Wild horse and livestock influences on vegetation and wildlife in sagebrush ecosystems: Implications for refining and validating Appropriate Management Level (AML)
USGS researchers are conducting a comprehensive study of wild horse and livestock records across the greater sage-grouse range to investigate impacts on vegetation and wildlife (specifically, sage-grouse and songbirds). Researchers will use these results to evaluate Appropriate Management Levels for wild horse and burros, and projections of vegetation productivity under changing conditions.
Background
After passage of the Wild and Free Roaming Horses and Burros Act of 1971, the Department of Interior, Bureau of Land Management (BLM) became the primary manager of wild horses across the west. This new management responsibility necessitated that BLM determine the number of horses allowed to graze within designated Herd Management Areas (HMAs) sustainably with other land uses, including livestock grazing and human recreation, while promoting wildlife conservation. At that time, grazing levels were determined for cattle by forage accounting methods, so a similar approach was applied to wild horses, providing an Appropriate Management Level (AML) to help avoid adverse effects from overgrazing. However, resource managers continue to refine this approach to assess grazing management while considering environmental factors. That is, instead of calculating grazing level based primarily on forage production, many other parameters can be used to provide a broader assessment of grazing influences on the ecosystem. Recent studies revealed a negative relationship between sage-grouse population trends and wild horse populations exceeding maximum AML in the Great Basin, and sage-grouse trends corresponded with timing and level grazing based on allotment records across Wyoming, given local vegetation productivity. A comprehensive analysis of effects from wild horse populations and livestock would assess their influence on a host of ecosystem parameters in addition to forage availability. Such an approach also could be sensitive to perturbations from externalities.
Our project goals and anticipated outcomes
We are evaluating the relative influence of horse abundance and livestock grazing on components of ecosystem health, from fine- and broad-scale metrics of vegetation cover and biomass productivity on population performance among avian wildlife species that depend on sagebrush habitats. We will compile datasets including relevant grazing indices (for example, BLM population estimates for wild horses and relative use by livestock), and vegetation and wildlife metrics summarized at multiple spatial scales. From these datasets, we will study the responses of vegetation and wildlife populations to grazing under a range of observed environmental conditions. We then will project responses of vegetation and wildlife to grazing under future scenarios.
This study will provide a comprehensive approach to evaluating current grazing practices and densities of grazers on shared public rangelands. Importantly, broad-based ecological information from this study will help managers respond to complexities resulting from changing habitat conditions.
