horses and burros living on land under its jurisdiction has been a continuing struggle for BLM. Because accurate estimates of free-ranging horse and burro populations are the foundation of scientifically based management of these animals, third parties have paid considerable attention to assessments of BLM’s methods for inventorying horses and burros over the history of the program (NRC, 1980, 1982; GAO, 1990, 2008). The committee received unfavorable comments during the study process from many members of the public regarding BLM’s reports of equid population estimates and assumed or reported population growth rates.

This chapter focuses initially on estimation of free-ranging horse and burro populations. It first distinguishes the difference between counting animals and estimating population size and discusses why this methodological distinction is important for management and transparency. It then reviews several classes of population-survey methods and their strengths, weaknesses, and applicability to free-ranging horses and burros. The section that follows evaluates information available on the methods used by BLM to inventory equid populations and report the results to the public and Congress when this study was conducted. Recent initiatives to improve BLM’s inventory procedures are then described with recommendations for strengthening the scientific validity and accuracy of the inventory program and enhancing communication of these important statistics to stakeholders. The second topic addressed in the chapter deals with population growth rates. A number of data sources that provide insight into growth rates of horse and burro populations are reviewed, and the results critiqued and synthesized. The chapter ends with a summary of the committee’s conclusions regarding BLM’s horse and burro inventory and reporting procedures and an assessment of typical population growth rates realized on western rangelands. The conclusions are then interpreted in the context of the challenges faced in managing free-ranging equid populations in the future.

ESTIMATING THE SIZE OF FREE-RANGING EQUID POPULATIONS

Since the inception of the Wild Horse and Burro Program, BLM’s population inventory program has involved attempting to survey completely the fixed areas occupied by free-ranging equids, known as HMAs, and to count all the animals detected. Those inventory surveys are commonly referred to as censuses in BLM reports; however, a census involves the perfect enumeration of every animal that occupies a given area of interest; that is, every animal is detected and counted. That is ideal, but counting free-ranging animal populations is an imperfect exercise. Topography, the extent of survey areas, vegetation structure, weather, animal behavior and coat color, the size of areas used by individual animals, the performance of aircraft used by observers, the skill and condition of observers, sun angle, cloud cover, and wind speed are some of the major factors that can influence the detectability of animals, which in turn affects the accuracy, efficiency, and effectiveness of survey methods (MacKenzie et al., 2006). For any given set of survey conditions, those factors can result in observers’ failure to detect animals that are present in a survey area or their unknowing detection and counting of the same animals on multiple occasions. Although animals can be missed or double-counted during the same survey, a large body of scientific literature on techniques for inventorying large mammals has demonstrated that failure to detect animals is overwhelmingly more common (Caughley, 1974a; Pollock and Kendall, 1987; Samuel et al., 1987). The first studies of probabilities of detection of free-ranging horses on western rangelands reported that in typical surveys only 7 percent of horses were undetected in flat, treeless terrain, but 50-60 percent were undetected in more rugged terrain with tree cover (Frei et al., 1979; Siniff et al., 1982). More recent studies of inventory



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