Desert Bighorn Sheep

The Desert Bighorn Sheep (Ovis canadensis nelsoni) is a subspecies of Bighorn Sheep that occurs in the desert Southwest regions of the United States.

Populations of the Desert Bighorn Sheep declined drastically with European colonization of the American Southwest beginning in the 1500s. As of 2004, Desert Bighorn Sheep numbers are extremely low, although the overall population trend has increased since 1960. These declines were followed by a period of population stabilization that was ascribed to conservation measures. The decline of Desert Bighorn Sheep probably mirrored the pattern of decline of the overall bighorn population. Desert Bighorn Sheep population trends have been upward since the 1960s when their population was estimated at 6,700-8,100. In 1980 Desert Bighorn Sheep populations were estimated at 8,415-9,040. A state-by-state survey was conducted a few years later and estimated the overall U.S. Desert Bighorn Sheep population at 15,980. The 1993 estimate of the population is 18,965-19,040.

Status and Trends by State

Arizona
Historically, Desert Bighorn Sheep occurred on all mountain ranges and plateau slopes in the southern, northern, and western sections of Arizona. In spite of early protection (beginning in the 1880s), researchers believed that bighorn populations declined until the 1950s.

Arizona began a limited hunting program in 1953 and reintroduction programs in 1958. The Arizona Game and Fish Department conducts annual helicopter surveys. The estimated 1960 population was at about 3,000-3,500. In 1993 the population had increased to an estimated 6,000.

California

Desert Bighorn Sheep occupied desert mountains in southeast California in historical times. California protected bighorn in 1883, and by 1960 the estimated population was at about 2,150-2,450. The state began transplanting in 1971 and permitted hunting beginning in 1986. In 1993 the populations were estimated at 4,300-4,325, with the breeds occupying about 50 mountain ranges.

The less common peninsular bighorn (O.c. cremnobates, under the 1940 definition) occurs in the desert mountains of southeast California from Palm Springs south to the Mexican border. From 1977 to 1993 this population declined from an estimated 1,171 to 400-425 individuals because of excessive lamb mortality. In 1992 the U.S. Fish and Wildlife Service proposed listing the peninsular bighorn as an endangered species. This subspecies also occurs southward into Mexico; populations there are larger. One survey estimated a population of 780-1,170 adult Desert Bighorn in northern Baja California.

Colorado
There is no scientific evidence that Desert Bighorn Sheep occurred historically in Colorado, although there is habitat in the state contiguous with Desert Bighorn Sheep habitat in Utah. Thus, Desert Bighorn Sheep probably occurred in the state, and became extirpated before subspecies' determinations could be made. The Colorado Division of Wildlife began transplanting Desert Bighorn Sheep in 1979. By 1993 populations containing approximately 475 bighorn had been established from the release of animals originally from Arizona and Nevada.

Nevada
Desert Bighorn Sheep historically occupied the central and southern portions of Nevada. Hunting the animals was prohibited from 1901 to 1952. Transplanting programs have been successful: between 1968 and 1988 more than 800 Desert Bighorn Sheep were transplanted. From these animals, 21 transplanted herds have been established. The 1960 estimated Nevada population was at 1,500-2,000 in 1960. The state began annual population trend counts in 1969. In 1993 the population was estimated at 5,294 animals, occupying 45 mountain ranges.

New Mexico
Although Desert Bighorn Sheep historically occupied mountain ranges and canyons in the southern part of New Mexico, by 1930 the animals were restricted to only four mountain ranges, and by the late 1940s were found in only two. In 1972 the state constructed the 3 km² (741 acres) Red Rock propagating enclosure and added brood stock. Transplants from the captive herd were subsequently made into the Big Hatchet, Peloncillo, and Alamo Hueco mountains.

The San Andres Mountain population was formerly the state's largest, but declined from 200 to fewer than 25 by 1991 because of psoroptic scabies. The 1960 New Mexican population was estimated at 400-500. In 1993 the estimated population was 295, of which 100 were at Red Rock.

Texas
Desert Bighorn Sheep appear to have occupied all the mountains in southwest Texas west of the Pecos River. In 1880 the population was estimated at 1,500 animals; some populations still existed in the late 1930s. By the mid-1950s all bighorns had become extirpated except for a small herd of 25; excessive hunting and competition with domestic livestock are believed to have been major factors in the final decline. In 1957 the Texas Game and Fish Department began a highly successful captive breeding and release program. By 1993 the free-ranging population was estimated at 310; 91 other sheep were in captivity.

Utah
Historically, Desert Bighorn Sheep occupied canyons and ranges in southern and eastern Utah. Significant population declines occurred in the 1870s, and the state did not permit hunting of bighorn from 1899 to 1967.

In 1967 limited hunting began, and in 1973 the state started an active transplant program. Between 1973 and 1990, over 250 Desert Bighorn Sheep were transplanted, establishing at least nine populations that augment four additional areas containing native populations. Utah, which has conducted aerial trend counts on bighorn since 1969, documented increasing populations statewide. Individual populations, however, have exhibited large increases and sudden declines. In 1993 the Desert Bighorn Sheep population was estimated at 2,200-2,250.

Future of Desert Bighorn Sheep

Since 1960 bighorn have increased in numbers, but their population levels are still low when compared with the estimates of pre-European numbers and the amount of available unoccupied habitat. The number of sheep in individual populations has fluctuated greatly. Population monitoring and efforts to restore Desert Bighorn Sheep must continue to ensure viable future populations.

References

• USGS Report: Our Living Resources - Desert Bighorn Sheep (http://biology.usgs.gov/s+t/noframe/r039.htm) by Henry E. McCutchen of the National Biological Service (adapted public domain text)