Sierra Nevada Bighorn Sheep
Sierra Nevada Bighorn Sheep
Ovis canadensis californiana
|Listed||January 3, 2000|
|Description||A wild mountain sheep.|
|Habitat||Inhabits alpine and subalpine zones during summer, and high, windswept ridges or lower-elevation sagebrush-steppe habitat in winter.|
|Food||Various herbaceous plants and shrubs.|
|Reproduction||Gives birth to 1 or 2 precocious kids, which are reared by the female parent.|
|Threats||Excessive hunting in the past, effects of introduced bovine diseases, and perhaps mortality caused by natural predators.|
The Sierra Nevada bighorn sheep is similar in appearance to other desert associated bighorn sheep. The species' pelage shows a great deal of color variation, ranging from almost white to dark brown, with a white rump. Males and females have permanent horns; the horns are massive and coiled in males, and are smaller and not coiled in females. As the animals age, their horns become rough and scarred with age, and will vary in color from yellowish-brown to dark brown. In comparison to many other desert bighorn sheep, the horns of the Sierra Nevada bighorn sheep are generally more divergent as they coil out from the base. Adult male sheep stand up to 3 ft (1 m) tall at the shoulder; males weigh up to 220 lbs (100 kg) and females 140 lbs (64 kg).
Several subspecies of bighorn sheep have been recognized on the basis of geography and differences in skull measurements. These subspecies of bighorn sheep include O. c. cremnobates (Peninsular bighorn sheep), O. c. nelsoni (Nelson bighorn sheep),O. c. mexicana (Mexican bighorn sheep), O. c. weemsi (Weems bighorn sheep), O. c. californiana (California bighorn sheep), and O. c. canadensis (Rocky Mountain bighorn sheep). However, recent genetic studies question the validity of some of these sub-species and suggest a need to re-evaluate overall bighorn sheep taxonomy. For example, Sierra Nevada bighorn sheep appear to be more closely related to desert bighorn sheep than the O. c. californiana found in British Columbia.
Bighorn sheep are primarily diurnal, and their daily activity shows some predictable patterns that consist of feeding and resting periods. Bighorn sheep are primarily grazers; however, they may browse woody vegetation when it is growing and very nutritious. They are opportunistic feeders selecting the most nutritious diet from what is available. Plants consumed include varying mixtures of graminoids (grasses), browse (shoots, twigs, and leaves of trees and shrubs), and herbaceous plants depending on season and location. Grass, mainly Stipa speciosa (perennial needlegrass), is the primary diet item in winter, but as spring green-up progresses, the bighorn sheep shift from grass to a more varied browse diet, which includes Mormon tea, California buckwheat, and bitterbrush.
Sierra Nevada bighorn sheep are gregarious, with group size and composition varying with gender and from season to season. Spatial segregation of males and females occurs outside the mating season, with males more than 2 years old living apart from females and younger males for most of the year. Ewes generally remain all their lives in the same band into which they were born. During the winter, Sierra Nevada bighorn sheep concentrate in those areas suitable for wintering, preferably Great Basin habitat at the very base of the eastern escarpment. Subpopulation size can number more than 100 sheep, including rams. By summer, these sub-populations decrease in size as more habitat becomes available. Breeding takes place in the fall, generally in November. Single births are the norm for North American wild sheep, but twinning is known to occur. Gestation is about 6 months.
Lambing occurs between late April to early July, with most lambs born in May or June. Ewes with newborn lambs live solitarily for a short period before joining nursery groups that average about six sheep. Ewes and lambs frequently occupy steep terrain that provides a diversity of slopes and exposures for escape cover. Lambs are precocious, and within a day or so, climb almost as well as the ewes. Lambs are able to eat vegetation within two weeks of their birth and are weaned between one and seven months of age. By their second spring, they are independent of their mothers. Female lambs stay with ewes indefinitely and may attain sexual maturity during the second year of life. Male lambs, depending upon physical condition, may also attain sexual maturity during the second year of life. Average lifespan is 9-11 years in both sexes, though some rams are known to have lived 12 to 14 years.
