Keywords

1 Introduction

Bighorn sheep (Ovis canadensis) and mountain goats (Oreamnos americanus) are herbivores in the family Bovidae (Feldhamer et al. 2020). Other mountain ungulates such as Dall’s (O. dalli) and Stone’s sheep (O. d. stonei), and mountain caribou (Rangifer tarandus) are not considered in this chapter, because they seldom occur on western rangelands. Ancestors of North American mountain sheep arose in Asia about 2.5 million years ago during the Villafranchian (Geist 1971; Valdez and Krausman 1999) and dispersed to North America via the Bering Land Bridge (Cowan 1940; Péwé and Hopkins 1967). The systematics and taxonomy of bighorn sheep are complex, but three clades currently are recognized: Sierra Nevada bighorn sheep (O. c. sierrae), desert bighorn sheep (O. c. nelsoni, O. c. mexicana), and Rocky Mountain bighorn sheep (O. c. canadensis) (Buchalski et al. 2016). Sierra Nevada and Rocky Mountain bighorn sheep diverged from desert bighorn sheep prior to or during the Illinoian glaciation ~ 315,000–94,000 years ago (Buchalski et al. 2016). By the Wisconsin glaciation (~ 40,000–23,000 years ago), fossils of Ovis were common (Guthrie 1968). Ancestors of mountain goats are also believed to have colonized western North America from Asia via the Bering Land Bridge during the Wisconsin glaciation (Rideout and Hoffman 1975). During the last glacial maximum, mountain goats were separated into northern, southern, and coastal refugial subpopulations (Nagorsen and Keddie 2000; Shafer et al. 2011b). Unlike bighorn sheep, subspecies have not been designated for mountain goats.

Bighorn sheep and mountain goats historically occupied suitable habitat across much of western North America; however, populations of these ungulates have declined since the latter part of the nineteenth century (Buechner 1960; Geist 1971). A downward trend in numbers of both species likely began with Euro-American settlement of western North America, and much attention has focused on unregulated market hunting, habitat loss or modification, and diseases contracted from domestic livestock as causes of that decline (Buechner 1960; Smith et al. 1991; Singer et al. 2000); some of these concerns remain. Primary challenges to conserving North American wild sheep on a continent-wide basis are maintaining habitat quality, reducing habitat loss, and managing disease (Krausman 2000; Bleich 2009b; Krausman and Bleich 2013). In this chapter, we discuss these mountain ungulates in areas where they overlap rangelands of western North America.

2 General Life History and Population Dynamics

2.1 Bighorn Sheep

Bighorn sheep are sexually dimorphic in size with males larger than females (Fig. 22.1; Weckerly 1998; Loison et al. 1999). Weight of adult male bighorn sheep from northern regions averages ~ 102 kg and adult females ~ 72 kg (Geist 1971; Festa-Bianchet et al. 1997; Shackleton et al. 1999; Krausman and Bowyer 2003), whereas desert-dwelling bighorn sheep are smaller (adult males =  ~ 70 kg; adult females =  ~ 48 kg) in size (Bleich et al. 1997; Krausman et al. 1999). Adult male bighorn sheep have large, curled horns used for ramming, head-to-head clashes, and for display to intimidate rivals, whereas horns of females are much smaller and not as strongly curled as those of males (Fig. 22.1; Geist 1971; Shackleton et al. 1999; Coltman et al. 2003). Bighorn sheep also possess conspicuous rump patches, which are thought to be used as an alarm signal and function primarily to promote group cohesion among conspecifics or as a signal to elicit predator evasion behavior within social groups (Hirth and McCullough 1977; Caro 2005). Additionally, bighorn sheep possess small litters with large-bodied precocial young—i.e., active and able to move independently shortly after birth (Fig. 22.1; Festa-Bianchet 1988b), are long-lived with long generation times, provide high maternal investment in young, and exhibit a low intrinsic rate of population increase (Festa-Bianchet 1988a; Shackleton et al. 1999; Gaillard et al. 2000). This suite of attributes responds strongly in a density-dependent manner, wherein reproduction and survival are negatively associated with population density in relation to the ecological carrying capacity of the environment (K; the number of individuals a particular area can support); as the population approaches K, reproduction and recruitment decline (Swenson 1985; Festa-Bianchet and Jorgenson 1998; Bowyer et al. 2014).

