UW research: Invasive grasses threaten mule deer, strategic action needed

Unchecked spread of cheatgrass and other annual invasive grasses could severely shrink mule deer habitat across northeast Wyoming, according to a new analysis led by University of Wyoming researchers. Using GPS-collar data from more than a hundred deer and high-resolution vegetation maps, the team found clear thresholds at which animals begin to abandon sagebrush country overrun by fast-spreading weeds. Without action, the models indicate that high-quality habitat could decline by roughly 62% within two decades. The same research also shows that targeted, proactive treatments can halt—and even reverse—those losses.

What the data reveals

Mule deer depend on sagebrush and other native perennials for energy-rich forage, particularly in summer and fall. In contrast, invasive annual grasses like cheatgrass green up early, then crash nutritionally and create dense carpets of dry fuel that foster frequent fires—conditions that degrade sagebrush communities and the food web mule deer rely on.

By integrating animal movement paths with vegetation cover layers from a rangeland mapping platform, researchers documented a strong behavioral response to weed encroachment. Deer used areas freely where annual invasives were sparse, but their use dropped once these grasses exceeded about 13% of sagebrush cover. When infestations surpassed roughly 20%, deer strongly avoided those habitats altogether.

Two futures: status quo vs. strategic defense

The team projected habitat outcomes under two scenarios. In a status-quo scenario where invasives continue spreading at current rates, high-quality mule deer habitat across northeast Wyoming would shrink dramatically in the next 20 years. In a proactive scenario—where land managers treat invasives and bolster native perennials—the models show that habitat quality can be maintained or improved.

Notably, the most effective strategy is to defend and reinforce core sagebrush areas and the lightly to moderately invaded lands surrounding them. By prioritizing these zones, treatments prevent small problems from becoming unmanageable and keep intact rangelands connected for migrating wildlife.

Why annual invasives hit mule deer so hard

• Poor nutrition timing: Cheatgrass delivers early-season green-up but senesces quickly, leaving little high-quality forage when deer need it most.
• Fire feedbacks: Dense, fine fuels promote more frequent and larger wildfires, which suppress slow-growing sagebrush and favor more weed expansion.
• Habitat fragmentation: As invasives spread, deer increasingly bypass affected patches, breaking up movement corridors and seasonal ranges.

How technology is sharpening conservation

This work underscores how modern tools are transforming rangeland stewardship. GPS collars record fine-scale movement patterns, while satellite-derived platforms track vegetation change across millions of acres. Combining those data streams allows researchers to pinpoint invasion thresholds, forecast outcomes under different management choices, and prioritize treatment zones where benefits will be greatest for wildlife.

These insights dovetail with regional conservation frameworks that identify core sagebrush landscapes and surrounding buffers. Aligning treatments with those priorities maximizes return on investment: defend intact strongholds, treat advancing fronts, and monitor recovery to adapt quickly.

What land stewards can do now

• Defend the cores: Focus first on intact sagebrush and adjacent areas with low to moderate invasion. Preventing spread is more cost-effective than reclaiming heavily infested sites.
• Use integrated treatments: Pair selective herbicides with reseeding of native perennials and adjust grazing timing to give desirable plants a competitive edge.
• Track change with remote sensing: Regularly review rangeland imagery and on-the-ground monitoring to detect new infestations early and verify treatment outcomes.
• Coordinate across boundaries: Invasives ignore fences. Align priorities among private landowners, tribes, agencies, and NGOs to treat at the landscape scale.
• Safeguard migration routes: Overlay treatment plans with current migration maps so work occurs where it will preserve connectivity and seasonal ranges.

The bottom line

Invasive annual grasses are reshaping the sagebrush biome, but their trajectory is not inevitable. Data from Wyoming’s mule deer make the trade-offs clear: when weeds pass specific thresholds, deer leave; when managers act early and strategically, high-quality habitat persists. With the tools now available—from GPS-collar insights to satellite vegetation maps—rangeland managers can target the right acres at the right time, protecting both wildlife and the rural economies tied to healthy sagebrush country.