LINKS TO USGS FORT COLLINS SCIENCE CENTER Counting Wild Horses Alternative Techniques Combining Techniques Contraception Collaborators Related Resources   Thriving Natural Ecological Balance The Wild and Free-Roaming Horse and Burro Act of 1971 requires that (1) wild horses, burros, and wildlife be in good health and reproducing at a rate that sustains the population; (2) the key vegetation is able to maintain its composition; (3) the soil is being protected; and (4) a sufficient amount of high quality water is available to all animals.   Population Modeling BLM Wild Horse Specialists use a statistical model developed by Dr. Stephen H. Jenkins, University of Nevada, which allows the user to see potential outcomes of management actions, based on age-sex ratios of the herd, contraceptive use, foal rate, survival rate, and removals.   Standard Gathers The herding of wild horses into a temporary holding corral in order to sort and remove animals that are in excess of the appropriate management level for a given Herd Management Area (HMA). Currently, the BLM conducts a gather of each HMA approximately every 4 years. Removed animals are sent to adoption centers for vaccinations, veterinary examination, and public adoption.

Counting America's Wild Horses and Burros: Better Estimates for Population Management

Source: U.S. Geological Survey, Fort Collins Science Center

Almost every management issue concerning wild horses and burros depends on accurate herd counts. Reliable, science-based population estimates are needed for maintaining everything from herd health to habitat carrying capacity to genetic diversity. The Wild Free-Roaming Horses and Burros Act of 1971, as amended, states that

…Congress finds and declares that wild free-roaming horses and burros are living symbols of the historic and pioneer spirit of the West; that they contribute to the diversity of life forms within the Nation and enrich the lives of the American people; and that these horses and burros are fast disappearing from the American scene. It is the policy of Congress that wild free-roaming horses and burros shall be protected from capture, branding, harassment, or death; and to accomplish this they are to be considered in the area where presently found, as an integral part of the natural system of the public lands. (PL 92-195, Sec. 1331, Congressional findings and declaration of policy)

The Bureau of Land Management (BLM) is the agency responsible for managing wild horses and burros on public lands. Pursuant to the Act’s requirements, the BLM’s overarching goal for wild horse and burro management is to achieve and maintain a "thriving natural ecological balance.”

In reality, the wild horse and burro population survey requirements for the BLM are daunting. The agency is responsible for managing 178 wild horse and 51 wild burro populations located across more than 88 million acres of public lands. Given the number and distribution of these populations, surveys of all burro herds and most wild horse herds are currently made from an aircraft.

A stated goal of the BLM Wild Horse and Burro Program is to conduct a population survey of each wild horse or wild burro herd (often referred to as herd management areas, or HMAs) at least every 4 years. With so few surveys, combined with the demand for reliable information on which to base management decisions, wild horse and burro managers need standardized, tested, defensible, cost-effective, yet easy-to-use aerial population estimation techniques for wild horse and burro herds in a range of habitat types and across a range of population sizes and densities. The accuracy and precision of current wild horse survey methods have not been rigorously tested. Thus, a statistically valid estimation technique with confidence intervals is needed.

Wild horse and burro managers are under increasing pressure to produce accurate and reliable population estimates of these nomadic animals. In order to plan any management removals, the BLM requires estimates of population size and growth, sex and age classification, and population modeling. Field trials on fertility control, using porcine zona pellucida (PZP), are being conducted in a large number of wild horse herds across the country. The use of fertility control, in conjunction with standard gathers, will demand better estimates of population size and more frequent (annual) monitoring of population growth.

Combining Resources

The U.S. Geological Survey, Fort Collins Science Center (FORT) has teamed up with the BLM and Colorado State University (CSU) to test aerial survey techniques for wild horses and burros in an effort to find a more accurate way to estimate population numbers. By introducing statistical sampling methodology and analysis, and adapting some well-known survey techniques, FORT scientists may be able to deliver a tool to help get to the bottom of the age-old question, “How many wild horses are there on the range?”

Aside from the BLM wild horse specialists who manage the individual herd areas, there are countless interest groups that can benefit from more accurate population surveys as well. The results affect how many grazing units are allowed for ranchers who wish to graze on public lands, how resources might be affected by potential competition with other wildlife species, and how many horses are allowed to stay on the range.

Combining Techniques

Counting wildlife from the air is not new science. Wildlife biologists have been using many techniques for years, but each species and each habitat has its own unique set of challenges. For this study, FORT and CSU scientists narrowed the field to four promising techniques for wild horses: mark-resight, simultaneous double-count, sightability bias correction model, and distance sampling. The problem with any one of these techniques is that, by itself, each has considerable drawbacks when applied to aerial counting of wild horses and the various types of terrain they inhabit. In an effort to create a synergistic effect and alleviate the limitations inherent to the individual techniques alone, the aerial survey team is investigating combinations of these techniques. This pioneering effort could affect the way biologists count many different species in the future.

Finding Synergy

A true census (counting every individual) of a wildlife population is seldom possible, but development of techniques to accurately estimate population numbers could reap benefits for wild horse and burro managers across the U.S. With accurate and defensible population estimates, managers can better maintain the thriving natural ecological balance of the wild horses and burros in their care. FORT and CSU biologists hope to uncover and refine just those techniques in order to assist the BLM with managing America’s "Living Legends."