- With no intervention, horse population will increase to self-limitation.
- Removal strategy perpetuates overpopulation.
- Better methodology for estimating population size needed.
THE U.S. Bureau of Land Management's (BLM) current practice of removing free-ranging horses from public lands promotes a high population growth rate, and maintaining them in long-term holding facilities is both economically unsustainable and incongruent with public expectations, according to a new report by the National Research Council (NRC).
The report says tools already exist for BLM to better manage horses and burros on healthy ecosystems, enhance public engagement and confidence and make the program more financially sustainable. It also provides evidence-based approaches that, if widely and consistently implemented, can improve the management of these animals on public lands in the western U.S.
The committee that wrote the report determined that most free-ranging horse populations are growing at 15-20% per year, meaning these populations could double in four years and triple in six years. With no intervention by BLM, the horse population will increase to the point of self-limitation, where both land degradation and high rates of horse mortality will occur due to inadequate forage and water supplies, NRC said.
In addition, periodic droughts — many of them severe — in western public lands cause immediate and often unpredicted effects. There is little, if any, public support for allowing these effects on the horse population or the land to happen, and both go against BLM's program mission, NRC said.
However, the report says the current removal strategy used by BLM perpetuates the overpopulation problem by maintaining the number of animals at levels below the carrying capacity of the land — protecting the rangeland and the horse population in the short term but resulting in continually high population growth and exacerbating the long-term problem.
To manage horse populations without periodic removals, widespread and consistent application of fertility control would be required, the committee determined. Three methods in particular — porcine zona pellucida (PZP) and GonaCon for mares and chemical vasectomy for stallions — were identified as effective approaches.
"The committee recommended these approaches based on the evidence of their efficacy with other populations, notably the horses on Assateague Island, but cautioned that scaling up use of these methods to the larger and more disseminated horse populations in the western U.S. will be challenging," said Guy Palmer, a veterinarian with Washington State University and chair of the study committee.
The committee also strongly recommended that BLM improve and standardize its methodology to estimate population size, stressing the importance of accurate counts as the basis for all management strategies. A large body of scientific literature suggests that the proportion of animals missed in current surveys ranges from 10% to 50%.
Additionally, an examination of the genetics and health of population groups as well as of the rangelands they occupy can be used to assure that both the animal populations and the ecosystem are being appropriately managed, NRC said. Developing an iterative process whereby public participants could engage with BLM personnel on data gathering and assessment would increase the transparency, quality and acceptance of BLM's decision-making process.
BLM sponsored the study. The National Academy of Sciences, National Academy of Engineering, Institute of Medicine and NRC make up the National Academies, which are private, nonprofit institutions that provide science, technology and health policy advice under a congressional charter.