Degree of Threat: High
Comments: Initial large declines were primarily the result of competition with domestic stock (e.g., cattle, sheep, burros), diseases and parasites introduced by domestic sheep, overhunting, and habitat loss (Cowan 1940; Buechner 1960; Sugden 1961; Stelfox 1971; Goodson 1982, Boyce et al. 1990, Valdez and Krausman 1999).
DOMESTIC STOCK: In Idaho, introduced bighorns avoided cattle and were more sensitive to cattle presence than were established populations, based on previous studies (Bissonette and Steinkamp 1996). Decline of desert populations has been attributed destruction of habitat and competition with domestic livestock and other native and domestic species (e.g., may be limited through resource competition with burros; Seegmiller and Ohmart 1981).
PARASITES AND DISEASE: Psoroptic scabies from domestic sheep devastated bighorn populations in the first half of the twentieth century (Boyce et al. 1990). The Peninsula ranges population (California and Baja California) has declined due to disease-caused low survival rate of juveniles (California Department of Fish and Game 1990). Many dieoffs (greater than 50% mortality over a few months) of herds have been reported over the last century; these are poorly understood, but seem to result from stress, which interacts with endemic lungworm infestations and lowers the animals' resistance to organisms such as PASTEURELLA bacteria. The animals subsequently die from acute bronchopneumonia (Ryder et al. 1992, Dunbar 1992, Schwantje 1988). See Bunch et al. (1999) for a general account of diseases and parasites affecting Bighorn Sheep.
HABITAT LOSS AND DEGRADATION: Loss and degredation of habitat, especially key winter forage sites, is a key threat (Valdez and Krausman 1999, Shackleton 1999, Krausman et al. 1999). Habitat degradation can occur through overgrazing by domestic stock, competition with exotic ungulates (e.g. Aoudad or Barbary, AMMOTRAGUS LERVIA), excessive off-road vehicle use, spread of rangeland weeds, and the usurpation of water sources (Simpson 1980, Valdez and Krausman 1999, Krausman et al. 1999). Fire suppression and resulting vegetation succession (encroachment of tall dense shrubland and forest) has been a major cause of habitat loss in Colorado and British Columbia (Davidson 1991, Cannings et al. 1999, Wakelyn 1987; see also Etchberger et al. [1989] for similar conclusion on the importance of fire in Santa Catalina Mountains, Arizona). Fragmentation of habitat reduces or eliminates genetic interchange among populations (Ramey 2000) and reduces the probability of recolonization following local extirpation; both these effects are especially of concern in small populations (fewer than 100 individuals), which are especially vulnerable to extirpation (Berger 1990).
OTHER: The social behavior and predator evasion strategy may have contributed to their decline (Miller and Gaud 1989). Feral honey bees may interfere with sheep attempting to drink at artificial water sources (guzzlers) in desert environments (Boyce et al. 2003).
Bighorn Sheep - Peninsular Ranges
Threatened in the United States by habitat loss, degradation, and fragmentation resulting from urban and commercial development (including proposed and approved renewable energy projects), invasive non-native plants (e.g., tamarisk), fire suppression (allows development of unsuitable thick vegetation), increased fire frequency (in some areas this facilitates invasion by non-native plants), and off-highway vehicle (OHV) activity; disease (in some instances contracted directly or indirectly from introduced domestic livestock); low recruitment (still a problem despite a recent increase in the bighorn sheep population); nonadaptive behavioral responses associated with residential and commercial development; high predation rates (by mountain lions and others) coinciding with low bighorn sheep population numbers in some areas; effects of non-native (e.g., ornamental) toxic plants; and climate change, particularly the ongoing and predicted trend toward warmer and drier conditions (USFWS 1998, 2011). Populations in Mexico are regarded as not very threatened (USFWS 1998).
Responses of bighorn sheep to human activity range from cautious curiosity to immediate flight or abandonment of habitat, as well as disruption of normal social patterns and resource use (USFWS 2011). Responses depend on the type of activity, an animal's previous experience with humans, size or composition of the bighorn sheep group, location of the sheep relative to elevation of the activity, distance to escape terrain, and distance to the activity (USFWS 2000).
The apparent extirpation of a subpopulation near the United States-Mexico border (Recovery Region 9) in the 1980s may have been caused construction of Interstate 8 in the mid-1960s, railroad activity, livestock grazing, poaching, and fire suppression (Rubin et al. 1998).
Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae)
"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).