Free-Ranging Horses and Burros in the Western United States
Since 1971, the Bureau of Land Management (BLM) of the U.S. Department of the Interior has been responsible for managing the majority of free-ranging horses and burros on arid federal public lands in the western United States. In the Wild Free-Roaming Horses and Burros Act of 1971 (P.L. 92-195), the U.S. Congress charged BLM1 with the “protection, management, and control of wild free-roaming horses and burros on public lands.” BLM was charged to protect the equids because, the legislation noted, “wild free-roaming horses and burros are living symbols of the historic and pioneer spirit of the West … and [they] are fast disappearing from the American scene.” In the mid-20th century, horse and burro populations were affected by competing uses for the land, including livestock grazing, and by roundups, from which the animals were often sold for slaughter (GAO, 1990). The protection provided in the 1971 legislation built on the “Wild Horse Annie Act” (P.L. 86-234), passed in 1959, which prohibited the use of motorized vehicles, including aircraft, to hunt free-ranging horses and outlawed the poisoning of watering holes on public lands.
The agency was also tasked with managing and controlling the population because of the multiple uses of public lands. Public lands provide habitat to horses and burros, but they are also used for recreation, mining, forestry, grazing for livestock, and habitat for wildlife, including mule deer, pronghorn, and bighorn sheep. Therefore, although the act stipulated that free-ranging horses and burros were “an integral part of the natural system of the public lands” and were to be managed “as components of the public lands,” it limited their range by definition to “their known territorial limits” in 1971. Such public lands were to be “devoted principally but not exclusively to [horse and burro] welfare in keeping with the multiple-use management concept of public lands.” In addition, horses and burros were to be managed at “the minimal feasible level.” Management should “achieve and maintain
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1 The Wild Free-Roaming Horses and Burros Act of 1971 also pertains to free-ranging horses and burros found on public lands administered by the U.S. Forest Service. This report focuses on animals managed by BLM, which is responsible for over 90 percent of the equid population on public lands in the western United States (GAO, 2008).
a thriving natural ecological balance on the public lands,” protect wildlife habitat, and prevent range deterioration.
The goal of protecting free-ranging horses and burros while managing and controlling them to achieve a vaguely defined thriving natural ecological balance within the multiple-use mandate for public lands has challenged BLM’s Wild Horse and Burro Program since its inception. Amendments to the Wild Free-Roaming Horses and Burros Act have not diminished the difficulty. BLM is to monitor the population size to determine where there is an excess of horses and burros; such a situation is to be identified when “a thriving natural ecological balance and multiple-use relationship” is threatened (P.L. 92-195 as amended by the Public Rangelands Improvement Act of 1978, P.L. 95-514). It is BLM’s responsibility to determine when that relationship is under threat and to remove animals to achieve balance. The legislation allows the destruction of old, sick, or lame animals. Excess animals removed from the range may be adopted. Those for which there is no adoption demand are to be “destroyed in the most humane and cost efficient manner possible”; however, the destruction of healthy, unadopted free-ranging horses and burros has been restricted either by a moratorium instituted by the director of BLM or by the annual congressional appropriations bill for the Department of the Interior in most years. Free-ranging horses and burros have successfully sustained populations in North America for over 300 years, and no large predator widely overlaps with their territory. Since 1989, adoptions have seldom exceeded the number of animals removed from the range; in the 2000s, the discrepancy neared a 2:1 ratio of animals removed to animals adopted (GAO, 2008). Thus, BLM’s effort to control horse and burro numbers by removing animals from the range has led to the stockpiling of “excess” horses and burros in holding facilities (Figures 1-1 and 1-2). In fiscal year 2012, more than 45,000 animals were in holding facilities, and their maintenance consumed almost 60 percent of the Wild Horse and Burro Program’s budget (BLM, 2012a).
With holding costs in 2010 projected to nearly double those in 2004 (Bolstad, 2011), the U.S. Senate Committee on Appropriations in 2009 instructed BLM to “prepare and publish a new comprehensive long-term plan and policy for management of wild horses and burros” (U.S. Congress, Senate, 2009). BLM responded with a proposed strategy designed around seven topics. With respect to science and research, one method for improving the use of science in its management of horses and burros was to “commission the [National Academy of Sciences] to review earlier reports and make recommendations on how the BLM should proceed in light of the latest scientific research” (BLM, 2011a).
