UW Study Finds Cheatgrass Could Choke Out Mule Deer Populations in Wyoming

In the sagebrush steppe ecosystems of northeast Wyoming, a silent invader is reshaping the landscape and threatening wildlife in the process. Cheatgrass (Bromus tectorum), an invasive annual grass, is spreading rapidly across the landscape, all the while altering habitats critical for local mule deer populations. A recent study from the University of Wyoming, published in Rangeland Ecology & Management, reveals how cheatgrass is driving mule deer away from their preferred habitats, and thus, contributing to population declines. 

“This is one of the first research studies to clearly assess the impacts of invasive annual grasses on mule deer habitat selection,” says Kurt Smith, lead author of the study. “The picture is grim if we sit back and do nothing. But there’s plenty of hope that we can maintain big game populations if we strategically treat cheatgrass and other invasives.”

Cheatgrass, introduced from Eurasia, is a highly competitive invasive species that thrives in disturbed environments. It outcompetes native plants, reduces forage quality, and increases wildfire frequency by creating dense, flammable monocultures. These changes degrade the sagebrush ecosystems that mule deer rely on for food, cover, and migration routes. Unlike native perennial grasses and shrubs, cheatgrass offers poor nutritional value, making it a suboptimal food source for herbivores like mule deer.

The University of Wyoming study, conducted in northeast Wyoming, used GPS collar data from over 100 mule deer and vegetation analysis via the Rangeland Analysis Platform to assess the impact of invasive annual grasses, particularly cheatgrass. The findings highlight a significant threat: as cheatgrass spreads, it reduces the availability of high-quality habitat, forcing mule deer to ultimately avoid these affected areas.

The study found that mule deer exhibit a nonlinear response to cheatgrass cover. At low levels (around 10-16%), mule deer tolerate the presence of cheatgrass, continuing to use these areas for foraging and movement. However, when cheatgrass coverage exceeds approximately 20%, deer strongly avoid these areas across all seasons. This avoidance effectively shrinks the available habitat, as mule deer prefer intact sagebrush landscapes with perennial grasses and shrubs that provide better nutrition and cover.

“Deer are super selective foragers, and if they can choose between native and nonnative, they will go where there’s something better,” says Jill Randall, Wyoming Game and Fish Department big game migration coordinator. “If cheatgrass is scattered in and among things, they will nibble, but 20% or above is pretty dominant. If they can go elsewhere and eat other native species, they will.”

This habitat loss compounds other stressors on mule deer populations, such as energy development, drought, and predation. In areas with less than 10% cheatgrass cover, mule deer behavior remains largely unaffected, but as coverage increases, the landscape becomes less hospitable, contributing to long-term population declines.

Using data spread across the past two decades, the researchers projected the future spread of cheatgrass under current trends. The results display an alarming trend, to say the least. According to researchers, within 20 years, up to 62% of the study area's high-quality mule deer habitat could become unusable if cheatgrass continues to invade at current rates. This loss would exacerbate existing declines in mule deer populations, which are already under pressure from multiple environmental and human-related factors.

These projections are sending a much needed sense of urgency in addressing the cheatgrass invasion. The cascading effects of not intervening could be detrimental, not only to mule deer, but a variety of other species and ecosystems as well.

Rather than simply highlighting the problem, UW researchers have tabled a number of strategies they believe would both aid in mitigating the impact of cheatgrass and help restore mule deer habitats. 

To combat the spread, researchers are advocating for a multifaceted approach. Targeted herbicide treatments, such as Rejuvra, which selectively eliminate annual grass seeds while preserving native perennials, are recommended for areas with low to moderate cheatgrass invasion, particularly near core sagebrush landscapes to help halt further spread. Additionally, the Sagebrush Conservation Design framework guides efforts to protect and expand intact habitats by prioritizing minimally invaded areas for restoration, aiming to reverse habitat loss and improve conditions for mule deer. The study also noted that collaborative partnerships among state agencies, landowners, and conservation groups are essential to fund and implement these large-scale restoration projects that should, by all means, boost shrub growth and attract more mule deer to treated areas.

The study’s authors went on to remind readers that the purpose of their findings is not simply to sound the alarm, but rather to inform, educate and initiate an appropriate response to a problem that today still remains manageable. 

“Sagebrush rangelands support rural economies through ranching, hunting and recreation,” said Jeremy Maestas, an ecologist with the USDA Natural Resources Conservation Service. “This research clearly shows that now is the time to defend and restore the sagebrush biome, not just for mule deer but for all of the people and wildlife who live here.”