Ovis canadensis sierrae

This population formerly was included in Ovis canadensis californiana, but Wehausen and Ramey (2000) transferred the population at the type locality of californiana (Yakima Co., Washington) to O. c. canadensis, and transferred more southern populations to O. c. nelsoni. The Sierra Nevada bighorn population, however, appears to be a lineage distinct from O. c. nelsoni (Ramey 1995, Wehausen and Ramey 2000). Wehausen et al. (2005) reviewed the taxonomy of bighorn sheep in the Pacific states and determined that correct name for the Sierra Nevada population is Ovis canadensis sierrae Grinnell, 1912. USFWS (2008) also reviewed available taxonomic information and agreed that the Sierra Nevada population should be recognized as a distinct subspecies (Ovis canadensis sierrae).

Small range in the Sierra Nevada, California; five disjunct populations totaling a few hundred individuals; imminently threatened by mountain lion predation and disease.

Sierra Nevada bighorn sheep inhabit portions of the southern Sierra Nevada along the eastern boundary of California in Tuolumne, Mono, Fresno, Inyo, and Tulare counties. Habitat occurs from the eastern base of the range at elevations as low as 4,790 feet (1,460 meters) and extends to above 14,100 feet (4,300 meters) in some areas(Wehausen 1980:3, 82, cited by USFWS 2008). Habitat is patchy, so the distribution is naturally fragmented (see USFWS 2008).

Historical range: eastern slope of the southern Sierra Nevada, and, for at least one subpopulation, a portion of the western slope, from Sonora Pass in Mono County south to Walker Pass in Kern County, a total distance of about 346 km (USFWS 1999). The extant range begins in the Lee Vining area in Mono County and extends south to the Mount Langley area in Inyo County, a linear distance of approximately 110 miles (177 kilometers) (USFWS 2008).

Of the 12 units encompassed by the critical habitat designation, 7 are currently occupied (USFWS 2008); these can be regarded as distinct occurrences or subpopulations.

"Factors limiting Sierra Nevada bighorn sheep recovery include disease, predation, low population numbers and limited distribution, availability of open habitat, and potential further loss of genetic diversity due to small population sizes and inadequate migration between them. Since the vast majority of Sierra Nevada bighorn sheep habitat is publicly-owned land, loss of habitat has not been a limiting factor. However, management of bighorn sheep habitat (e.g., fire suppression) can result in habitat alterations and loss of key dispersal corridors connecting herds, which could be limiting factors." [Source: USFWS 2007]

Existing subpopulations are very small and are imminently threatened by mountain lion predation (USFWS 1999, 2000), which may need to be managed in some areas. As the numbers of Sierra Nevada bighorn sheep increase with recovery, the need for mountain lion control specifically for the benefit of Sierra Nevada bighorn sheep should be reduced and eventually eliminated (USFWS 2008). Continued suppression of fires in Sierra Nevada bighorn sheep range is a threat as habitat succession alters the abundance of suitable bighorn sheep habitat and increases bighorn sheep vulnerability to mountain lion predation (see USFWS 2008).

Although die-offs of bighorn sheep due to disease have occurred unrelated to domestic sheep (Miller et al. 1991: 534-540, cited by USFWS 2008)), a major contributing factor responsible for the decline of Sierra Nevada bighorn sheep populations over the years is thought to be the introduction of diseases by domestic livestock. Clifford et al. (2007:18, cited by USFWS 2008) indicated concern regarding the probability of a respiratory disease case occurring from disease transmission between domestic sheep and Sierra Nevada bighorn sheep, especially in the northern part of bighorn sheep range. Domestic grazing allotments within the vicinity of Sierra Nevada bighorn sheep habitat should be reviewed and activities should be modified as necessary to prevent competition and contact between the domestic livestock (sheep and goats) and bighorn sheep.

Domestic livestock (sheep, goats, cattle) grazing practices that result in overgrazing or allow for contact between domestic sheep, domestic goats, and Sierra Nevada bighorn sheep is a threat. Domestic livestock could compete with Sierra Nevada bighorn sheep for forage at some level in designated critical habitat units (USFWS 2008).

Some population units require special management considerations or protection to address impacts from development activities, including road construction and maintenance within Sierra Nevada bighorn sheep habitat (USFWS 2008).

Patented mining claims occur within habitat used by the Sierra Nevada bighorn sheep, but the area of the claims is small. Mining activities and associated facilities threaten bighorn sheep by causing the loss of vegetation structure required for foraging activities; the destruction of habitats used for escape, bedding, lambing, or connectivity between ranges; and the disturbance due to ongoing mining activities. Disturbance could modify bighorn sheep behavior or cause them to flee an area (USFWS 2008).

It remains unclear how significantly Sierra Nevada bighorn sheep may be affected by human disturbance. Increases in human uses of bighorn sheep habitat, including recreational activities such as rock and ice climbing, mountaineering, ski touring, hiking, camping, pack station establishment, snowmobiling, and off-road vehicle use may cause detrimental disturbance to Sierra Nevada bighorn sheep in some areas (USFWS 2008). Impacts to the habitat could occur through trampling and reduced vegetation structure due to grazing by pack animals (USFWS 2008).

Management actions to protect Sierra Nevada bighorn sheep habitat from catastrophic, naturally occurring events may be necessary. Events such as wildfires and avalanches could temporarily destroy large areas that provide summer or winter foraging habitat (USFWS 2008).