The committee formed by the National Research Council of the National Academy of Sciences in response to BLM’s request was given a long statement of task that required a variety of expertise (Box 1-1). The charge called on the Committee to Review the Bureau of Land Management Wild Horse and Burro Management Program to investigate the annual rates of growth in the animal populations, the implications of genetic diversity for their long-term health, and how they interact with the environment. It also asked the committee to assess the effects of management actions, such as treating animals with contraceptives or removing animals from the range, and to evaluate BLM’s tools for measuring the effects. Agency methods for determining the number of animals living on the range and the number of animals appropriate for the range were also to be examined. Finally, the committee was tasked to identify options that could address stakeholder concerns making use of the best available science.
FIGURE 1-1 Horse population reported by the Bureau of Land Management (BLM), horses removed from the range, and horses in holding facilities, 1996-2012 (for years available).
DATA SOURCE: Horse population data from BLM (1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2005a,b, 2006a, 2007a, 2008a, 2009a, 2010, 2011b, 2012b); horse removal data provided by BLM; holding-facilities data from BLM (2004, 2006b, 2007b, 2008b, 2009b, 2011c, 2012c).
FIGURE 1-2 Burro population reported by the Bureau of Land Management (BLM), burros removed from the range, and burros in short-term holding facilities, 1996-2012 (for years available). NOTE: There are no long-term holding facilities for burros.
DATA SOURCE: Burro population data from BLM (1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2005a,b, 2006a, 2007a, 2008a, 2009a, 2010, 2011b, 2012b); burro removal data provided by BLM; holding-facilities data from BLM (2004, 2006b, 2007b, 2008b, 2009b, 2011c, 2012c).
BOX 1-1
Statement of Task
At the request of the Bureau of Land Management (BLM), the National Research Council (NRC) will conduct an independent, technical evaluation of the science, methodology, and technical decision-making approaches of the Wild Horse and Burro Management Program. In evaluating the program, the study will build on findings of three prior reports prepared by the NRC in 1980, 1982, and 1991 and summarize additional, relevant research completed since the three earlier reports were prepared. Relying on information about the program provided by BLM and on field data collected by BLM and others, the analysis will address the following key scientific challenges and questions:
- Estimates of the wild horse and burro populations: Given available information and methods, how accurately can wild horse and burro populations on BLM land designated for wild horse and burro use be estimated? What are the most accurate methods to estimate wild horse and burro herd numbers and what is the margin of error in those methods? Are there better techniques than BLM currently uses to estimate population numbers? For example, could genetics or remote sensing using unmanned aircraft be used to estimate wild horse and burro population size and distribution?
- Population modeling: Evaluate the strengths and limitations of models for predicting impacts on wild horse populations given various stochastic factors and management alternatives. What types of decisions are most appropriately supported using the WinEquus model? Are there additional models BLM should consider for future uses?
- Genetic diversity in wild horse and burro herds: What does information available on wild horse and burro herds’ genetic diversity indicate about long-term herd health, from a biological and genetic perspective? Is there an optimal level of genetic diversity within a herd to manage for? What management actions can be undertaken to achieve an optimal level of genetic diversity if it is too low?
- Annual rates of wild horse and burro population growth: Evaluate estimates of the annual rates of increase in wild horse and burro herds, including factors affecting the accuracy of and uncertainty related to the estimates. Is there compensatory reproduction as a result of population-size control (e.g., fertility control or removal from herd management areas)? Would wild horse and burro populations self-limit if they were not controlled, and if so, what indicators (rangeland condition, animal condition, health, etc.) would be present at the point of self-limitation?
To accomplish the committee’s comprehensive charge, members were appointed on the basis of their scientific research and experience with the questions involved in the statement of task. Experts were selected from the fields of behavioral ecology, conservation biology, genetics, natural-resources management and range ecology, population ecology, reproductive physiology, sociology, veterinary medicine, and wildlife ecology. (The committee members’ biographies are in Appendix A.) The committee also retained a consultant who had expertise in equine reproduction.
The committee’s study was the first examination of BLM’s Wild Horse and Burro Program by the National Research Council in over 20 years. The National Research Council had published three reports on free-ranging horses and burros under BLM’s jurisdiction. The first two reports, Wild and Free-Roaming Horses and Burros: Current Knowledge and Recommended Research, Phase I Final Report (1980) and Wild and Free-Roaming Horses and Burros: Final Report (1982), completed the first and third phases of a three-phase study mandated
- Predator impact on wild horse and burro population growth: Evaluate information relative to the abundance of predators and their impact on wild horse and burro populations. Although predator management is the responsibility of the U.S. Fish and Wildlife Service or State wildlife agencies and given the constraints in existing federal law, is there evidence that predators alone could effectively control wild horse and burro population size on BLM land designated for wild horse and burro use?
