Ecosystem Modeling of the Pryor Mountain Wild Horse Range

Coughenour, Michael

INTRODUCTION The US Bureau of Land Management (BLM) is responsible for the maintenance of a thriving ecological balance on the Pryor Mountain Wild Horse Range (PMWHR) and has deemed it necessary to conduct periodic management removals of wild horses since 1970 to halt and prevent range degradation (USBLM 1997). The PMWHR includes lands which are administered by the National Park Service (NPS), specifically, portions of the Bighorn Canyon National Recreation Area (BCNRA), as well as lands administered by the BLM and the U.S. Forest Service (Hall 1972; US BLM 1984) . It has been federally mandated that wild horses have access to the NPS lands, by inclusion of the BCNRA in the designated PMWHR. However, in setting the appropriate numbers of wild horses, other NPS management objectives must be weighed, including protection of native plant communities and wildlife. In the past, horse carrying capacities have been calculated using a widely accepted method that sums up total forage biomass, applies a proper use factor of 50% utilization. The appropriate number of horses on the PMWHR must be explained in terms of measurable and predictable changes brought about by different horse densities in the abundance and composition of soils, plants and wildlife, and in the condition and welfare of the horses. These responses depend on plant species, the levels and patterns of herbivory, climate, soil properties, and their distributions on the landscape. Forage varies annually, and spatially over the landscape. Horse distributions vary seasonally, and parts of the landscape are used more heavily than others. Although it is unlikely that these wild horses will ever be allowed to self-regulate, there is a need to estimate the results if that were to occur. A general approach is necessary, which can assess the effects of herbivory on soils, plants, and animals in a dynamic and spatially heterogenous landscape. Here, a neutral, ecosystem modeling approach was used, which makes no specific assumptions about management objectives, levels of proper use, ideal range conditions, or equilibrial biotic communities. The objectives of this study (Coughenour 1999) were to: (a) evaluate a general, ecosystem modeling approach to support more informed policies for managing large ungulates; and (b) assess the effects of different numbers of horses on ecosystem structure and function on the PMWHR. This is an alternative approach than traditional approaches to evaluating carrying capacity based solely on forage supply or animal population dynamics.