Fig. 22.1
2 photos. 1. Two adult male bighorn sheep with curved horns. 2. A female bighorn sheep and a lamb standing on the mountain rocks.

Two adult male Rocky Mountain bighorn sheep (top), and an adult female Rocky Mountain bighorn sheep and lamb (bottom) during spring, Utah

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The sexes of bighorn sheep spatially segregate from one another for a portion of the year, thus using different areas in mountainous rangelands (Bleich et al. 1997; Bowyer 2004; Whiting et al. 2010a). Indeed, sexes of desert-dwelling bighorn sheep may segregate into mountain ranges separated by ~ 15 km to balance the needs for crucial resources against risk of predation (Bleich et al. 1997). Young typically are born in the spring while the sexes are segregated (Fig. 22.1; Whiting et al. 2011b, 2012); desert-dwelling bighorn sheep, however, have a protracted birthing period (Bleich et al. 1997; Rubin et al. 2000). Females allocate substantial maternal care to their single young, which they birth and rear in precipitous terrain (Geist 1971; Festa-Bianchet 1988c; Bleich et al. 1997) that contains fewer predators than areas occupied by males during sexual segregation (Bleich et al. 1997). Females also may defend young by attacking predators, especially coyotes (Canis latrans; Berger 1978b; Bleich 1999). The sexes of bighorn sheep follow differing strategies for lowering the risk of predation—males increase group size and females move closer to escape terrain (e.g., steep slopes, cliffs, and rock outcroppings) to lower predation risk (Bleich et al. 1997; Bowyer 2004; Schroeder et al. 2010). Tradeoffs between acquiring essential resources and avoiding predation are well-documented for bighorn sheep; these ungulates, especially females, may forego areas of high-quality forage to avoid predators (Festa-Bianchet 1988d; Berger 1991; Bleich et al. 1997). Mountain lions (Puma concolor) also are an important predator and can have substantial effects on survival and population growth in small populations of bighorn sheep (Ross et al. 1997; Johnson et al. 2013; Rominger 2018).

Male and female bighorn sheep exhibit important differences in the morphology and physiology of their digestive tracts that lead to males having larger rumens than females, and, as a result, are better adapted to digesting less-nutritious forages. Females, with smaller rumens than males, require high-quality forages necessary to support the high costs of late gestation and lactation; such differences foster sexual segregation (Barboza and Bowyer 2000, 2001). These differences and many other life-history characteristics of bighorn sheep are associated with their population ecology.

2.2 Mountain Goats

Mountain goats are sexually dimorphic in size with males larger than females. Adult male mountain goats weigh 90–181 kg and adult females weigh 59–111 kg (Côté and Festa-Bianchet 2003). Mountain goats exhibit specialized morphological and behavioral adaptations that enable them to inhabit steep and rugged environments characterized by severe climatic conditions (Fig. 22.2). For example, soft padded hooves surrounded by a hard keratinous sheath combined with a vertically oriented narrow body and muscular shoulders enable athletic and sure-footed locomotion in rugged, cliffy terrain—habitat that is preferentially used to reduce the risk of predation (Festa-Bianchet and Côté 2008). Like bighorn sheep, the population biology of mountain goats is linked to the seasonal availability of nutritional resources, and this species also exhibits sexual segregation (Festa-Bianchet and Côté 2008). For example, mountain goat parturition occurs during late May and early June and coincides with green-up of highly nutritious forage (Pettorelli et al. 2007; Festa-Bianchet and Côté 2008). During the summer growing season mountain goats accumulate fat and protein reserves needed to survive long winters characterized by severe nutritional deficiency. Thus, summer and winter weather can play an important role in mediating nutritional condition and can exert strong effects on individual growth, reproduction, and survival (Pettorelli et al. 2007; Festa-Bianchet and Côté 2008; White et al. 2011). Consequently, these specialized ungulates exhibit a slow life-history strategy with late age of maturity (age at first reproduction = 4–5 years, body mass asymptote = 4–6 years) and may not reproduce annually to mitigate the effects of reproductive costs on probability of survival (Festa-Bianchet and Côté 2008); such relationships can be associated with density-dependent processes (Houston and Stevens 1988; Bowyer et al. 2014). Consequently, mountain goat populations have low growth rates—i.e., 1–4% (Hamel et al. 2006; Rice and Gay 2010; White et al. 2021a) and are sensitive to weather conditions, especially in northern coastal environments that can be prone to episodic, severe snowfall (White et al. 2011).