- Population control: What scientific factors should be considered when making population control decisions (roundups, fertility control, sterilization of either males or females, sex ratio adjustments to favor males, and other population control measures) relative to the effectiveness of control approach, herd health, genetic diversity, social behavior, and animal well-being?
- Fertility control of wild horses: Evaluate information related to the effectiveness of fertility control methods to prevent pregnancies and reduce herd populations.
- Managing a portion of a population as nonreproducing: What scientific and technical factors should BLM consider when managing for wild horse and burro herds with a reproducing and nonreproducing population of animals (i.e., a portion of the population is a breeding population and the remainder is nonreproducing males or females)? When managing a herd with reproducing and nonreproducing animals, which options should be considered: geldings, vasectomized males, ovariectomized mares, or other interventions? Is there credible evidence to indicate that geldings or vasectomized stallions in a herd would be effective in decreasing annual population growth rates, or are there other methods BLM should consider for managing stallions in a herd that would be effective in tangibly suppressing population growth?
- Appropriate Management Level (AML) establishment or adjustment: Evaluate BLM’s approach to establishing or adjusting AML as described in the 4700-1 Wild Horses and Burros Management Handbook. Based upon scientific and technical considerations, are there other approaches to establishing or adjusting AML BLM should consider? How might BLM improve its ability to validate AML?
- Societal considerations: What are some options available to BLM to address the widely divergent and conflicting perspectives about wild horse and burro management and to consider stakeholder concerns while using the best available science to protect land and animal health?
- Additional research needs: Identify research needs and opportunities related to the topics listed above. What research should be the highest priority for BLM to fill information and data gaps, reduce uncertainty, and improve decision-making and management?
by Congress in the Public Rangelands Improvement Act of 1978 (P.L. 95-514).2 Those reports were the product of one study committee, the Committee on Wild and Free-Roaming Horses and Burros, which was convened from 1979 to 1982. The third report, Wild Horse Populations: Field Studies in Genetics and Fertility (1991), was undertaken by a separate committee, the Committee on Wild Horse and Burro Research, in accordance with congressional appropriations in fiscal year 1985 to fund another study. The Committee to Review the Bureau of Land Management Wild Horse and Burro Management Program was asked to build on the findings in those three reports. Appendix B contains a summary of findings of the earlier studies that overlap with the statement of task for the Committee to Review the Bureau of Land Management Wild Horse and Burro Management Program.
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2 The second phase of the study consisted of research projects recommended by the committee in its first report.
Six information-gathering meetings took place during the study process (Appendix C). In addition to a presentation from BLM, the committee heard from experts in fertility control, predation, behavioral ecology, and genetics of free-ranging horses and burros. It also received presentations of research on free-ranging horses and burros by the U.S. Geological Survey and the U.S. Fish and Wildlife Service, on the use of adaptive management to address natural-resources issues, on tools for communicating science effectively, and on methods for engaging the public in assessment and decision-making on scientific issues. The committee heard from many interested parties at four public-comment sessions and received numerous written submissions on research and stakeholder concerns related to free-ranging horses and burros and to BLM’s management of the animals (Box 1-2).
The committee based its findings and conclusions on a number of sources. In addition to the information gathered at its meetings, committee members examined peer-reviewed scientific literature on free-ranging horses and burros, particularly literature published since the previous National Research Council reports were completed. The committee analyzed data on free-ranging horse and burro populations and genetics that it received from BLM and from E. Gus Cothran of Texas A&M University, respectively, in response to submitted inquiries (Appendix D). It also synthesized responses from BLM, Stephen Jenkins, and Charles de Seve regarding population modeling and from BLM on establishing herd population levels. When it was relevant, the committee also consulted gray and unpublished literature to inform its analysis.
BOX 1-2
Divergent Opinions on Appropriate Management of Free-Ranging Horses and Burros
The management of free-ranging horses and burros on public lands is a long-standing source of contention among stakeholder groups. During the course of its review, the committee heard from BLM and from many interested parties about the struggle of managing horses and burros in accordance with a thriving natural ecological balance and the multiple-use mandate. The intent of the Wild Free-Roaming Horses and Burros Act was interpreted differently by various stakeholders, and many critiques of BLM’s implementation of the law were offered.
In a presentation to the committee, BLM outlined its mandate under the current law. Among the law’s stipulations are that animals are to be managed on land on which they were found in 1971, the land is to be managed for multiple uses, and excess animals are to be removed immediately if appropriate management levels are exceeded.
