A U.S. Geological Survey (USGS) study published in the Journal of Wildlife Management estimates that greater sage-grouse could decline by as much as 70% by 2034 in areas that are occupied by free-roaming horses if the horse populations continue to increase unchecked at current rates. Sage grouse populations have experienced declines in arid sagebrush environments where there is habitat loss. Increasing numbers of free-roaming horses in these sagebrush ecosystems are causing habitat degradation, with wildfire and other disturbances also playing a role in the sage grouse declines.

“While this study highlights the challenges of maintaining biodiversity in sagebrush environments, it also provides valuable insights into the options for wildlife management,” said David Applegate, associate director exercising the delegated authority of the USGS Director. “It’s an example of how the USGS provides landscape-scale science and data about resource conditions, trends and interactions that state and federal wildlife managers can trust to help make informed and effective decisions.”

In areas where free-roaming horses are at or below the appropriate management levels (AMLs) established by the Bureau of Land Management (BLM) the impacts to sage grouse populations were similar to areas without horses. According to the researchers, “This suggests that the maximum AMLs are effective at neutralizing the adverse impacts that horse populations have on sage-grouse populations and that free-roaming horses have the potential to coexist with native wildlife under the right management approach.” A related analysis also determined that for every 50% increase in horse abundance above AML, sage grouse abundance is likely to decline by 2.6%.

“Preserving the integrity of sagebrush ecosystems within the paradigm of multiple-use is a common goal of management agencies tasked with public lands stewardship,” said Peter Coates, a research wildlife biologist with the USGS Western Ecological Research Center and lead author of the study. “The results of the study indicate that coexistence is possible for free-roaming horses and sage-grouse if horse populations are maintained below established AMLs.”