Wednesday, January 19, 2011
Feral horses, cows, and the greater sage-grouse share the same habitat in many places within the shrub-steppe biome. The sage-grouse is not yet listed under the Endangered Species Act, but management practices implemented to protect them and their habitat are critical to prevent them from having to be listed. Their population decline is largely a result of habitat loss due to the depletion of the shrub-steppe biome that the sage-grouse depend upon for survival. Their habitat is being damaged by changes in fire regimes, invasive alien species, and the conversion of land for agricultural uses and development (Crawford et al. 2004). Although there are no empirical data to support the hypothesis, it is often suggested that overgrazing and the resulting decline in herbaceous understory have resulted in the decreasing population of sage-grouse (Wambolt et al. 2002).
Since the height of grasses and other vegetative cover is an important component of sage-grouse nesting sites, grazing may have a direct effect on this native species if vegetation is clipped too short. The sage grouse are thought to be an umbrella species, a species that when protected can protect others as well, for the vertebrate inhabitants of the shrub-steppe. The greater sage-grouse are an important indicator for the health of rangelands (Rowland et al. 2006, Wambolt et al. 2002).
Though the rangelands that house the sage-grouse are home to many grazing species, the two that exert the greatest influence, and that are surrounded by the greatest controversy, are cattle and feral horses. Research has found that feral horses have direct influences in sagebrush communities on structure and composition of vegetation, as well as influence on soils by increasing penetration resistance through compaction (Beever & Aldridge, Unpublished). Though horses are unique in that they are an alien species federally protected under the Free-Roaming Wild Horse and Burro Act of 1971 to be managed as an “integral part of the natural system” (Public Law 92-195), the lands they roam are also mandated for multiple uses by the Federal Land Policy and Management Act of 1976 (Public Law 94-579). This includes the highly controversial use of livestock grazing, which allows livestock to compete for forage with the feral horses and wildlife. The livestock exert direct influences of their own onto sagebrush communities by affecting biodiversity and ecosystem processes, such as nutrient cycling. Grazing reduces herbaceous understory and increases woody vegetation (Beck & Mitchell 2000), which can have an impact on habitat for sage-grouse.
Livestock grazing is managed by changing the location, time, and intensity of herds, and by limiting use of certain areas. Livestock owners who graze their cattle on public lands can be told to turn their cows out at a later date to avoid overconcentration of grazing on leks, which are grassy areas surrounded by sagebrush where male grouse attract females during the breeding season. Feral horses, on the other hand, are not managed as either wildlife or livestock. Their population is controlled by round-ups that remove excess numbers from the range. Round-ups occur approximately every four years. This is often the time it takes for a horse population to double as their growth rates are as high as 20% a year (Eberhardt et al. 1982, Garrott et al. 1991). These differences in management can have implications for how each species impacts sage-grouse habitat.
This thesis will address issues pertaining to the management of large ungulates and how their grazing affects sage-grouse habitat within the shrub-steppe biome and will provide the Bureau of Land Management (BLM) in Oregon state with field research on the sage-grouse habitat located in the Burns District’s Riddle Mountain herd management area adjacent to the Kiger HMA (HMA) (See Map 1). These are the herd management areas that are home to the famous Kiger horse, a breed of mustang that still holds onto its genes that date back to the original horse brought by the Spanish in the 16th century. The appropriate management level (AML) for this herd area, determined by forage availability, is 33-56 horses. As of my last conversation with the Burns District Rangeland Specialist, Bill Anderson, there were about 100 head of excess horses in the HMA (Anderson, 2010). According to the recent Oregon Fish and Wildlife draft plan to maintain and enhance sage-grouse habitat, the BLM is strongly encouraged to prioritize round-ups in horse areas that are both above AML and include known populations of sage-grouse (Hagen, Anthony, & Oregon 2010). Along with horses grazing on Riddle Mountain, Anderson said there are nine hundred head of cattle are turned out to graze in July. Therefore, my study will provide data relevent to when grazers are at maximum and determine if too many grazers have significant impact to vegetation that will harm sage-grouse. With my results, management can adjust to prevent detrimental utilization of ranges in the Riddle Mountain area.
My objective is to provide data that will allow me to determine if these large omnivores have a measurable impact on the shrub-steppe biome. I will also determine which grazer is more damaging to the vegetation by setting up cages, where cattle will go under to graze within, but horses will not (Anderson, 2010). In the Riddle Mountain HMA, there are known grazing leases, horses, and sage-grouse leks. This study will fill a data gap in the current body of research, providing direct evidence of the presence or lack of grazing impacts by wild and domestic ungulates on these habitats. My hope is that this study will be useful in determining if there are any appropriate measures that can be taken to prevent degradation to the shrub-steppe biome.
My research will add to the collection of data gathered concerning the impact of feral horses and livestock on public lands, and it will have an impact on management decisions regarding the preservation of the shrub-steppe habitats that are inhabited by many species, including the greater sage-grouse.