Some parties who participated in public-comment sessions expressed concern that rangeland health was adversely affected because the population of horses and burros often exceeded appropriate management levels. This perspective considered competition between equids and wildlife to be detrimental to wildlife. It was also pointed out that livestock, which have grazing rights on public lands, do not remain on the land all year, unlike horses.
Other participants in the public sessions of committee meetings communicated that horses and burros were unfairly limited in their range and in their numbers. From that point of view, appropriate management levels were too low to maintain genetically healthy herds, and horses and burros were restricted to too few acres of public land. For example, the number of acres on which livestock are allowed is much greater than that of the Herd Management Areas (the land allocated to horses and burros). Many participants asserted that the horse is a reintroduced wildlife species and fills a niche in its ecosystem. Concern was also expressed about the stress placed on animals during gathers (roundups) and in holding. There were many requests for BLM to provide more robust and transparent evidence to support its management decisions.
Most commenters agreed that the operation of the program was excessively expensive and that management could be improved to reduce costs and increase the welfare of all animals on the range.
BOX 1-3
Describing Horses and Burros Under Different Management Regimes
In the literature that the committee reviewed, there were many nuances regarding the management regimes of horse and burro populations and other animals. To clarify the differences, the committee defines the terms that are used in the report here.
Free-Ranging. Although the 1971 legislation calls horses and burros in the western United States free-roaming, the committee chose to use the term free-ranging to reflect the purposeful and spatially adaptive uses of the rangelands that the horses and burros inhabit. Such populations are allowed to use spatially extensive habitats in ways that increase access to forage, improve their physiological condition, and increase the probability of their own and their population’s viability. (In many of the contraceptive studies reviewed by the committee, treatments were applied to free-ranging horses that had been gathered from the range and held captive for study.)
Semi–Free-Ranging. The committee uses this term to refer to populations of horses and burros that are confined to limited areas, for example, in fenced reserves or protected areas that are nevertheless expansive enough for the animals to move freely over larger areas than typical farms or ranches.
Domestic. For the purposes of the report, domestic describes an animal that is kept by humans, typically as a companion animal or as livestock. This is different from definitions based on presumed inherited effects of domestication in ancestral bloodlines. The report terminology distinguishes between domestic donkeys and free-ranging burros.
The committee did not limit itself to research and data on free-ranging horses and burros in the western United States. It also consulted studies on free-ranging horses and burros on the barrier islands off the East Coast of the United States, particularly the herds on Assateague Island and Shackleford Banks.3 Those populations are not under BLM management and are not subject to the Wild Free-Roaming Horses and Burros Act of 1971, but results of research on the herds, which in many cases have been studied much more often and thoroughly than BLM herds, were relevant to the conclusions drawn by the committee. Germane studies of the biology, physiology, and behavioral ecology of domestic horses and burros, Przewalski’s horses (wild horses native to central Asia), free-ranging horse and burro populations in other countries, native equid species on other continents, and free-ranging ungulates in the United States and elsewhere were also assessed (Box 1-3).
The committee’s statement of task was extensive but did not encompass all issues and challenges pertaining to the Wild Horse and Burro Program. The committee’s tasks pertained to management issues related to horses and burros on the range. It was not asked to examine BLM procedures and actions related to gathers—the roundups that BLM conducts to administer such management actions as adjusting sex ratios on the range, treating animals with contraceptives, and removing animals from the range. The committee’s tasks did not include investigation of the effects of gathers on the welfare of gathered horses and
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3 Several free-ranging horse and burro herds are resident on barrier islands off the East Coast of the United States and Canada. The herds on Assateague Island (Maryland) and Shackleford Banks (North Carolina) are managed by the U.S. Department of the Interior’s National Park Service (NPS). There are free-ranging equid populations in the United States that are not under the jurisdiction of BLM. Some are managed by other federal agencies, such as NPS and the U.S. Fish and Wildlife Service; Indian reservations, state agencies, and local entities are also responsible for some herds.
burros. The welfare of animals in holding facilities or of animals that leave the program through adoption or sale was also not part of the study’s charge. A critique of the legal framework under which the horses and burros are managed (including the number of acres on which BLM manages the animals), an examination of BLM’s legal authority to use euthanasia, and specific recommendations for program budget allocations were similarly not within the scope of the study.
The committee was not tasked with examining issues within BLM that may affect how the Wild Horse and Burro Program functions. One example is related to livestock grazing. The agency’s multiple-use mandate includes administering grazing allotments on public lands to private owners of livestock. Whether livestock or equids do or should receive preferential treatment by BLM when rangeland is allocated or when the number of animals on the range is adjusted to keep rangelands healthy was not within the study’s scope. Another example is BLM’s internal organizational structure. The committee was not asked to examine how the organizational hierarchy of national, state, and field offices and the responsibilities of and working relationships between these levels pertain to BLM’s effectiveness in managing horses and burros.
