species—such as bears (Woods et al., 1999; Sawaya et al., 2012), mountain lions (Ernest et al., 2000), tigers (Sugimoto et al., 2012), wolves (Stenglein et al., 2010), coyotes (Kohn et al., 1999), and mountain gorillas (Guschanski et al., 2009)—genetic surveys have provided information about not only population sizes but sex ratios, levels of genetic diversity, and relatedness.
Although to the committee’s knowledge the genetic-tag method has not been used for free-ranging horses, the necessary preliminary work to develop methods of preserving and genotyping DNA from horse dung has been done. There was no need to estimate population size for the Assateague Island National Seashore herd because individual horses are carefully monitored by park management, but the National Park Service sought information about relatedness among individuals to assess and inform its management regime. In a collaborative study with scientists at the Smithsonian Institution, methods of preserving horse dung were tested, and a representative set of microsatellite loci was optimized (Eggert et al., 2010). Potential disadvantages of this method include the time needed for genotyping and data analysis and the difficulties that may be encountered in finding a laboratory willing to conduct the work at a reasonable cost.
Herd Management Area Survey Information Requested and Received by the Committee
The committee initially requested the most recent 12 years of records (2000-2011) on all HMAs so that it could evaluate the methods and procedures used by BLM to estimate sizes of free-ranging horse and burro populations at the time of its study. Because BLM publishes annual national statistics on the numbers of horses and burros on western public rangelands, the committee assumed that requested records would include an estimate of the population of each HMA for each year. Actual surveys of the number of animals occupying a given HMA are usually not conducted annually (BLM, 2010), so the committee expected only a subset of years for each HMA to include records of actual animals counted on the basis of some survey procedure and estimates for the intervening years to be based on previous inventories. For years when counts were conducted, the committee requested the approximate date of the count, the survey platform used (e.g., ground, fixed-wing aircraft, helicopter), and whether the inventory covered the entire HMA or used some sort of sampling regimen whereby a portion of the HMA was surveyed and the results were extrapolated to obtain a population estimate for the entire HMA.
Previous research on techniques for surveying free-ranging horses and burros (Frei et al., 1979; Siniff et al., 1982; Walter and Hone, 2003; Laake et al., 2008; Lubow and Ransom, 2009) and many other large mammal species (Caughley, 1974a; Pollock and Kendall, 1987; Samuel et al., 1987) has demonstrated that not all animals are detected on surveys. Thus, survey results require the estimation of detection probability and adjustment of the number of animals counted to account for the proportion of animals that were undetected. The committee also asked whether the number of animals counted was adjusted to produce the population estimate for a given year. The committee was informed that populations in years in which no counts were conducted were estimated by multiplying the previous year’s population estimate by some assumed population growth rate until another count was conducted (Box 2-1; BLM, personal communication, December 2011). If the HMA had experienced a gather and removal of horses in the intervening year, the number of animals removed was incorporated into the later year’s population estimate. Thus, for years in which no count was performed for the HMA, the committee requested that BLM report