News Notes

Overabundance of Wild Horses Harms Sage Grouse

When wild horse populations are allowed to grow beyond numbers set by the Bureau of Land Management (BLM), greater sage-grouse numbers suffer.

Free-roaming horses on BLM and Forest Service lands in the West are protected under the Wild and Free-Roaming Horses and Burros Act of 1971. Management on BLM lands is based upon a maximum appropriate management level of 26,785 horses and burros. However, as of March 2023, BLM lands were occupied by nearly 83,000 free-roaming equids, whose annual growth rates can average 20% per year. Over 80% of the BLM’s horse management areas have populations exceeding their appropriate management levels.

Grazing by free-roaming horses can reduce shrub density, vegetation cover and abundance of key resources used by sage-grouse, the researchers said. About 12% of the West’s sage-grouse habitat is within BLM herd management areas. In Wyoming, sage-grouse co-occur in about 99% of BLM herd management areas—11% of the state’s total sage-grouse habitat.

This research focused on sage-grouse in Wyoming’s Atlantic Rim, Red Desert, Jeffrey City and Stewart Creek areas. The scientists captured and monitored 995 adult female grouse, and monitored 1,075 nests, 372 broods and 136 juveniles. They also looked at the numbers of free-roaming horses in those areas. For comparison, grouse were captured and monitored in areas outside of BLM horse management areas as well.

“There was strong or moderate evidence that overabundant free-roaming horses negatively affected nest, early and late brood, and juvenile (grouse) survival,” the researchers wrote, noting relative survival declines ranging from 8.1% to 18.3%.

Notably, survival rates for adult female sage-grouse in areas where horses didn’t exceed appropriate management levels were similar to survival rates in areas outside horse management areas, where exposure to free-roaming horses was assumed to be zero.

The overabundance of 1 iconic species is harming another, the researchers concluded. MeV

For more information:
Beck JL, Milligan MC, Smith KT, et al. Free-roaming horses exceeding appropriate management levels affect multiple vital rates in greater sage-grouse. J Wildl Manage. 2024 Sept. 18. Doi: 10.1002/jwmg.22669 https://wildlife.onlinelibrary.wiley.com/doi/10.1002/jwmg.22669.