In addition, as became evident from public comments at information-gathering sessions and submitted written comments, the statement of task did not include questions that are of concern to many stakeholders. The study did not investigate such topics as the relevance of the evolutionary origin of the horse species in North America and the logistical and economic feasibility of establishing ecosanctuaries for horses and burros. The study did not examine the procedures that BLM uses to gather horses and burros, so it did not explore whether alternative methods of gathering equids could be used. Furthermore, the report does not comment on whether the number of free-ranging horses and burros deemed appropriate by BLM or the area of range available to equids should be increased or decreased.
As a committee established under the auspices of the National Research Council, the Committee to Review the Bureau of Land Management Wild Horse and Burro Management Program was constituted to answer science-based questions. Although the answers to science-based questions inform policy decisions, it is the role of decision-makers to weigh the values associated with the possible outcomes of management actions. National Research Council committees are also not constituted to be bodies of legal review or critique.
Therefore, many of the questions alluded to above were not within the prerogative of the committee. Horses and burros removed from the range by culling or by gathering and moving them to long-term holding facilities are not managed on the range and thus were not within the committee’s statement of task. The report’s findings on the effects of population control on herd health, genetic diversity, and social behavior (Chapters 4 and 5) would apply to horses and burros remaining on the range if a herd were culled; in contrast, policy decisions to cull on or near the range or to remove animals to long-term holding facilities permanently to control animal populations are value judgments. Similarly, the answers to questions related to the numbers of animals of any species on the range are determined by the public’s values, both economic and emotional, concerning not only equids but livestock, wildlife, rangeland conditions, and other natural resources. Science can inform what effects different combinations of species and population levels may have on the range, but science cannot say what decisions should be made.
Acts of Congress are policy decisions. The committee recognized that a complicated legal framework affects how free-ranging horses and burros are managed and that the complexity of the framework may create an impediment to effective management. However, it is the role of members of Congress, as representatives of their constituents, to promulgate
or amend laws. In the report, the committee commented only by way of description on the legal framework under which horses and burros are managed.
Because of the existing legal framework that protected horses and burros at the time of the study, the committee did not investigate whether the horse should be considered a reintroduced species because of its evolution in North America. Previous National Research Council reports (NRC, 1980, 1982) examined the question and reported that the dearth of information regarding changes in the horse, Equus caballus, since domestication, which occurred after the species crossed the land bridge into Eurasia, and changes in the environment and the complex of species in North America since the Pleistocene epoch,4 when E. caballus inhabited the continent, made the designation of the horse as a reintroduced species difficult to assess. Discoveries about the evolutionary and genetic history of the horse have been made since those reports (see Weinstock et al., 2005), but uncertainty remains regarding the degree of similarity or change in the morphology and behavior between modern horses and ancestral horses from Pleistocene North America. In the context of the committee’s study, free-ranging horses and burros under BLM management, whether or not they are considered a species reintroduced into North America, are protected by the Wild Free-Roaming Horses and Burros Act and therefore have the protection stipulated in the law, that is, to their known territorial limits as of 1971.
Regarding evaluations of gather techniques, the effects of gathers on horses and burros, and the condition of animals placed in long-term holding facilities, such a study would be better conducted by a committee specifically constituted with the expertise to assess animal welfare. The treatment of animals during gathers and in holding facilities has been studied by the Government Accountability Office (2008) and by a task force of the American Association of Equine Practitioners (2011). Investigating the circumstances of animals that leave the program through sale or adoption is more appropriate for a body that has auditing authority. The committee was not asked to assess the viability of ecosanctuaries, so such expertise was not included in the committee’s makeup.
Though the committee did not address the aforementioned issues directly, it recognized that increasing costs of gathering animals and holding them indefinitely drove Congress to ask BLM to develop a long-term plan for managing free-ranging horses and burros. The committee was also aware that concerns for animals gathered and placed in holding facilities or released from the program through adoption or sale cause much of the stakeholder frustration with the Wild Horse and Burro Program. In fulfilling its statement of task, which sets forth how BLM can use science to improve management of animals on the range, the committee had the goal in this report to provide BLM with tools that could help the agency to decrease the use of and spending on contentious practices and to manage healthy populations on the range.
