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High Density of Free-Roaming Horses Linked to Decline of Greater Sage-Grouse

Study explores detrimental effects of large horse populations on chicks and juvenile birds

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
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
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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Mountain Journal is a nonprofit, public-interest journalism organization dedicated to covering the wildlife and wild lands of Greater Yellowstone. We take pride in our work, yet to keep bold, independent journalism free, we need your support. Please donate here. Thank you.
Sophie Tsairis
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.
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