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Because of history, context, and periodic introductions, most wild horse herds are not truly isolated populations. The National Academies of Sciences report to the BLM (2013) recommended that single HMAs should not be considered isolated genetic populations. Rather, managed herds of wild horses should be considered as components of interacting metapopulations, connected by interchange of individuals and genes due to both natural and human-facilitated movements. The same is true for wild burro herds. Herds in the larger metapopulation have a background of shared domestic breed heritage, and natural and intentional movements of animals between herds. When needed, WHB management has historically used introductions from other HMAs to augment observed heterozygosity, which is a measure of genetic diversity, to reduce the risk of inbreeding-related health effects. Introducing a small number of fertile animals every generation (about every 8-10 years) is a standard management technique that can alleviate potential inbreeding concerns (BLM 2010).

Most herds have been sampled for genetic diversity once or more. Reports from those samples can provide insights into the degree of connectivity between a sampled herd, and other BLM-managed herds. The WHB program research coordinator can help field offices with interpretation of those data, as evidence of genetic connections. Specifically, Appendix F of the 2013 NAS report is a table showing the estimated 'fixation index' (Fst) values between 183 pairs of samples from wild horse herds. Fst is a measure of genetic differentiation, in this case as estimated by the pattern of microsatellite allelic diversity analyzed by Dr. Cothran’s laboratory. Low values of Fst indicate that a given pair of sampled herds has a shared genetic background. The lower the Fst value, the more genetically similar are the two sampled herds. Values of Fst under approximately 0.05 indicate virtually no differentiation, values of 0.10 indicate very little differentiation, and only if values are above about 0.15 are any two sampled subpopulations considered to have evidence of elevated differentiation (Frankham et al. 2010). For most herds, documented Fst values indicate that they exist in a highly connected metapopulation that includes horse (or burro) herds in many other HMAs.

Natural Resources Council of the National Academies of Sciences (NAS). 2013. Using Science to Improve the BLM Wild Horse and Burro Program: A Way Forward. See Chapter 5: “Genetic diversity in free-ranging horse and burro populations.”

For an explanation about the Fst fixation index, see:

Frankham, R., J. D. Ballou, and D. A. Briscoe. 2010. Introduction to conservation genetics, second edition. Cambridge University Press, New York, New York.