STATUS OF FREE-RANGING HORSES AND BURROS UNDER BUREAU OF LAND MANAGEMENT JURISDICTION
At the time the committee conducted its review, BLM reported that 31,453 horses and 5,841 burros were on the range (BLM, 2012b). The animals live on Herd Management Areas (HMAs),5 rangeland that they inhabited in 1971 and that BLM has found to have adequate forage, water, cover, and space to support them. In 2012, there were 179 HMAs, 171 of which contained equids. Figure 1-3 shows HMAs designated by BLM for use by horses, burros, or
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4 The Pleistocene epoch ended 11,700 years before the present.
5 U.S. Forest Service herd areas are referred to as Wild Horse (or Burro) Territories.
FIGURE 1-3 Herd Management Areas (HMAs) in 2012.
NOTE: The HMAs are categorized by the species that the Bureau of Land Management (BLM) manages in an area. Burros may live in some HMAs that are managed only for horses and vice versa. HMAs discussed often in the report are circled on the map.
DATA SOURCE: Mapping data provided by BLM. Species data from BLM (2012b).
both. Recognizing that the proximity of some HMAs to one another allows animals to move from one HMA to another, BLM began to manage some groups of HMAs as complexes, or larger units, in the late 2000s. In 2012, 93 HMAs were parts of complexes (Figure 1-4).
HMAs are in 10 states; almost half the 179 are in Nevada (Table 1-1). BLM reported in 2012 that almost 60 percent of the free-ranging horse population was in Nevada, followed by Wyoming and Utah, at 11 and 10 percent, respectively. Over 50 percent of the burros on the range were in Arizona and 25 percent were in Nevada (Figure 1-5).
As required by the Wild Free-Roaming Horses and Burros Act (as amended), BLM sets an appropriate management level (AML) for each HMA, the numeric population range at which the agency has determined a herd can be maintained in healthy condition without adversely affecting a thriving natural ecological balance. When establishing an AML, BLM must also consider other federal acts pertaining to public lands, including the Federal Land Policy and Management Act of 1976 (P.L. 94-579), the Wilderness Act of 1964 (P.L. 88-577), the National Historic Preservation Act of 1966 (P.L. 89-665), the Clean Water Act of 1972 (P.L. 92-500), the Endangered Species Act of 1973 (P.L. 93-205), and the Forest and Rangeland Renewable Resources Planning Act of 1974 (P.L. 93-378). The requirements of these acts as
FIGURE 1-4 Herd Management Areas (HMAs) managed together or with Wild Horse or Burro Territories as complexes.
NOTE: Blank HMAs are not managed as part of a complex. The complex codes in the legend correspond to the following HMAs:
| CA1 | Buckhorn, Coppersmith | |
| CA2 | Round Mountain (managed by the U.S. Forest Service with the Devil’s Garden Plateau Wild Horse Territory) | |
| CA3 | Fort Sage (California), Fort Sage (Nevada) | |
| CA4 | High Rock, Nut Mountain, Wall Canyon, Bitner, Fox Hog ID1 Black Mountain, Hard Trigger ID2 Four Mile (Idaho), Sand Basin NM1 Carracas Mesa (managed by the U.S. Forest Service) | |
| NV1 | Stone Cabin, Saulsbury, Hot Creek, Reveille (managed by the Bureau of Land Management with Monitor Wild Horse Territory) | |
| NV2 | Pancake, Sand Springs West (managed by the Bureau of Land Management with Monte Cristo Wild Horse Territory) | |
| NV3 | Johnnie, Red Rocks, Wheeler Pass (managed by the Bureau of Land Management with Spring Mountain Wild Horse Territory) | |
| NV4 | Fish Lake Valley (managed by the Bureau of Land Management with U.S. Forest Service Wild Horse Territory) | |
| NV5 | Triple B, Maverick-Medicine (managed by the Bureau of Land Management with Cherry Springs Wild Horse Territory) | |
| NV6 | Antelope, Antelope Valley, Goshute, Spruce-Pequop | |
| NV7 | Owyhee, Little Owyhee, Little Humboldt, Rock Creek, Snowstorm Mountain |
| NV8 | Blue Wing Mountains, Seven Troughs, Lava Beds, Nightingale Mountains, Kamma Mountains, Shawave Mountains | |
| NV9 | Diamond, Diamond Hills North, Diamond Hills South | |
| NV10 | Callaghan, Rocky Hills, Bald Mountain | |
| NV11 | Buffalo Hills, Fox-Lake Range | |
| NV12 | Seven Mile, Fish Creek, Little Fish Lake, North Monitor (managed with Butler Basin and Little Fish Lake Wild Horse Territories) | |
| NV13 | Roberts Mountain, Whistler Mountain | |
| NV14 | Montgomery Pass (managed by the U.S. Forest Service) | |