Sierra Nevada bighorn sheep inhabit the alpine and subalpine zones during the summer, using open slopes where the land is rough, rocky, sparsely vegetated and characterized by steep slopes and canyons. Most of these sheep live between 10,000 and 14,000 ft (3000 and 4200 m) in elevation in summer. In winter, they occupy high, windswept ridges, or migrate to the lower elevation sagebrush-steppe habitat as low as 4,800 ft (1463 m) to escape deep winter snows and find more nutritious forage. Bighorn sheep tend to exhibit a preference for south-facing slopes in the winter. Lambing areas are on safe steep, rocky slopes. These sheep prefer open terrain where they are better able to see predators. For these reasons, they usually avoid forests and thick brush if possible.
The historical range of the Sierra Nevada bighorn sheep includes the eastern slope of the 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, California, a total distance of about 215 mi (346 km). By the turn of the century, about 10 out of 20 historical subpopulations survived. The number dropped to five subpopulations at mid-century, and down to two subpopulations in the 1970s, near Mount Baxter and Mount Williamson in Inyo County. Currently, five subpopulations of Sierra Nevada bighorn sheep occur at Lee Vining Canyon, Wheeler Crest, Mount Baxter, Mount Williamson, and Mount Langley in Mono and Inyo counties, three of which are reintroduced subpopulations established from sheep obtained from the Mount Baxter sub-population from 1979 to 1986.
The current and historical habitat of the Sierra Nevada bighorn sheep is almost entirely on public land managed by the U. S. Forest Service, Bureau of Land Management, and National Park Service (NPS). The Sierra Nevada is located along the eastern boundary of California, and peaks vary in elevation from 6,000-8,000 ft (1829-2438 m) in the north, to over 14,000 ft (4267 m) in the south adjacent to Owens Valley, and then drop rapidly in elevation in the southern extreme end of the range. Most precipitation, in the form of snow, occurs from October through April.
Historically, bighorn sheep populations occurred along and east of the Sierra Nevada crest from Sonora Pass (Mono County) south to Walker Pass (Olancha Peak). Sheep apparently occurred wherever appropriate rocky terrain and winter range existed. With some exception, most of the populations wintered on the east side of the Sierra Nevada and spent summers near the crest.
Five subpopulations remain that include a total of nine female demes (i.e., local populations). These demes are defined by separate geographic home range patterns of the females: Mount Langley— eight ewes, Mount Williamson—three ewes, Black Mountain—five ewes, Sand Mountain—five ewes, Sawmill Canyon—two ewes, Wheeler Ridge—17 ewes, Mount Gibbs—two ewes, Tioga Crest—one ewe, Mount Warren—five ewes. Of these, the Mount Williamson, Black Mountain, and Tioga Crest demes appear not to use low elevation winter ranges at all, and they will probably go extinct as a result. The Black Mountain deme was previously part of the Sand Mountain deme (part of the Mount Baxter subpopulation) and became a separate deme after winter range abandonment occurred in the late 1980s. The five remaining ewes in this deme appear not to know of the Sand Mountain winter range, which lies considerably north of their home range. They were almost certainly all born after winter range abandonment on Sand Mountain. This deme has shown a steady decline in size.
There are six female demes that may persist, but all are still very vulnerable to extinction due to small size. Of the two ewes and lamb that spent February, 1998 at the mouth of Sawmill Canyon (another Mount Baxter subpopulation deme), only a ewe and a lamb remained when last seen there in 1998. Shortly after they were last seen, evidence of a mountain lion was found on the rocks where they had been weathering a month of severe winter storms. When the normal summer range of this deme of females was investigated twice last summer, it was difficult to find evidence of any sheep remaining. This deme may contain only a single remaining ewe, or none.
