INTRODUCTION
By exploring bighorns’ historical population dynamics, one can better understand how much is at stake with the risk of bighorn-domestic sheep disease transmission. “[Bighorns] have made the transition from relative abundance to one of the rarest ungulates in North America” (Valdez and Krausman 1999, 19-21). Euro-American settlement caused precipitous decline of bighorn populations, but from the mid-twentieth century to current times, wildlife managers have worked hard to restore bighorns (Toweill and Geist 1999).
FORMER ABUNDANCE & HISTORIC RANGE
Statements from Western settlers and explorers “indicate that bighorn were once more widely distributed and numerous than they are presently” (Valdez and Krausman 1999, 240). Different estimates of bighorn numbers in “pristine times” range from 500,000-4,000,000. In 1991, about 185,000 wild sheep lived in North America. The vast majority of these (nearly 100,000) were Dall’s sheep, and there were about 71,000 bighorns (Valdez and Krausman 1999). Rocky Mountain bighorns used to be more widely distributed (Toweill and Geist 1999). According to Toweill and Geist, “many scientists who have examined archaeological evidence and reviewed accounts left by explorers believe that bighorn sheep may well have been the most common big game animal in mountainous regions” (1999, 67).
Toweill and Geist also state: “Prior to settlement, California bighorn sheep were widespread . . . . They occupied nearly all of the relatively dry mountain ranges, basins and canyons west of the Rocky Mountains to the Cascades of British Columbia, Washington, and Oregon and were the bighorn sheep characteristic of the Great Basin” (1999, 123-124). Biologist Vernon Bailey noted that their range also included lava beds and buttes east of the Cascades (Toweill and Geist 1999).
Desert bighorns once ranged further into Mexico (Toweill and Geist 1999). Their relatives, the Sierra bighorns, used to live further north and south. They ranged from the Sonora Pass region down to Olancha Peak. In historic times, the Sierra may have hosted over 1,000 bighorns (USFWS 2007).
DECLINE
In the 1800s and early 1900s, bighorn numbers in the continental U.S. dramatically declined. Unregulated hunting was one of the most direct causes (Valdez and Krausman 1999). People killed bighorns for food, trophies, and the wild game market (ODF 2003). Decline contributors also included habitat loss, alteration, and disturbance from human activities (Valdez and Krausman 1999).
Some of the most significant decline factors related to livestock. Cattle and domestic sheep damaged habitat through overgrazing in the southwestern U.S. and Mexico in the early 1800s and in the northwestern U.S. in the late 1800s and early 1900s (Valdez and Krausman 1999). Competition with livestock for forage reduced bighorn numbers. However, livestock disease (especially from domestic sheep) was more devastating to bighorns than competition for food (Toweill and Geist 1999).
With Western settlement, shepherds herded domestic sheep over vast stretches of bighorn habitat (ODFW 2003). For example, by 1820, an estimated 3,000,000 domestic sheep grazed in central and northern New Mexico (some of which was Rocky Mountain bighorn range). Additionally, from 1911-1920, more than 10,000 domestic sheep grazed the Standley Allotment in northeast Oregon’s Wallowa Mountains (Coggins 2010).
All bighorn subspecies sharply declined because of Euro-American settlement. Rocky Mountain bighorns were extirpated from: New Mexico by 1902, Washington by 1917, Oregon by 1945, and Nevada by 1946. Utah likely had fewer than 100 Rocky Mountain bighorns by the mid-1930s. In the continental U.S., California bighorns were completely wiped out. They were extirpated from: California by about 1913, Oregon by 1917, Washington by 1925, and Idaho and Nevada by 1940. Only two populations of desert bighorns persisted in New Mexico by 1955. The last sighting of native desert bighorns in Texas occurred in 1960. Only one viable population of desert bighorns survived in Utah by the mid-1960s (Toweill and Geist 1999). By the 1970s, Sierra bighorns lived in only two populations that together totaled about 250 animals. Bighorns in the Sierra reached a low point of about 100 animals in 1995 (USFWS 2007).
The Desert Bighorn Council’s technical staff summarizes why bighorn decline was especially damaging compared to that experienced by other big game animals:
“Following enormous population declines in the late 1800s and early 1900s, bighorn populations did not recover, in contrast to other wildlife species such as mule deer (Odocoileus hemionus) and elk (Cervus elaphus). Bighorns have demonstrated much less tolerance than other native North American ungulates to poor range conditions, interspecific competition, overhunting, and stress caused by loss of habitat. Furthermore, they have shown a much greater susceptibility to diseases” (1990, 33).
RESTORATION
Efforts to protect bighorns began with hunting regulations instituted in the 1800s. For example, in 1861, Nevada prohibited bighorn hunting from January to July. The state also fully closed bighorn hunting from 1901-1952 (NDOW 2001). In 1872, Montana passed its first hunting regulation affecting bighorns (a season limitation), and in 1895, the state “specified a bag limit of 8 sheep” (MFWP 2010, 12). In 1878, California passed legislation protecting all its bighorns from hunting (USFWS 2007). Oregon protected its bighorns from hunting in 1911 (ODFW 2003).