| NV15 | Hickison Summit (managed by the U.S. Forest Service with the Hickison Wild Burro Territory) | |
| NV16 | Calico Mountains, Black Rock East, Black Rock West, Granite Range, Warm Springs Canyon | |
| OR1 | Coyote Lake, Alvord Tule Springs, Sand Springs, Sheepshead/Heath Creek | |
| OR2 | Kiger, Riddle Mountain | |
| OR3 | Murderer’s Creek (managed by the U.S. Forest Service with Murderer’s Creek Wild Horse Territory) | |
| UT1 | Choke Cherry (Utah), Mt. Elinor (Utah), Eagle (Nevada) | |
| UT2 | Bible Springs, Four Mile (Utah), Tilly Creek | |
| UT3 | North Hills (managed by the Bureau of Land Management with the North Hills Wild Horse Territory) | |
| WY1 | Adobe Town, Salt Wells Creek | |
| WY2 | Divide Basin, Lost Creek, Stewart Creek, Antelope Hills, Green Mountain, Crooks Mountain | |
| WY3 | Dishpan Butte, Muskrat Basin, Conant Creek, Rock Creek Mountain | |
| WY4 | White Mountain, Little Colorado | |
| DATA SOURCE: Mapping data and complex information provided by the Bureau of Land Management. | ||
TABLE 1-1 Herd Management Areas, by State, 2012
| State | Number of Herd Management Areas | Number of Herd Management Areas with Free-Ranging Equids |
| Arizona | 7 | 7 |
| California | 21 | 19 |
| Colorado | 4 | 4 |
| Idaho | 6 | 6 |
| Montana | 1 | 1 |
| Nevada | 85 | 79 |
| New Mexico | 2 | 2 |
| Oregon | 18 | 18 |
| Utah | 19 | 19 |
| Wyoming | 16 | 16 |
| Total | 179 | 171 |
| SOURCE: Bureau of Land Management (2012b). | ||
FIGURE 1-5 Number of equids reported by the Bureau of Land Management (BLM) for each Herd Management Area in 2012.
DATA SOURCE: Mapping data provided by BLM. Population data from BLM (2012b).
they pertain to free-ranging horse and burro management are discussed in Chapter 7 (see section “The History of Appropriate Management Levels”).
Table 1-2 shows the upper bounds of AMLs and the estimated population of each species in each state. Often, when populations exceed the upper bound of AML, BLM conducts a gather. After a gather, a healthy animal may be released back to the range, released back to the range after being gelded or treated with a contraceptive, or removed to a short-term holding facility. Animals removed from the range may be put up for adoption.6 An animal that is not adopted is ultimately moved to a long-term holding facility, where it remains. In 2010, BLM removed 9,042 animals from the range (BLM, email communication, December 11, 2011). As of September 2012, it held 14,238 animals in short-term holding facilities and 33,623 in long-term holding facilities (BLM, 2012c).
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6 At times during the lifetime of the law, BLM has had the authority to sell animals without limitation. During 2005-2010, it sold roughly 650 animals a year (Bolstad, 2011). At the time the study was conducted, BLM had authority to sell animals, although legislation to remove the authority had been proposed.
TABLE 1-2 Upper Limits of Appropriate Management Levels and Population Estimates of Horses and Burros by State, 2012
| State | Appropriate Management Levels | Population Estimates | ||
| Horse | Burro | Horse | Burro | |
| Arizona | 240 | 1,436 | 502 | 3,194 |
| California | 1,585 | 478 | 1,965 | 939 |
| Colorado | 812 | 0 | 967 | 0 |
| Idaho | 617 | 0 | 640 | 0 |
| Montana | 120 | 0 | 170 | 0 |
| Nevada | 11,964 | 814 | 18,425 | 1,456 |
| New Mexico | 83 | 0 | 108 | 0 |
| Oregon | 2,690 | 25 | 2,093 | 35 |
| Utah | 1,786 | 170 | 3,040 | 217 |
| Wyoming | 3,725 | 0 | 3,543 | 0 |
| Total | 23,622 | 2,923 | 31,453 | 5,841 |
| SOURCE: Bureau of Land Management (2012b). | ||||
Because a thorough review of the literature on horse and burro biology was conducted in the 1980 National Research Council report, this report begins with questions pertinent to the statement of task. Information from the 1980 report on the social organization of free-ranging horses and burros is summarized briefly in Box 1-4, and equid life history is explained further in later chapters. Although burros are discussed in this report, the management of horses is featured more comprehensively as more studies have been conducted on free-ranging horses than on burros. Also, at the time this report was published, BLM estimated that it managed over 30,000 horses and fewer than 6,000 burros on the range. Thus, the committee inferred that managing horses was the more pressing issue for BLM and that its review should devote more attention to horses than to burros.