The Sand Mountain deme has had only four ewes in it for almost all of the 1990's. During the summer of 1998, Dr. John Wehausen documented a yearling female with them, thus the total of five ewes listed above. However, the four adult ewes must now be approaching the ends of their lives, making this deme also very vulnerable to extinction, even if they have been showing some increased winter range use. Without successful births and recruitment of female lambs into this deme, this deme will quickly experience a decline.
Subpopulations of Sierra Nevada bighorn sheep probably began declining with the influx of gold miners to the Sierra Nevada in the mid-1880s, and those losses have continued through the 1900s. By the 1970s, only two subpopulations of Sierra Nevada bighorn sheep, those near Mount Baxter and Mount Williamson in Inyo County, are known to have survived. Specific causes for the declines are unknown. Market hunting may have been a contributing factor as evidenced by menus from historic mining towns such as Bodie, which included bighorn sheep. However, with the introduction of domestic sheep in the 1860s and 1870s, wild sheep are known to have died in large numbers in several areas from disease contracted from domestic livestock. Large numbers of domestic sheep were grazed seasonally in the Owens Valley and Sierra Nevada prior to the turn of the century, and disease is believed to be the factor most responsible for the disappearance of bighorn sheep subpopulations in the Sierra Nevada. Experiments have confirmed that bacterial pneumonia, carried normally by domestic sheep, can be fatal to bighorn sheep.
By 1979, only 220 sheep were known to exist in the Mount Baxter subpopulation, and 30 in the Mount Williamson subpopulation. Conservation efforts by several Federal and State agencies from 1970 to 1988 were aimed at expanding the distribution of Sierra Nevada bighorn sheep by translocating sheep back into historical habitat. Sheep were obtained from the Mount Baxter subpopulation and transplanted to three historic locations. Consequently, Sierra Nevada bighorn sheep now occur in five subpopulations in Mono and Inyo counties: Lee Vining Canyon, Wheeler Crest, Mount Baxter, Mount Williamson, and Mount Langley. The Sierra Nevada bighorn sheep population reached a high of about 310 in 1985-86. Subsequently, population surveys have documented a declining trend.
Habitat throughout the historic range of Sierra Nevada bighorn sheep remains essentially intact; the habitat is neither fragmented nor degraded. However, by 1900, about half of the Sierra Nevada bighorn sheep populations were lost, most likely because of introduction of diseases by domestic livestock, and illegal hunting. Beginning in 1979, animals from the Mount Baxter subpopulation were translocated to reestablish subpopulations in Lee Vining Canyon, Wheeler Crest, and Mount Langley in Mono and Inyo counties. Currently, Sierra Nevada bighorn sheep are limited to five subpopulations. Almost all of the historical and current habitat is administered by federal agencies. Some small parcels of inholdings within the species' range are owned by the Los Angeles Department of Water and Power. Also, there are some patented mining claims in bighorn sheep habitat, but the total acreage is small.
During the period of the California gold rush (starting about 1849), hunting to supply food for mining towns may have played a role in the decline of the population. Besides being sought as food, Sierra Nevada bighorn sheep were also killed by sheepmen who considered wild sheep as competitors for forage with domestic sheep. The decimation of several wildlife species in the late 1800s prompted California to pass legislation providing protection to deer, elk, pronghorn antelope, and bighorn sheep.
Disease is believed to have been the major contributing factor responsible for the precipitous decline of Sierra Nevada bighorn sheep starting in the late 1800s. Bighorn sheep are host to a number of internal and external parasites, including ticks, lice, mites, tapeworms, roundworms, and lungworms. Most of the time, parasites are present in relatively low numbers and have little effect on individual sheep and populations.
Cattle were first introduced into the Sierra Nevada in 1860s but were replaced with domestic sheep that could graze more extensively over the rugged terrain. Large numbers of domestic sheep were grazed seasonally in the Sierra Nevada prior to the turn of the century, and the domestic sheep would use the same ranges as the wild sheep, occasionally coming into direct contact with them. Both domestic sheep and cattle can act as disease reservoirs. Scabies, most likely contracted from domestic sheep, caused a major decline of bighorn sheep in California in the 1870s to the 1890s and caused catastrophic die-offs in other parts of their range.
