Invasive Annual Grasses Threaten Mule Deer Habitat

RESEARCHERS STRESS DEFENDING AND GROWING CORE SAGEBRUSH AREAS IN PRIORITY MULE DEER HABITAT

Invasive annual grasses (IAGs)—such as cheatgrass, medusahead, and ventenata—are the leading cause of sagebrush degradation, according to the Sagebrush Conservation Design. Efforts like the Institute for Managing Annual Grasses Invading Natural Ecosystems (IMAGINE) Tech Transfer Partnership and WLFW’s Framework for Conservation Action in the Sagebrush Biome are improving how land managers and landowners protect and expand sagebrush cores against the negative impacts of IAGs.

While these programs address sagebrush rangeland health, research gaps remain—particularly in understanding how the West’s big game, especially mule deer, respond to increasing IAG infestations.

New research from WLFW-affiliated scientist Jerod Merkle, a biology professor at the University of Wyoming, reveals how these iconic ungulates react when IAGs dominate rangelands.

IAGs, like cheatgrass, green up early in spring and dry out quickly. They outcompete native grasses and forbs by using precious soil moisture and sunlight before native plants begin growing. Once dry, they become highly flammable, fueling severe rangeland fires. After fire, they rapidly recolonize burned areas, creating a destructive cycle: cheatgrass → fire → more cheatgrass → more fire.

Although wildlife and livestock may consume IAGs when other forage is scarce, their high silica content and low nutritional value make them a poor food source compared to native plants.

While Wyoming’s mule deer habitat has benefited from targeted conservation, little work has focused on protecting these areas from IAGs. Merkle and his team set out to understand how mule deer respond to rising IAG cover within key ranges.

They found mule deer began avoiding habitat when IAG cover reached 10-14% and strongly avoided areas once IAG cover approached 20%. The researchers also modeled future scenarios. In a “no action” case—where managers do not defend sagebrush cores—IAG spread at current rates would cause more than half the study area to experience major declines in mule deer habitat quality within 20 years. By contrast, targeted IAG treatments could significantly improve habitat, especially in priority areas.

From an ecosystem management perspective, the researchers emphasize that defending sagebrush cores before IAGs gain a foothold is the most effective strategy. IAG treatment efficacy wanes and costs become increasingly burdensome as infestations worsen, making prevention and early treatment key tactics for maintaining sagebrush rangeland health. Focusing efforts on maintaining and enhancing areas with high ecological integrity allows land managers to more effectively mitigate the negative impacts of IAGs on sagebrush rangelands, while simultaneously benefiting focal species such as mule deer.

MULE DEER RESPONSE TO INVASIVE ANNUAL GRASSES: IMPLICATIONS FOR STRATEGIC MANAGEMENT IN SAGEBRUSH PRIORITY AREAS

Abstract: Invasive annual grasses (IAG) are a widespread threat to the sagebrush steppe (Artemisia spp. L) rangelands and associated wildlife of the western United States. Multiple ungulate species including mule deer (Odocoileus hemionus Rafinesque) rely on sagebrush steppe seasonally. While substantial efforts have been made to conserve critical mule deer habitat, less attention has been given to mule deer habitat affected by IAGs and there is limited information about how mule deer respond to IAG invasions.

We evaluated mule deer resource selection in a sagebrush grassland community impacted by IAGs in northeast Wyoming. We then used empirical model estimates to forecast how IAG management could impact mule deer habitat in the future following a strategic IAG framework focused on defending and growing sagebrush core areas most threatened by IAGs. We found that mule deer responded to IAGs in a nonlinear pattern across all seasons and strongly avoided areas once cover exceeded approximately 20%. When projecting results 20 yr into the future, we found that over half of the study area is expected to experience significant declines in mule deer habitat quality if IAGs continue to spread at the same rate observed over the past two decades. However, with targeted IAG treatments, we predicted widespread improvements in mule deer habitat, particularly in priority areas where ecological integrity can be restored with future IAG management.

Our findings reinforce the emerging notion that ecosystem-based frameworks designed to defend and grow intact sagebrush steppe through strategic management efforts also have the potential to benefit species of conservation interest. As current conservation efforts to mitigate IAGs are not progressing fast enough to address the magnitude of the IAG problem in sagebrush across the west, strategic management efforts will be necessary to maintain important habitats for numerous sagebrush occurring wildlife.

Citation:  Kurt T. Smith, Brian A. Mealor, Jerod A. Merkle, “Mule Deer Response to Invasive Annual Grasses: Implications for Strategic Management in Sagebrush Priority Areas”, Rangeland Ecology & Management, Volume 103, 2025, Pages 128-137.

Permanent URL:  https://doi.org/10.1016/j.rama.2025.07.012

Acknowledgements:  Data collection was funded by the Wyoming Game and Fish Department. This work was supported by USDA-NRCS, the Knobloch Family Foundation, and University of Wyoming. We express sincere gratitude to H. Sawyer who shared mule deer location data for this research.