Successful management of horses and burros requires knowing how many animals live on the range. BLM often receives criticism about the validity of the reported number of animals and therefore asked the committee to review its methods for estimating the size of the population of free-ranging horses and burros under its jurisdiction. The committee was also charged with evaluating the estimated population growth rate that BLM uses, another issue that is highly contentious between some stakeholders and BLM. Chapter 2 analyzes data provided to the committee by BLM and reviews the literature on population survey techniques to address this task.
Population processes, such as population growth and self-limitation, affect population size. They can be influenced by the density of a population or by independent factors, such as climate or, in the case of free-ranging horses and burros, management decisions. Chapter 3 examines how density-dependent and density-independent factors and management actions may affect the population processes of free-ranging horses and burros. Changes in the size of a population due to density, climate, predation, and management actions are specifically studied.
BLM has used the contraceptive porcine zona pellucida in mares since 2004, but it has been administered to so few animals that it has had no effect on population size. Since the earlier National Research Council reports were published (NRC, 1980, 1982,
BOX 1-4
Social Organization of Free-Ranging Equids
Equids organize themselves socially in a variety of forms. Two dominant forms are harem organization and territorial organization. A harem, also known as a band, consists of a dominant stallion, subordinate adult males and females, and offspring. The group is strongly bonded, although bands are not entirely stable. Typically, adults in the group are not close genetic relatives. Movement among bands is not uncommon; it often occurs when a stallion is displaced, when a stallion defeats a competitor for a mare, or when females reach maturity. Harem organization is common in free-ranging horses; an average band size is five animals. Occasionally, bands come together to form temporary aggregations or herds. In territorial organization, a male typically defends a territory and mates with females that enter the area. The mother-offspring relationship is the only stable bond. Burros typically display this form of social organization. Temporary groups of bachelor males exist in both organization patterns.
1991), considerable progress has been made in developing and testing fertility control for wild animal populations, both free-ranging and captive. Chapter 4 investigates the fertility- control options for mares and stallions that are available to BLM. The on-the-range feasibility and efficacy of each method is assessed, and the effect of potential widespread application of these methods on population processes is evaluated.
Chapter 5 summarizes the research on genetic diversity in free-ranging horse and burro populations in the western United States. Much work has been conducted since the earlier National Research Council reports were published, and genetic-testing capabilities have advanced. The chapter examines the relevance of genetic diversity to long-term herd health of ungulates in general and of free-ranging horses and burros in particular. It presents methods for maintaining healthy levels of genetic diversity. It also reviews the science on the minimum population size needed for viability and explores the different ways in which free-ranging horse and burro populations could be managed for genetic diversity, for example: In terms of genetics, should a population be defined as the animals on an HMA, the animals on an HMA complex, or the entire population of free-ranging horses or burros?
Anticipating the effects of a management action can help decision-makers to select the most efficient and productive course of action when managing animal populations. Chapter 6 reviews population models that are or could be used by BLM to project the effects of management actions (such as removals from the range, contraceptive treatments, and changes in the sex ratio) on the population dynamics of a herd or a larger population. The components necessary for a modeling framework that would comprehensively address the Wild Horse and Burro Program’s challenges are detailed.
The Wild Free-Roaming Horses and Burros Act charges BLM with establishing AMLs and managing populations to protect and restore a thriving natural ecological balance of all wildlife species, particularly endangered species, and to protect rangelands from deterioration. The agency must also consider the capacity of an area to support equids in a healthy condition and the multiple-use objective of BLM management when determining AMLs. In Chapter 7, the committee examines the process that BLM has designed for establishing and adjusting AMLs, as published in its Wild Horses and Burros Management Handbook in June 2010. The chapter also reviews alternative approaches that BLM might use to set and validate AMLs.
As alluded to in Box 1-2, there are strong and often divergent stakeholder opinions regarding the management of horses and burros, and BLM has often been criticized for its procedures by parties holding conflicting opinions and values. Chapter 8 explores ways in which BLM can use participatory approaches to find greater convergence on management objectives and actions that use the best science available. The issue of the horse as native to North America is also addressed in the chapter.
Chapter 9 uses the report’s findings to suggest a sustainable path forward for the Wild Horse and Burro Program built on scientific research.
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