Die-offs from pneumonia contracted from domestic sheep is another important cause of losses. In 1988, a strain of pneumonia, apparently contracted from domestic sheep, wiped out a reintroduced herd of bighorn sheep in Modoc County. Native bighorn sheep cannot tolerate strains of respiratory bacteria, such as Pasteurella species, carried normally by domestic sheep and close contact with domestic animals results in transmission of disease and subsequent deaths of the exposed animals. Bighorn sheep can also develop pneumonia independent of contact with domestic sheep. Lung-worms of the genus Protostrongylus are often an important contributor to the pneumonia disease process in some situations. Lungworms are carried by an intermediate host snail, which is ingested by a sheep as it is grazing. Lungworm often exists in a population, but usually doesn't cause a problem. However, if the sheep are stressed in some way, they may develop bacterial pneumonia, which is complicated by lungworm infestation. Bacterial pneumonia is usually a sign of weakness caused by some other agent such as a virus, parasite, poor nutrition, predation, human disturbance, or environmental or behavioral stress that lowers the animal's resistance to disease. Bighorn sheep in the Sierra Nevada carry lungworms, but the parasite loads have been low, and there has been no evidence of any clinical signs of disease or disease transmission.
Currently, domestic sheep grazing allotments are permitted by the U. S. Forest Service in areas adjacent to Sierra Nevada bighorn sheep subpopulations. Domestic sheep occasionally escape the allotments and wander into bighorn sheep areas, sometimes coming into direct contact with bighorn sheep. For example, in 1995, 22 domestic sheep that were permitted on Forest Service land wandered away from the main band and were later found in Yosemite National Park, after crossing through occupied bighorn sheep habitat. Other stray domestic sheep, in smaller numbers, have been known to wander up the road in Lee Vining Canyon into bighorn sheep habitat. Based on available information, and given the susceptibility of bighorn sheep to introduced pathogens, disease will continue to pose a significant and underlying threat to the survival of Sierra Nevada bighorn sheep until the potential for contact with domestic sheep is eliminated.
Predators such as coyote, bobcat, mountain lion, gray fox, golden eagle, and free-roaming domestic dogs prey upon bighorn sheep. Predation generally has an insignificant effect except on small populations such as the Sierra Nevada bighorn sheep. Coyotes are the most abundant large predator sympatric with bighorn sheep populations and are known to have killed young Sierra Nevada bighorn sheep. In the late 1980s, mountain lion predation of Sierra Nevada bighorn sheep increased throughout their range. This trend has continued into the 1990s.
Predation by mountain lions probably was a natural occurrence and part of the natural balance of this ecosystem. From 1907 to 1963, the State provided a bounty on mountain lions; the State also hired professional lion hunters for many years. The bounty most likely kept the mountain lion population reduced such that bighorn sheep predation was rare and insignificant. Between 1963 and 1968, mountain lions were managed as a nongame and nonprotected mammal, and take was not regulated. From 1969 to 1972, lions were re-classified as game animals. A moratorium on mountain lion hunting began in 1972 and lion numbers likely increased. In 1986, the species was again classified as a game animal, but the California Department of Fish and Game hunting recommendations were challenged in court in 1987 and 1988. In 1990, a State-wide ballot initiative passed into law prohibiting the killing of mountain lions except if humans or their pets or livestock are threatened. Another ballot measure, Proposition 197, which would have modified current law regarding mountain lion management failed to pass in 1996, largely because of the public's concern that the change may allow mountain lion hunting. With the removal of the ability to control the mountain lion population, lion predation has become a significant limiting factor for the Sierra Nevada bighorn sheep.