Fig. 22.2
2 photos. A. An adult female goat with a kid standing on the mountain landscape. B. An adult male goat, standing on the snow-covered mountain rocks.

An adult female mountain goat and kid during late-winter (top) and an adult male mountain goat on low-elevation winter range (bottom), Alaska

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Mountain goats are vulnerable to predation by large carnivores, such as wolves (C. lupus) and brown (grizzly, Ursus arctos) or black bears (U. americanus; Fox and Streveler 1986; Festa-Bianchet and Côté 2008), but the specialized adaptations of these ungulates for using rugged mountain terrain mitigate predation-risk. Nonetheless, inhabiting rugged terrain can involve nutritional costs leading to trade-offs between safety and acquisition of forage resources (Hamel and Côté 2007). The presence of large carnivores also can elicit indirect effects including increases in endocrine stress responses that can negatively influence reproduction (Dulude‐de Broin et al. 2020). Life in extreme environments can also lead to increased sensitivity to stochastic factors, with events such as avalanches as an important source of mortality in some areas of coastal Alaska (White et al. 2011).

3 Population Status

3.1 Bighorn Sheep

Bighorn sheep are associated with habitats as diverse as the frigid and wind-swept ridges in the alpine regions of the highest mountains in North America to hot, arid areas below sea level in some inland desert basins (Fig. 22.3). Historically, the distribution of bighorn sheep extended eastward from British Columbia (Cowan 1940; Buechner 1960) to the badlands of North Dakota and South Dakota and southward into Mexico (Krausman et al. 1999). The distribution of ~ 48,000 Rocky Mountain bighorn sheep closely follows the Rocky Mountains from northern British Columbia southward to northern New Mexico (Krausman and Bowyer 2003). The ~ 39,000 desert bighorn sheep occupy habitat across much of the Great Basin, Mojave, Sonoran, and Chihuahuan deserts. Sierra Nevada bighorn sheep have a restricted distribution and are endemic to the Sierra Nevada of eastern California (Wehausen and Ramey 2000). Bighorn sheep occupying the peninsular ranges of southern California are considered a distinct population segment that is listed as endangered by the federal government; Sierra Nevada bighorn sheep are recognized as a valid subspecies of bighorn sheep and also are listed as endangered by the federal government (USFWS 2000, 2007).

Fig. 22.3
A map of western U S A is categorized based on the distribution of species of bighorn sheep and mountain goats with ecoregions including Apache Highlands, Aspen Parkland, Black Hills, Cascade Mountains, Colorado and Columbia Plateau, Great Basin, and Mojave Desert.

Distributions of bighorn sheep and mountain goats overlain on rangeland ecoregions in the western USA and Canada (Map credit: M. Solomon)

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3.2 Mountain Goats

The current distribution of mountain goats (80,000–120,000 individuals) extends eastward from coastal Alaska to the Rocky Mountains and south from Alaska, Yukon and the Northwest Territories to Montana, Idaho, and Washington, and includes a northernmost and geographically isolated population of native mountain goats in the Mackenzie Mountains of Yukon and Northwest Territories (Fig. 22.3; Festa-Bianchet and Côté 2008). In coastal Alaska and British Columbia, mountain goat populations almost exclusively occur on mainland mountain ranges; but apparently native populations historically and currently occur on some islands (Shafer et al. 2011a, b). Mountain goats have been successfully introduced into non-native ranges in the western USA (Montana, Oregon, Colorado, Utah, Nevada, South Dakota, Wyoming, and Washington–Olympic Peninsula) as well as into several non-native ranges in Alaska (Kodiak Island, Revillagigedo Island).

