Inbreeding & Loss of Genetic Diversity

Page 144 USING SCIENCE TO IMPROVE THE BLM WILD HORSE AND BURRO PROGRAM.

“In small populations or populations that suffer size bottlenecks,1 allelic diversity is lost relatively quickly through random genetic drift, but heterozygosity is less affected. In small populations that are isolated, inbreeding is inevitable and occurs within only a few generations. Whereas inbreeding does not change allele frequencies, it results in a change in the proportion of individuals that carry two alleles at a locus that are identical by descent and decreases heterozygosity. Thus, it is important to measure and monitor allelic diversity, observed and expected heterozygosity (Ho and He), and coefficients of inbreeding (Fis) in managed populations. Genetic diversity in a population results from a number of evolutionary forces: mutation, natural selection, gene flow, and genetic drift. Although mutation is the ultimate source of all genetic variation, mutation rates of most genes are low and cannot replenish diversity quickly once it is lost (Lande, 1995). The effects of natural selection depend on whether it is directional, stabilizing, or balancing selection. Regardless of the kind of natural selection exerted on a population, when a population is small, only strong selection will affect the level of diversity (Frankham et al., 2010). In contrast, the recruitment of even a small number of unrelated breeding individuals into a population (gene flow) can increase genetic diversity or prevent its loss. Genetic drift—random change in allele frequencies between generations—is a strong force in small populations and can result in rapid loss of genetic diversity (Frankham et al., 2010).”

The NAS Report supports the need to manage for genetic diversity

and to prevent in-breeding in free-roaming horses.

The BLM videos at Mustangs – The WH & B Act – WH &B Program – Gathers often talk about the need to manage the wild horses and they do have good herd management programs when the funding is available. In the late 1970s horses on the South Steens HMA were what the old timers may have referred to as mustangy, but with good herd management by the BLM the horses on the South Steens are a fine group of horse today. Same story on the Palomino Buttes HMA and I am sure the same is true on many of the HMAs. Just as the free-roaming horses were managed by ranchers and horse runners before 1971, they are now managed by the Wild Horse & Burro Program. Both the old-time horse runners and the present managers of the free-roaming horses and burros on the public lands know from experience the horses need to be well managed or they will become mustangy. The NAS (National Academy of Sciences) Report reached the same conclusions using science.

In the Great Basin, the number of acres it takes to support a free-roaming horse will not allow the densities needed to support genetically healthy and diverse herds on healthy lands. The acres it takes to support a single horse for a year on the public lands in the Great Basin will support 30 or more horses for a year in the grasslands of eastern Kansas and Oklahoma. With the densities supported on the grasslands natural selection could be used as the main tool to promote genetic diversity using management only as needed. Gathers would still be needed to track genetic diversity and to remove the excess horse. If birth control is used as the main population control, genetic may diversity becomes an issue again.

The Great Basin is not a natural habitat for horses. The more acres it takes to support a horse for a year increases the need for management to keep healthy horses on healthy ranges. Management would be less stressful on the horses in the grasslands and be less of a financial strain. The option of removing the horses from public lands and placing the wild horse program in the grasslands of Kansas and Oklahoma would have many benefits.