The increased presence of mountain lions appears to have changed Sierra Nevada bighorn sheep winter habitat use patterns. In two bighorn sheep subpopulations, one in the Granite Mountains of the eastern Mojave Desert, and the other was the Mount Baxter subpopulation in the Sierra Nevada, mountain lions reduced the subpopulation in the Granite Mountains to eight ewes between 1989 and 1991, and held it at that level for 3 years, after which lion predation decreased and the bighorn sheep sub-population increased at 15 percent per year for 3 years. All the mortality in that subpopulation was attributed to mountain lion predation. The Mount Baxter bighorn sheep subpopulation abandoned its winter ranges, presumably due to mountain lion predation. Forty-nine sheep were killed by lions on their winter range between 1976 and 1988 out of an average subpopulation size of 127 sheep. These mortalities from mountain lion predation represented 80 percent of all mortality on the winter range, and 71 percent for all ranges used. There is also evidence that many of the bighorn sheep killed were prime-aged animals.
The bighorn sheep on Mount Baxter moved to higher elevations possibly to evade lions. By avoiding the lower terrain and higher quality forage present during the spring, sheep emerge from the winter months in poorer condition. Consequences from the change in habitat use resulted in a decline in the Mount Baxter subpopulation due to decreased lamb survival, because lambs were born later and died in higher elevations during the winter. This may have also been the case with the Lee Vining subpopulation decline, when the bighorn sheep ran out of fat reserves at a time when they should have been replenishing their reserves with highly nutritious forage from low elevation winter ranges. Because of the winter habitat shift by the bighorn sheep, the Mount Baxter subpopulation has declined significantly. With the large decline of bighorn sheep on Mount Baxter, the total population of Sierra Nevada bighorn sheep has now dropped below what existed when the restoration program began in 1979, according to Dr. Wehausen. In a 1996 survey on Mount Williamson, there was no evidence of groups of sheep, and this subpopulation was the last one found using its low-elevation winter range in 1986. Mountain lion predation may have led to the extirpation of this subpopulation, one of the last two native subpopulations of Sierra Nevada bighorn sheep.
The Sierra Nevada bighorn sheep population is critically small with a total of only about 100 sheep known from five subpopulations. There is no known interaction between the separate subpopulations. The Sierra Nevada bighorn sheep currently is highly vulnerable to extinction from threats associated with small population size and random environmental events.
Although inbreeding depression (decline of genetic vigor) has not been demonstrated in the Sierra Nevada bighorn sheep, the number of sheep occupying all areas is critically low. The minimum size at which an isolated group of this species can be expected to maintain itself without the deleterious effects of inbreeding is not known. Researchers have suggested that a minimum effective population size of 50 is necessary to avoid short-term inbreeding depression, and 500 to maintain genetic variability for long-term adaptation. Small populations are extremely susceptible to demographic and genetic problems. Small populations suffer higher extinction probabilities from chance events such as skewed sex ratio of offspring, (e.g., fewer females being born than males). For example, the Mount Langley subpopulation has been declining. In 1996-97, out of a subpopulation of 4 ewes and 10 rams, 5 lambs were born, of which 4 were female. Although a positive event for this subpopulation, it could have been devastating if the female:male ratio of offspring had been reversed.
Small, isolated groups are also subject to extirpation by naturally occurring random environmental events, e.g., prolonged or particularly heavy winters and avalanches. In 1995, for example, a dozen sheep died in a single avalanche at Wheeler Ridge. Such threats are highly significant because currently the subpopulations are small and it is also common in bighorn sheep for all members of one sex to occur in a single group. During the very heavy winters in the late 1970s and early 1980s, there was no notable mortality in the subpopulations because they were using low elevation winter ranges.
Competition for critical winter range resources can occur between bighorn sheep and elk and/or deer. However, competition between these species does not appear significant since deer and bighorn sheep readily mix on winter range, and the habitat overlap between elk and bighorn sheep is slight.
In addition to disease, mountain lion predation, and random natural events, other factors may contribute to bighorn sheep mortality. For example, two subpopulations (Wheeler Ridge and Lee Vining) have ranges adjacent to paved roadways exposing individuals from those subpopulations to potential hazards. Bighorn sheep have been killed by vehicles in Lee Vining Canyon on several occasions.
