Free-roaming horses may be a charismatic symbol of the Western U.S., but new research shows that when their numbers grow, they tend to gallop roughshod over one of the region’s most imperiled birds.
A University of Wyoming study published in the November Journal of Wildlife Management documents how sage grouse numbers plummet when the wild horse population exceeds three times the number the federal government says the land can support. The problem is further complicated by the fact that it couples a free-roaming mammal that’s managed by the federal government — and defended by legions of staunch advocates — with sage grouse, a flamboyant chicken-sized bird that dances at open-air discos known as leks. The birds have been put forward as a candidate for the endangered species list almost 10 times in the last couple of decades.
“Overabundant horses simplify habitat and make it less suitable for sage grouse,” said Jeffrey Beck, the study’s lead author and a rangeland ecology professor at the University of Wyoming.
The study, which examined 15 field seasons’ worth of data from several areas in southwest Wyoming, found that sage grouse nests, chicks and juveniles all declined as horse numbers grew.
“There are healthy horse herds and healthy ecosystems out there, but in places that haven’t seen rain in 20 years, you’re really seeing conflict,” said Nicki Frey, a Utah State University wildlife extension professor who was not involved in the study.
“There are healthy horse herds and healthy ecosystems out there, but in places that haven’t seen rain in 20 years, you’re really seeing conflict.”
The Bureau of Land Management is responsible for managing free-roaming horses on Western public lands, a mission it has historically accomplished by rounding them up and sending them to sale barns across the country. But Beck says that as lawsuits to protect the horses increased and appetite to work with them decreased, the BLM found itself with so many animals that it was forced to pay to feed them somewhere else. In fact, a majority of the herd management areas in Beck’s study held more horses than the agency said the habitat could support, with some holding as much as three — even up to seven — times that number.
This impacts sage grouse in a variety of ways, Beck says. Horses trample the sagebrush that grouse need for nesting, for example, and they tend to gather at water, degrading riparian areas and adversely affecting the hydrology.
Beck did not consider the impact of public-land grazing, because, he said, the BLM accounts for the impact of livestock when it’s setting the population thresholds for horses.
“Even in areas with no cattle, there are issues with grouse and horses,” Frey added.
The study found that when horse numbers reach the BLM’s recommended threshold, sage grouse nest success drops by 4%. Triple the horse numbers, and twice as many sage grouse nests are likely to fail. That’s a big loss, Beck said, given that even in a good year, only about half of the nests succeed.
Horses overlap with sage grouse in only about 12% of the bird’s range, said Tom Christiansen, the Wyoming Game and Fish Department’s retired sage grouse biologist. And horses are far from the bird’s only problem: Sage grouse in states like Wyoming, where about 40% of Western grouse live, struggle with everything from oil, gas and mining and West Nile virus to prairie wildfires that burn sagebrush and usher in invasive species like cheatgrass. Horses, he said, are just “another issue in a long list of issues for sage grouse.”
They still seem to have a big impact on the grouse, though. Last year, the Eastern Shoshone and Northern Arapaho tribes on the Wind River Reservation in central Wyoming, using their tribal sovereignty, took matters into their own hands and rounded up about 6,500 horses. By the end of 2023, Tribal Fish and Game Director Art Lawson told a Wyoming legislative committee, the operation was a success, “The results are pretty much overnight,” he said.
“We’ve been doing a lot of tribal youth projects, getting kids outdoors,” Lawson said. “And we’re actually having to dodge sage grouse on the highways now because they’re everywhere.”