Governments also reserved habitat for bighorns, though some of these efforts failed to prevent extirpation. For example, in 1915, the Steens Mountain Game Refuge was established in Oregon to protect California bighorns (ODFW 2003). Also, in 1936, the U.S. Fish and Wildlife Service created the Desert National Wildlife Refuge in Nevada to protect desert bighorns (NDOW 2001).
Thanks to efforts largely initiated in the 1960s, wildlife managers have restored bighorns to many former ranges. Restoration was carried out with: trapping/transplanting actions, habitat management, installation of water sources, and biological research. Rocky Mountain and California bighorns have been established in former Audubon’s bighorn habitat, Rocky Mountain and desert bighorns’ ranges have expanded, and managers have largely restored California bighorns to the U.S. with transplants from British Columbia (Toweill and Geist 1999). Sierra bighorns had increased to about 465 animals by 2011 (Torres 2011). Regarding North American wild sheep in general, Toweill and Geist state: “Even though there have been many failures and setbacks, herds have grown dramatically” (1999, 198).
More organized advocacy for bighorn sheep (especially from hunters) and efforts to separate bighorns from domestic sheep have also contributed to ongoing restoration (Toweill and Geist 1999). Nonetheless, according to the U.S. Forest Service, in 2006, “the total number of bighorn sheep in the U.S. [was] thought to be less than 10% of presettlement numbers . . . . and most existing populations [were] relatively isolated and small, composed of fewer than 100 individuals. Over half of existing bighorn populations [were] the result of translocations” (USDA 2006, 2). Furthermore, disease (some of it possibly from domestic sheep) killed about 890 bighorns in the continental U.S. as recently as the winter of 2009-2010 (WAFWA 2010).
REFERENCES
Coggins, Victor L. 2010. Historic bighorn sheep disease outbreaks in western North America and mountain sheep extirpation from Oregon. In proceedings of Northern Wild Sheep and Goat Council’s 17th Biennial Symposium, Hood River, OR. June 7-11.
Desert Bighorn Council Technical Staff. 1990. Guidelines for the management of domestic sheep in the vicinity of desert bighorn habitat. In transactions of DBC’s 34th Annual Meeting, Hermosillo, Sonora, Mexico. April 4-6.
Montana Department of Fish, Wildlife, and Parks (MFWP). 2010. Montana Bighorn Sheep Conservation Strategy: 2010. Helena. http://fwpiis.mt.gov/content/getItem.aspx?id =397 46 (accessed October 15, 2011). [govt. doc.]
Nevada Division of Wildlife (NDOW). 2001. Nevada Division of Wildlife’s Bighorn Sheep Management Plan: October 2001. Reno. http://www.ndow.org/about/pubs/plans/bighorn_ management_plan.pdf (accessed October 15, 2011). [govt. doc.]
Oregon Department of Fish and Wildlife (ODFW). 2003. Oregon’s Bighorn Sheep & Rocky Mountain Goat Management Plan: December 2003. Salem. http://www.dfw.state.or.us/ wildlife/management_plans/docs/sgplan_1203.pdf (accessed October 15, 2011). [govt. doc.]
Torres, S. 2011. Status of Bighorn Sheep in California, 2011. Desert Bighorn Council. http://www.desertbighorncouncil.org/meetings/PowerPointPres/CA%20Status%20Report% 202011.pdf (accessed January 17, 2012).
Toweill, Dale E., and Valerius Geist. 1999. Return of royalty: Wild sheep of North America. Missoula, MT: Boone and Crockett Club and Foundation for North American Wild Sheep.
U.S. Forest Service (USFS). 2006. Risk Analysis of Disease Transmission Between Domestic Sheep and Bighorn Sheep on the Payette National Forest. McCall, ID. http://www.mwvcrc.org/bighorn/payette bighornreport.pdf (accessed October 15, 2011). [govt. doc.]
U.S. Fish and Wildlife Service (USFWS). 2007. Recovery Plan for the Sierra Nevada Bighorn Sheep. Sacramento. http://www.nps.gov/yose/naturescience/loader.cfm?csModule= security/getfile&PageID= 382279 (accessed October 15, 2011). [govt. doc.]
Valdez, Raul, and Paul R. Krausman, eds. 1999. Mountain sheep of North America. Tucson: The University of Arizona Press.
Western Association of Fish and Wildlife Agencies (WAFWA). 2010. WAFWA Wild Sheep Working Group recommendations for domestic sheep and goat management in wild sheep habitat: July 21, 2010. N.p.: WAFWA. http://www.wafwa.org/documents/wswg/WSWG ManagementofDomesticSheepandGoatsinWildSheepHabitatReport.pdf (accessed May 17, 2012).