Free-roaming horses across America's West have made for some highly contentious disagreements. In Wyoming, a recent study indicates that an overpopulation of horses is causing harm to greater sage-grouse in the Cowboy State. Here, horses from the Adobe Town herd management area in south-central Wyoming gaze toward the lens. Photo by Jacob Hennig
by Sophie Tsairis
A recently published study from the University of Wyoming reveals that high populations
of free-roaming, nonnative horses are overrunning the sagebrush-steppe
ecosystem, contributing to the decline of greater sage-grouse, a species
already facing significant stressors. The research points to a negative
connection between high densities of horses and the percentage of sage-grouse
chicks hatching and young birds surviving to adulthood.
Jeffrey Beck, a University of
Wyoming ecologist who led the research published in the Journal of Wildlife
Management, told Mountain Journal the issue with horses is that their population often
exceeds the maximum goal of land managers.
In Wyoming, approximately 11
percent of sage-grouse habitat is within BLM herd management areas, and 80
percent of these areas currently have horse populations exceeding target goals.
Beck noted that the issue arises when horse populations surpass these limits.
In 1971, Congress passed the Wild Free-Roaming Horses and
Burros Act, declaring wild horses and
burros symbols of the historic and pioneer spirit of the West. The act tasked
the BLM and U.S. Forest Service with the responsibility of determining and
maintaining appropriate management levels.
According to BLM,
free-roaming horse herds can double in 4-5 years, quickly outgrowing the
capacity of the landscape to support them.
In Wyoming, approximately 11 percent of sage-grouse habitat is within BLM herd management areas, and 80 percent of these areas currently have horse populations exceeding target goals.
Current management strategies to control horse
populations include using birth control and removing excess animals from the
range. However, the agency often faces legal challenges from wild horse
advocacy groups, complicating
efforts to maintain population levels.
“Since 1971,” according to the agency, “the
BLM has removed approximately 37,000 animals from public rangelands in
Wyoming as part of its efforts to maintain healthy horses and burros on
healthy public rangelands. Animals removed from public rangelands are offered
to the public for adoption; unadopted animals are cared for on open pastures
for the rest of their lives.”
A free-roaming horse takes in a covey of sage-grouse on the Adobe Town HMA. Photo by Jacob Hennig
Beck's team conducted a comprehensive study tracking
nearly 1,000 GPS-collared sage-grouse in southern and central Wyoming. They
compared survival rates in areas with and without horses, examining factors
like nest success, brood survival and the survival of juvenile and adult birds.
The results were telling: in areas where horse populations exceeded management
targets drastically, sage-grouse nests, chicks and juveniles suffered
significant mortality. However, adult sage-grouse appeared unaffected by the
presence of horses.
According to the study,
population estimates of free‐roaming horses for the HMAs ranged from 59–700
percent of the maximum appropriate management level.
“The high densities of horses
are breaking up the landscape, increasing bare land and leaving birds vulnerable
to predators,” Beck said.
His recommendation for land managers is to prioritize
keeping horse populations within or below the target numbers, since the
research indicates that when horses are managed properly, their numbers do not
have a large impact on sage-grouse populations.
“What has been interesting
about the Great Basin and Wyoming study is that the BLM’s appropriate
management level seems to work for sage-grouse, as long as [horse populations]
don’t exceed their numbers,” he said.
Beck hopes to build on this
research in coordination with the U.S. Geological Survey and biologists to
establish what the effects of free-roaming horses on nests, broods and
juveniles mean for sage-grouse population growth rates.
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About Sophie Tsairis
Sophie Tsairis is a freelance writer based in Bozeman, Montana. She earned a master's degree in environmental journalism from the University of Montana in 2017.
