Ovis canadensis nelsoni

Wehausen and Ramey (2000) examined variation in skull and horn characters in populations from the Great Basin north to British Columbia and Alberta and concluded that: (1) O. c. auduboni does not warrant recognition separate from O. c. canadensis; (2) populations of O. c. "californiana" in British Columbia and Washington are best treated as O. c. canadensis; (3) extirpated native populations of O. c. "californiana" in northeastern California, Oregon, northern Nevada, and southwestern Idaho should be included in O. c. nelsoni (recognizing that some transition to O. c. canadensis probably occurred along that northern boundary); and (4) O. c. californiana is restricted to the central and southern Sierra Nevada in California. However, the type locality of O. c. californiana is Yakima Co., Washington, a locality now assigned to O. c. canadensis; thus, the Sierra Nevada population is not part of O. c. californiana, but is either O. c. canadensis or a distinct subspecies with a different name. The evidence of Wehausen and Ramey (2000) that this population is fixed for a unique mtDNA haplotype would indicate the latter. Wehausen et al. (2005) and USFWS (2008) reviewed available taxonomic information and concluded that the Sierra Nevada population should be recognized as a distinct subspecies (Ovis canadensis sierrae).

Ramey (1995) examined mtDNA variation in O. canadensis populations in the southwestern United States and adjacent northern Baja California and found a lack of concordance between mtDNA haplotype distributions and current subspecies designations. Additionally, Ramey (1993) and Wehausen and Ramey (1994) found no morphological support for the separation of desert sheep into four subspecies. Ramey (1995) suggested that populations formerly considered to be O. c. nelsoni, O. c. mexicana, O. c. cremnobates, and O. c. weemsi probably should be recognized as a single polytypic subspecies on the basis of morphological and mtDNA data. As such, the subspecies would be O. c. nelsoni Merriam, 1897, and it includes bighorn sheep populations in the Peninsular Ranges of southern California and Baja Califdornia. A study of nuclear DNA markers is needed to test this taxonomic arrangement (Ramey 1995).

As of 2011, Integrated Taxonomic Information System (www.itis.gov; accessed 23 August 2011) did not recognize any subspecies of Ovis canadensis.

This taxon has a moderate range in western North America and occupies suitable habitat in many parts of this range. Significant declines over historical population and range extent levels have left populations at levels far below past numbers. As a highly managed taxon, most herds are actively managed and are stable or increasing although road mortality and loss of production due to human disturbance at spring lambing areas in California remain serious threats.

This taxon occurs in central and western Nevada south through southeastern California east to the margins of Arizona and north into the basin areas of southcentral Utah. The range extent, as estimated by prescribing a minimum convex polygon around the range polygons as described by Smith (2014), is about 85,500 km2.

There are currently an estimated 220 herds of this taxon within its prescribed range which, for purposes of this assessment, are considered the extant occurrences.

Threats to this taxon include loss and degradation of habitat from urban and agricultural development, habitat fragmentation and direct mortality from major highways and roads, diseases associated with contact with domestic sheep, availability and timing of summer water supplies, and the effects of climate change. Urban development and human disturbance are a primary threat factor in the California Peninsula range with over 100 acres of essential habitat lost to urban development in 2018. Urban development introduces other population stressors including increased vehicle traffic, and potential impacts from pools and fencing. In other parts of this taxon’s range anthropogenic activities including mining and recreational activities contribute to habitat fragmentation. Direct mortality from vehicle collisions is considered a threat in some areas and apparently increasing in key areas impacting movement between subpopulations and impeding recovery efforts. Historical surface water sources (Tinajas, seeps, and springs) are more frequently dry during the mid- to late-summer months from competition with other sources including invasive plants and introduced horses and burros reducing availability for this taxon. In many parts of the southwest, wildlife managers provide artificial water supplies to support populations. Disease is a primary concern throughout this taxon’s range. Poorly managed livestock, domestic animals from encroaching urban development and recreational users, and recent outbreaks of pneumonia contribute to this being a chronic threat factor. Climate change impacts likely include temperature and precipitation extremes becoming more frequent and water availability less predictable due to more frequent and prolonged drought. Stewart et al (2016) ranked the California peninsula bighorn sheep as moderately vulnerable in 3 of 4 assessed climate change scenarios and highly vulnerable in the fourth.