3.3 Population Monitoring

Two common methods for estimating population abundance of bighorn sheep are aerial surveys (Bleich et al. 1990a; Stockwell et al. 1991; Bates et al. 2021) and resight surveys performed from the ground (McClintock and White 2007; Johnson et al. 2010; Taylor et al. 2020). Helicopter surveys have been used increasingly during the past 20 years to monitor populations of bighorn sheep (Krausman and Hervert 1983; Bleich et al. 1994; McClintock and White 2007). Additionally, photographs of collared bighorn sheep from motion-sensor cameras set at water sources can be used to estimate population abundance (Perry et al. 2010; Taylor et al. 2020, 2022). Mark-recapture methods based on collection of fecal DNA also have been used to estimate population abundance (Schoenecker et al. 2015). Reproduction in bighorn sheep can be estimated by visual observation during the birthing period (Festa-Bianchet et al. 2000; Whiting et al. 2010b, 2011b). Survival often is quantified from animals with radio collars and by using mark-resight or known-fate analyses (Neal et al. 1993; Shannon et al. 2014).

Mountain goats are challenging to monitor because of the rugged and often remote environments they inhabit. Size and composition of mountain goat populations are often estimated using aerial survey techniques (fixed- and rotor-wing aircraft) and have involved use of mark-resight, sightability, and distance-sampling models to derive estimates (Poole 2007; Rice et al. 2009; Schmidt et al. 2019), but uncorrected minimum counts also have been used (McDonough and Selinger 2008). In highly accessible areas, ground-based methods involving direct observation or genetic mark-recapture (i.e., fecal DNA analyses) have been used to derive population estimates (Gonzalez-Voyer et al. 2001; Poole et al. 2011; Belt and Krausman 2012). Survival and reproduction typically are estimated using mark-resight or known-fates analyses involving marked animals (Smith 1986; Festa-Bianchet and Côté 2008; White et al. 2011, 2021b).

4 Habitat Associations

4.1 Bighorn Sheep

Bighorn sheep are well known for their dependence on steep, rugged terrain of variable elevations and ecoregions in western rangelands, whether in mountains or major river canyons, and adjacent foothills, all of which are generally characterized by sparse vegetation (Krausman and Bowyer 2003). Often bighorn sheep use habitat that is characterized by slopes > 20%, within 1000 m of escape terrain, and in areas of limited vegetational cover (Smith et al. 1991; Bleich et al. 1997; Andrew et al. 1999; Robinson et al. 2020; Lowrey et al. 2021). Bighorn sheep select the most appropriate terrain available in a particular area, and managers view scores derived from habitat models in a relative, rather than in an absolute, context (Andrew et al. 1999). These ungulates rely heavily on their visual acuity and open terrain to detect predators (Geist 1971; Risenhoover and Bailey 1985), and typically occupy areas in which they are well-adapted to detect and evade, or less apt to encounter, predators (Berger 1978a; Bleich et al. 1997). Hence, the distribution of bighorn sheep is restricted largely to mountains, canyons, and river corridors across the western portion of North America (Krausman et al. 1999; Krausman and Bowyer 2003). Migration to and from seasonal ranges is important for this species (Geist 1971; Jesmer et al. 2018; Spitz et al. 2020).

Depleted abundance and distribution compared with pre-European settlement and close association with steep, rugged, and sparsely vegetated areas has resulted in bighorn sheep having a naturally fragmented distribution across mountainous and canyon areas of western North America (Schwartz et al. 1986; Bleich et al. 1990b). As a result, bighorn sheep populations are typically small (e.g., 30 animals) but may number up to several hundred or more individuals occurring in remote and spatially isolated areas (Berger 1990; Epps et al. 2005; Donovan et al. 2020). Metapopulations are the primary foundation for habitat management and conservation of bighorn sheep (Bleich et al. 1990b, 1996; DeCesare and Pletscher 2006). A metapopulation is defined as the total population in a geographic area that is comprised of smaller subpopulations that are interconnected genetically and demographically by periodic movements of individual bighorn sheep (DeCesare and Pletscher 2006; Malaney et al. 2015; Epps et al. 2018). The subpopulations that comprise a metapopulation are expected to exhibit population dynamics independent of each other, and local extinctions are expected to occur; these are offset by colonization events involving individuals that move among isolated habitats, whether occupied or not, within the metapopulation. Thus, the viability of a bighorn sheep metapopulation depends upon the persistence of the subpopulations of which it is comprised (Bleich et al. 1996; DeCesare and Pletscher 2006), and colonization events must occur more frequently than extinction events.

