Expert Report

Each report is produced by a committee of experts selected by the Academy to address a particular statement of task and is subject to a rigorous, independent peer review; while the reports represent views of the committee, they also are endorsed by the Academy. Learn more on our expert consensus reports.

This report reviews the science that underpins the Bureau of Land Management's oversight of free-ranging horses and burros on federal public lands in the western United States, concluding that constructive changes could be implemented. The Wild Horse and Burro Program has not used scientifically rigorous methods to estimate the population sizes of horses and burros, to model the effects of management actions on the animals, or to assess the availability and use of forage on rangelands. Evidence suggests that horse populations are growing by 15 to 20 percent each year, a level that is unsustainable for maintaining healthy horse populations as well as healthy ecosystems. However, promising fertility-control methods are available to help limit this population growth. In addition, science-based methods exist for improving population estimates and predicting the effects of management practices in order to maintain genetically diverse, healthy populations, and estimating the productivity of rangelands. Greater transparency in how science-based methods are used to inform management decisions may help increase public confidence in the Bureau of Land Management.

Webinar Materials
June 18: Playback
June 26: Playback

Additional materials are available online, including a FAQ, an interactive map, and the presentation given at the webinars.

Key Messages

  • Management of free-ranging horses and burros is not based on rigorous population-monitoring procedures. At the time of the committee's review, most Herd Management Areas did not use inventory methods or statistical tools common to modern wildlife management. Survey methods used to count animals were often inconsistent and poorly documented and did not quantify the uncertainty attached to counts.
  • On the basis of information provided to the committee, the statistics on the national population size cannot be considered scientifically rigorous. The links between BLM's estimates of the national population size and its actual population surveys – the data that underlie these estimates – are obscure. The procedures used to develop population estimates for the Herd Management Areas from counts of animals are not standardized and frequently not documented. It seems that the national statistics are the product of hundreds of subjective, probably independent judgments and assumptions by range personnel about the proportion of animals counted during surveys, population growth rates, and other factors. As a result BLM's reported annual population statistics, which are based on the assumption that all animals are detected and counted, probably underestimate the actual number of animals on the range.
  • Developing and using a centralized relational database that captures all data on animal counts and removals generated by BLM's field offices and by animal processing and holding facilities would provide a clear connection between the actual data collected and the reported statistics. To improve transparency and public confidence, survey data at the Herd Management Area level, as well as procedures used to modify it to generate population estimates, and should be made available to the public.
  • The majority of free-ranging horse populations on public rangelands in the western United States are growing 15 to 20 percent a year. The committee reviewed the ages of horses removed from the range during the years 1989 to 2011 and found that these data can provide a reasonable assessment of the general growth rate of the horse populations. That growth rate was supported by the published literature the committee reviewed.
  • Management practices are facilitating high rates of population growth. Free-ranging horse populations are growing at high rates because BLM's removals hold populations below levels affected by food limits. If population density were to increase to the point that there was not enough forage available, it could result in fewer pregnancies and births and lower young-to-female ratios and survival rates. Decreased competition for forage through removals may instead allow population growth, which then drives the need to remove more animals.
  • Predators will not typically control population growth rates of horses. Because predators like mountain lions and wolves are not abundant in Herd Management Areas, the potential for predators to affect free-ranging horse populations is limited. Mountain lions require habitats different from those favored by horses, and the committee was unable to find any examples of wolf predation on free-ranging horses in the United States.
  • The most promising fertility-control methods for free-ranging horses or burros are porcine zona pellucida (PZP) vaccines and GonaConTM vaccine for females and chemical vasectomy for males. This conclusion is based on criteria such as delivery method, availability, efficacy, duration of effect, and potential for side effects. Although applying these methods usually requires gathering horses and burros, that process is no more disruptive than the current method of population control – gathering and removal -- without the further disruption of removing animals. Considering all the current options, these three methods, either alone or in combination, offer the most acceptable alternative to removing animals for managing population numbers.
  • Management of horses and burros as metapopulations is necessary for their long-term genetic health. Genetic studies of horses on 102 Herd Management Areas show that the genetic diversity for most populations is similar to those of healthy mammal populations, although genetic diversity is not static and could change over time. Little is known about the genetic health of burros; the few studies that have been conducted reported low genetic diversity compared to domestic donkeys. To achieve an optimal level of genetic diversity, managers could consider the collective populations of several Herd Management Areas as a single population. Management options include intensively managing individuals according to their genetic makeup within Herd Management Areas, translocating horses and burros among these areas, or both.
  • Recording the occurrence of diseases and clinical signs and the age and sex of the affected animals would allow BLM to monitor the distribution and prevalence of genetic conditions that affect population health. Such data have not been recorded and integrated to date. Some conditions that cause genetic mutations in horses are not lethal, so it is possible for the mutations to increase in frequency in Herd Management Areas, especially if inbreeding occurs. Surveillance of these mutations would be possible if blood or hair samples are collected from horses during gathers. Over time, regular sampling would reveal whether a particular Herd Management Area has a higher occurrence of a given mutation that might affect the fitness of the herd.
  • It is unclear whether or how the results of the WinEquus model are used in management decisions, and the input parameters are not transparent. BLM currently includes the results of WinEquus, a program that simulates how horse populations would change with management actions, in its gather plans and environmental assessments. However, WinEquus results depend on the values of input parameters – for example, age-specific foaling rates or the sex and the age composition of a herd – and various management options selected by the user when setting up the simulations. These parameters were rarely provided in gather plans and environmental assessments. In addition, in most of the reviewed documents, there was no explanation or interpretation of WinEquus output, making it difficult to determine if results were used to make management decisions or were offered as justification for decisions that were made independently.
  • The Wild Horses and Burros Management Handbook lacks specificity. Issued by BLM in 2010, the handbook provides some degree of consistency in goals, allocation of forage, and general habitat considerations. Currently the handbook lacks the specificity needed to adequately guide managers on establishing and adjusting Appropriate Management Levels – the number of horses and burros BLM deems appropriate for a given Herd Management Area . It does not provide sufficient detail on how to conduct various kinds of assessments. In addition, the handbook does not clarify the important legal definitions related to implementing and assessing management strategies for free-ranging horses and burros, leaving these concepts uninformed by science and open to multiple interpretations.
  • How Appropriate Management Levels are established, monitored, and adjusted is not transparent to stakeholders, supported by scientific information, or amenable to adaptation with new information and environmental and social change. Appropriate Management Levels are a focal point of controversy between BLM and the public. Standards for transparency, quality, and equity are needed in establishing these levels, monitoring them, and adjusting them. The public should be able to understand the methods used and how they are implemented, and to access the data used to make decisions. In addition, data and methods used to inform decisions must be scientifically defensible. Appropriate Management Levels must be adaptable based on environmental change, changes in social values, or the discovery of new information.
  • Resolving conflicts with polarized values and opinions regarding land management rests on principles of transparency and public participation in decision making. Participatory decision-making processes foster the development of a shared understanding of the ecosystem, an appreciation for others' viewpoints, and the development of good working relationships. Thus, BLM should develop an iterative process between public deliberation and scientific research and co-design the participatory process with representatives of the public.