If the spread of cheatgrass and other annual invasive grasses is left unchecked, mule deer will lose key habitat in northeast Wyoming, a new University of Wyoming study suggests.
The study, published this week in the journal Rangeland Ecology & Management, found that mule deer avoid places where invasive annual grasses such as cheatgrass are overtaking sagebrush and grassland habitats. If the spread of these weeds goes unchecked, the study predicts that mule deer may lose more than half of their high-quality habitat in northeast Wyoming in the next two decades.
Fortunately, the research also shows that targeted management of invasive annual grasses can reverse this trend.
“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 new study and senior research scientist at UW. “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.”
Mule deer rely on healthy sagebrush and other native perennial plants for food. Cheatgrass and other invasive grasses are less nutritious and provide poor-quality habitat for these big game, especially in the summer and fall, the researchers note.
To track how mule deer used the landscape, including in areas containing invasive grasses, Smith and his colleagues analyzed the movement of more than 100 animals with GPS collars. Then, they overlaid the animals’ movements with the type of vegetation cover using the Rangeland Analysis Platform.
Mule deer prefer habitats with low levels of invasive annual grasses, the researchers found. However, once invasive annual grasses covered more than 13% of sagebrush lands, mule deer began to use those areas less. When invasive grasses covered more than 20% of the land, mule deer strongly avoided those areas.
After compiling deer movement and vegetation data, the researchers forecasted what the future might look like under two scenarios: one where invasive grasses continued to spread across the sagebrush biome at current rates, and another where active management actions (like applying herbicides) reduced the weeds and allowed native perennials to recover.
If nothing is done to protect core sagebrush areas from the spread of these weeds, invasive annual grasses could potentially reduce high-quality mule deer habitat across northeast Wyoming by 62% in the next 20 years, the results suggest.
“Mule deer are already facing habitat loss and fragmentation across the West. Doing nothing isn’t an option,” says Jerod Merkle, Knobloch Professor of Migration Ecology and Conservation at UW and senior author of the study. “Luckily, we now have the tools, the science and the broad support to combat the spread of invasive annual grasses in Wyoming and beyond.”
According to the new research, if targeted treatments are applied in places already prioritized by existing conservation frameworks like the Sagebrush Conservation Design, the potential loss of habitat can be completely reversed.
These frameworks prioritize treating areas with only low to moderate levels of invasive grasses, as well as places near still-intact, core sagebrush landscapes.
“The predicted maintenance of high-quality mule deer habitat is stunning if we proactively defend core sagebrush from invasive annual grasses,” says Brian Mealor, director of the University of Wyoming’s Institute for Managing Invasive Grasses Invading Natural Ecosystems (IMAGINE) and co-author of the study. “It gives a lot of hope that we can protect wildlife while also improving rangelands as a whole.”
Partners across the West are collaborating on novel, win-win solutions for landowners and land managers interested in conserving sagebrush rangelands and the wildlife they support.
For instance, IMAGINE and the USDA Natural Resources Conservation Service’s Working Lands for Wildlife provide tools, funding, and technical support for managing invasive annual grasses. The Wyoming Migration Initiative at UW—in collaboration with the U.S. Geological Survey, state wildlife management agencies and Tribes—provides migration maps and other science-based strategies that help prioritize where to treat weeds to best benefit the West’s iconic big game species.
“Sagebrush rangelands support rural economies through ranching, hunting, and recreation,” says Jeremy Maestas, an ecologist with the USDA Natural Resources Conservation Service (NRCS). “This research clearly shows that now is the time to defend and restore the sagebrush biome, not just for mule deer but for all the people and wildlife who live here.”
The research was supported by the Wyoming Game and Fish Department, USDA NRCS Working Lands for Wildlife, Western EcoSystems Technology, Inc., the Knobloch Family Foundation, and the University of Wyoming.
To read the full paper, visit https://bit.ly/mule-deer-iag-2025. Contact Merkle at jmerkle@uwyo.edu or (307) 766-5448 with questions.
