Pryor Mountain Wild Horse Range Survey and Assessment

Ricketts, Matthew

The Pryor Mountain Wild Horse Range (PMWHR) was created in 1968 by a U.S. Department of the Interior Secretarial Order. This designation was the second of its kind in the United States, and directed that management of the wild horses be within a balanced program that considers all public values without any impairment to the land’s productivity. The PMWHR covers about 39,651 acres managed by the Bureau of Land Management (BLM), U.S. Forest Service (USFS), and the National Park Service (NPS). The area lies within two states, Montana and Wyoming. Due to widespread concern about the ability of the PMWHR to support wild ungulate populations, the BLM, as the lead management agency, asked the USDANatural Resources Conservation Service (NRCS) to conduct a comprehensive inventory and assessment of the health of the PMWHR and to determine what a sustainable number of horses would be. The area is located approximately 47 miles south of Billings, Montana and nine miles north of Lovell, Wyoming. The PMWHR is extremely diverse and complex topographically, geologically, and ecologically. It varies in environment and elevation from a sagebrush / saltshrub dominated cold desert (six inches MAP-mean annual precipitation) at about 3,850 feet in Wyoming, to a subalpine setting with subalpine fir (Abies lasiocarpa) and open meadows (27 inches MAP) in Montana at about 8,750 feet. NRCS methodology was used to inventory rangeland condition (similarity index), rangeland trend, and health. Ecological sites were identified and mapped in 1981 and provided the baseline for the inventory (BLM and SCS 1981). Thirteen new ecological site descriptions were developed to adequately address the complex and unique nature of the PMWHR. Three transects per section were installed on average to evaluate similarity index, apparent ecological trend, species composition by weight, noxious weed cover, biological crust cover, plant community type, and available forage production. One rangeland health assessment, along with a one hundred-point cover transect, was done per section. The similarity index averages about 30 percent across the entire PMWHR, apparent trend is down on 76 percent of the transects, severe erosion is occurring on approximately 57 percent of the landscape, and range health is functioning at a moderate to a moderate-extreme departure from the historic climax plant community. As of 2003 the PMWHR supports 161 feral horses and 100 bighorn sheep year around, along with 350 mule deer during the winter months with most leaving before summer. Dietary overlap between feral horses and other large ungulates is considered minimal. The present animal unit months (AUMs) of usable forage for feral horses was determined using geographical information system (GIS) and global positioning system (GPS) technology to determine slope classes used and to create grazability models based on travel distances to water. Grazability is the percentage of usable forage in an area allocated for use in order to maintain plant health. It is a relative term considering animals’ grazing preference of areas including variables such as distance from water, ecological sites, slope steepness, aspect, and species preference. Slopes over 30 percent were essentially unused during the months of May through Pryor Mountain Wild Horse Range Survey and Assessment 2 November, while slopes over 50 percent were essentially unused during the months of December through April.

Pryor Mountain Wild Horse Range Survey and Assessment

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