies. This comprehensive management strategy must include explicit consideration of genetics, wildlife health, cattle introgression, data management, local unit management issues, partner/stakeholder engagement (Chapter 5). As we complete this project, the DOI Bison Working Group is concurrently working to finalize a bison health report that will describe considerations and issues for management of healthy DOI bison populations, and recommendations for coordination among DOI management units to better support bison health. This forthcoming DOI bison health report, in conjunction with the results of this project, will be key to informing a bison metapopulation strategy. Addressing the complexity of managing bison for conservation purposes will call for new and sustained levels of coordination and communication between DOI agencies and other bison conservation stakeholders, and traditional management models at the individual herd or even bureau level will need to be adapted to encompass broad species conservation goals that support continental conservation across multiple jurisdictions while respecting the variations in local management purpose and capacity.

The next several years offer unprecedented opportunity to capitalize on active engagement and partnerships to make meaningful, impactful, and durable gains in the conservation of bison in North America. With an articulated vision, sound scientific foundations, and committed internal and external partnerships, DOI bureaus are now well equipped to implement a new approach to bison conservation: a cooperative, multi-scaled stewardship model to preserve and protect our national bison heritage and to promote ecological and cultural restoration of bison to North America.

Part of a series of articles titled Wildlife in the Badlands.

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Last updated: November 6, 2023