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).”
Copyright © National Academy of Sciences. All rights reserved.
Using Science to Improve the BLM Wild Horse and Burro Program: A
Way Forward
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.