Currently, there is a large lion occupying the winter range areas used by members of the Mount Langley deme. These ewes have been using that winter range enough over the past three winters to be showing a subpopulation increase (recruitment of five lambs for four ewes in the past 2 years). This lion could easily reverse that trend by killing multiple members of this deme and discouraging them from using this winter range.
With the likely extinction of some of the existing demes, the remaining demes become all the more important to the persistence of this distinct population segment. There has been no pattern as to which demes survive. All population dynamics over the past 15 years have been unanticipated. Individual mountain lions can do enormous damage to any of these small demes, as can catastrophic events such as snow avalanches. The current larger size of the Wheeler Ridge deme does not preclude it from experiencing a sudden decline, as the Mount Warren deme experienced in 1998.
Every deme is critical to the species survival and every remaining female in every deme is critically important to the persistence of their demes.
Lastly, the potential for contact with domestic sheep and the transmission of disease could, by itself, eliminate an entire deme. Domestic sheep continue to stray into Sierra Nevada bighorn sheep habitat. Recently, domestic sheep have come in close proximity to the resident bighorn sheep on numerous occasions, but, by good fortune, domestic sheep have not come into contact with bighorn sheep during these events.
Vulnerability to demographic problems must be viewed as a combination of immediate threats of predation, changed habitat use due to the presence of mountain lions, the resultant decline in ewe nutrition and lamb survivorship, exposure to environmental catastrophes, and the transmission of disease from domestic sheep.
Conservation and Recovery
In April, 1999 U. S. Fish and Wildlife Service (FWS) issued an emergency rule listing this sheep as endangered because of increased predation and disease.
The Sierra Nevada bighorn sheep restoration program used the Mount Baxter subpopulation as the source of reintroduction stock from 1979 to 1988. The three reintroduced subpopulations at Lee Vining Canyon, Wheeler Mountain, and Mount Langley all suffered from mountain lion predation shortly after translocation of sheep. The Lee Vining Canyon subpopulation lost a number of sheep to mountain lion predation, threatening the success of the reintroduction effort. The subpopulation was supplemented with additional sheep and the State removed one mountain lion each year for three years, which helped reverse the decline of this sub-population. Also, because domestic sheep are preyed upon by mountain lions, livestock operators who have a Federal permit to graze their sheep on Forest Service land can get a depredation permit from the State, and have the U. S. Department of Agriculture, Wildlife Services, remove the mountain lion. The Lee Vining Canyon subpopulation occurs in the general area where domestic sheep are permitted, and has benefitted for the last four or five years from the removal of two to three mountain lions per year that were preying on domestic sheep.
In response to a very rapid decline in population numbers, in 1876 the State legislature amended an 1872 law that provided seasonal protection for elk, deer and pronghorn to include all bighorn sheep. Two years later, this law was amended, establishing a four-year moratorium on the taking of any pronghorn, elk, mountain sheep or female deer. In 1882, this moratorium was extended indefinitely for bighorn sheep. In 1971, California listed the California bighorn sheep as "rare." The designation was changed to "threatened" in 1984 to standardize the terminology of the amended California Endangered Species Act, and upgraded the species to "endangered" in 1999.
The California Fish and Game Code provides for management and maintenance of bighorn sheep. The policy of the State is to encourage the preservation, restoration, and management of California's bighorn sheep. The California Department of Fish and Game supports the concept of separating livestock from bighorn sheep, by creating buffers, to decrease the potential for disease transmission.
Since the Sierra Nevada bighorn sheep was listed by the State of California in 1971, the California Department of Fish and Game has undertaken numerous efforts for the conservation of the sheep, including but not limited to—(1) intensive field studies; (2) reestablishment of three additional sub-populations in historical habitat; (3) creation, in 1981, of the Sierra Nevada Bighorn Sheep Interagency Advisory Group, including representatives from Federal, State, and local resource management agencies which has produced the Sierra Nevada Bighorn Sheep Recovery and Conservation Plan (1984) and a Conservation Strategy for Sierra Nevada Bighorn Sheep (1997); and (4) culling four mountain lions that were taking Sierra Nevada bighorn sheep, which played a significant role in the efforts to reestablish one subpopulation.
