As cheatgrass spreads across the West, a new study finds that the annual invasive grass is impacting mule deer movement patterns in Wyoming – and will likely affect mule deer habitat in the future unless something’s done.

In the study, Mule Deer Response to Invasive Annual Grasses: Implications for Strategic Management in Sagebrush Priority Areas, published in the journal Rangeland Ecology and Management, researchers analyzed movement patterns from 115 GPS-collared mule deer where range maps found a variation in plant cover, according to WyoFile. 

In areas with less than 10% of cheatgrass, deer stuck around. If the grass spread to 10% to 16%, deer stopped frequenting the area – and areas with over 20% of the invasive plant deer avoided altogether. 

Cheatgrass is an annual invasive grass that was brought to North America by European settlers in the 1800s, according to Working Lands for Wildlife. It was introduced as a way to provide enough forage for grazing livestock since native grasses and forbs couldn’t keep up. Today, it overuns native vegetation in sagebrush landscapes and dominates portions of Utah, Nevada, Idaho, Oregon and California. It’s easy fuel for wildfires because, at maturity, it’s long and dry, and “short-circuits a lot of the important ecological cycles,” meaning loss of important habitat for a lot of wildlife. Not to mention it’s a poor equivalent for forage as it doesn’t stay green enough to provide nutrition to grazing animals. 

Basically? Cheatgrass is bad news, but so common, that it’s found in 49 states. 

“Deer are super selective foragers, and if they can choose between native and non-native, they will go where there’s something better,” said Jill Randall, Wyoming Game and Fish Department’s big game migration coordinator, regarding the results of the study. “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.”

One solution offered by the authors of the study involves using an EPA-approved herbicide called Rejuvra, which is already being used on some rangelands in the West, according to WyoFile. 

Rejuvra kills cheatgrass’s annual seeds, but leaves perennial grasses “relatively untouched.” It’s an expensive solution, according to Randall, but “critical for maintaining sagebrush ecosystems” used by mule deer, sage grouse, lizards and other wildlife. In areas where Rejuvra has been used, game cameras have shown mule deer returning to areas once the cheatgrass has been removed.

To read the full study, click here.