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

Numbers count

The Wild Free-Roaming Horses and Burros Act of 1971, as amended, states that

… 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 U.S. Bureau of Land Management (BLM)¹ and the USDA Forest Service are responsible for managing the vast majority of wild horses and burros on public lands, although a few U.S. Fish and Wildlife Service and National Park Service units also support wild horses and burros. 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.”

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. In reality, the wild horse and burro population survey requirements for the BLM are daunting. The agency is responsible for 199 Herd Management Areas (HMAs) located across more than 34 million acres of public lands in the U.S. West. Given the number and distribution of these populations, surveys of all burro herds and most wild horse herds are currently made from 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 at least every 4 years. Given 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 habitats 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 is needed.

Combining resources

The U.S. Geological Survey, Fort Collins Science Center (FORT) has teamed 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 by adapting some well-known survey techniques, FORT scientists are endeavoring to deliver a tool to help get to the bottom of the age-old question, “How many wild horses are on the range?”

Aside from the BLM wild horse specialists who manage the individual herds, there are countless interest groups that can benefit from more accurate population surveys as well. Survey results affect how many grazing units are allowed for ranchers who wish to graze their livestock 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 some 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 modeling, and distance sampling. Each of these techniques has drawbacks when applied to aerial counting of wild horses in 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 manage the wild horses and burros in their care. FORT and CSU biologists are engaged in testing and refining such techniques in order to support the BLM in managing America’s "living symbols of the historic and pioneer spirit of the West."

¹We focus here on management by the BLM, our primary research partner.