Mountain lion hunting has not occurred in California since 1972. As a result of passage of Proposition 117 in 1990 prohibiting the hunting or control of mountain lions, the California Department of Fish and Game does not have the authority to remove mountain lions to protect the Sierra Nevada bighorn sheep and secure their survival.
Federal agencies have adequate authority to manage the land and activities under their administration to benefit the welfare of the bighorn sheep. Steps are being taken to enhance habitat through prescribed burning to improve forage and maintain open habitat, and to retire domestic sheep allotments that run adjacent to bighorn sheep habitat. For example, 650 acres were burned in 1997 in Lee Vining Canyon to reduce mountain lion hiding cover, and there are plans to do more burns in other areas on Forest Service land. However, in some cases, because of conflicting management concerns, conservation efforts are not proceeding as quickly as necessary. Although efforts have been underway for many years, the Forest Service has been unable to eliminate the known threat of contact between domestic sheep and the Sierra Nevada bighorn sheep by either eliminating adjacent grazing allotments, or modifying allotments such that a sufficient buffer zone exists that would prevent contact between wild and domestic sheep.
In 1971, the State, in cooperation with the Forest Service, established a sanctuary for the Mount Baxter and Mount Williamson subpopulation of Sierra Nevada bighorn sheep, called the California Bighorn Sheep Zoological Area. About 41,000 acres (16,600 hectares) of Forest Service land was set aside for these two subpopulations. At the time, it was felt that the reason for the species' decline was related to human disturbance. The sanctuary was designed to regulate human use in some areas, and reduce domestic sheep/wild sheep interaction by constructing a fence below the winter range of the Mount Baxter subpopulation along the Forest Service boundary. Adjacent summer range on NPS land was also given a restrictive designation to reduce human disturbance. The Zoological Area continues to receive special management by the Forest Service; it encompasses land designated as wilderness and mountain sheep habitat.
Protection of the Sierra Nevada bighorn sheep requires reduction of the threat of mountain lion predation, particularly during the months of April and May when bighorn sheep attempt to use low elevation winter ranges to obtain necessary nutrition after lambing, and ewes and lambs are most vulnerable to lion predation. The sheep's endangered status allows FWS to remove mountain lions that threaten Sierra Nevada bighorn sheep. Removal of mountain lions does not necessarily involve lethal techniques.
Protection of the Sierra Nevada bighorn sheep also requires reduction of the threat of disease transmission from domestic sheep by preventing domestic sheep from coming into contact with bighorn sheep. The FWS will work with the Forest Service to reduce the threat of disease transmission by domestic sheep. Reduction of this threat may involve elimination of grazing allotments adjacent to bighorn sheep habitat, or modifying allotments to create a sufficient buffer zone that would prevent contact between domestic sheep and bighorn sheep.
U. S. Fish and Wildlife Service
Ventura Fish and Wildlife Office
2493 Portola Rd. Suite B
Ventura, California 93003-7726
Telephone: (805) 644-1766
Fax: (805) 644-3958
U. S. Fish and Wildlife Service. 20 April 1999. "Emergency Rule To List the Sierra Nevada Distinct Population Segment of California Bighorn Sheep as Endangered." Federal Register 64 (75): 19300-19309.
"Sierra Nevada Bighorn Sheep." Beacham's Guide to the Endangered Species of North America. . Encyclopedia.com. (April 24, 2019). https://www.encyclopedia.com/environment/science-magazines/sierra-nevada-bighorn-sheep
"Sierra Nevada Bighorn Sheep." Beacham's Guide to the Endangered Species of North America. . Retrieved April 24, 2019 from Encyclopedia.com: https://www.encyclopedia.com/environment/science-magazines/sierra-nevada-bighorn-sheep
Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).
Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.
Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:
Modern Language Association
The Chicago Manual of Style
American Psychological Association
- Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
- In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.