Bighorn sheep diets are dominated by grasses and sedges; however, these ungulates exhibit seasonal variation in diet composition including browse (Fig. 22.4; Bleich et al. 1997; Krausman et al. 1999; Shackleton et al. 1999). In spring and summer, bighorn sheep eat mostly forbs, sedges, and grasses (Wikeem and Pitt 1992; Krausman et al. 1999; Shackleton et al. 1999). During winter, consumption of shrubs and senescent grasses also occurs (Singer and Norland 1994; Shackleton et al. 1999). Desert bighorn sheep also forage on prickly pear (Opuntia spp.) and other cactus species (Mammillaria spp. and Ferocactus spp.). Also, differential use of forage occurs between male and female bighorn sheep, especially when the sexes are segregated, with males consuming more graminoids (Bleich et al. 1997). Bighorn sheep may consume soil during spring and summer to acquire sodium, calcium, magnesium, and other minerals (Holl and Bleich 1987; Krausman et al. 1999). Among trace minerals, selenium may be especially important, given its fundamental role in virtually all physiological processes and because it varies widely in abundance across geographic areas (Flueck et al. 2012; Bleich et al. 2017). Additionally, water sources (artificial and natural) are important features in areas occupied by bighorn sheep (Fig. 22.4; Bleich et al. 2006; Whiting et al. 2009, 2011a), but the development of artificial water sources is a contentious issue for bighorn sheep management on rangelands of the western USA (Rosenstock et al. 1999, 2001; Bleich 2009a). Much of the opposition to provision water sources has its origin in the 1964 Wilderness Act, which opponents of water developments invariably invoke to prevent development of this essential resource because it ‘degrades’ legislated wilderness (Bleich 2005, 2016). Ironically, grazing and water developments for domestic livestock in wilderness areas are acceptable, and bighorn sheep and many other species of wildlife are dependent on those surface waters. Water development specifically to benefit bighorn sheep, however, has been opposed at virtually every opportunity (Bleich 2009a, 2016), in large part because of the failure of wilderness legislation to have been based more on ecological values than on less tangible benefits (Bleich 2016).

Fig. 22.4
Two photos of the mountain landscape with a herd of sheep grazing on the sparse grass.

Bighorn sheep foraging on low-elevation shrub and grass winter range (top), and bighorn sheep waiting to access a small, natural water seep (underneath the large rock at the right) in Utah (bottom)

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4.2 Mountain Goats

Mountain goats exhibit strong selection for steep, rugged habitats proximal to escape terrain (i.e., slopes > 40–50 degrees), provided adequate forage resources are available; mountain goats uncommonly use habitats greater than 400 m from escape terrain (Festa-Bianchet and Côté 2008; Shafer et al. 2012; White and Gregovich 2017; Lowrey et al. 2018). This strategy is well-suited to minimize risk of predation from wolves and brown or black bears (Fox and Streveler 1986; Sarmento and Berger 2020). Mountain goats exhibit ecotypic variation in seasonal migratory behavior and habitat selection (Hebert and Turnbull 1977). For example, in the north Pacific coastal regions, mountain goats generally migrate from alpine summer ranges to low-elevation, forested winter ranges because of the wet, heavy snowpack that occurs at high elevations within this region (Shafer et al. 2012; White and Gregovich 2017). In drier and colder interior regions, however, mountain goat seasonal movements are limited, and animals tend to winter in high-elevation alpine habitats and use windblown ridges with exposed vegetation or tree-line habitats (Festa-Bianchet and Côté 2008; Poole et al. 2009; Richard and Côté 2016). In interior regions, mountain goats often are sympatric with bighorn sheep, and can exhibit substantial niche overlap (DeVoe et al. 2015; Lowrey et al. 2018).

Mountain goats consume a wide variety of forage types (Côté and Festa-Bianchet 2003) but exhibit distinct seasonal variation in diet composition (Saunders 1955). Following green-up, mountain goats commonly consume forbs, sedges, and grasses in alpine summer ranges. During winter, however, shrubs, lichen litterfall, and even conifer needles are consumed when other lower-growing forages are buried under snow. In some interior ranges, senesced grasses and sedges also can be used on wind-blown alpine slopes. During spring and summer, mineral licks represent an important resource for mountain goats in interior mountain ranges (Hebert and Cowan 1971; Singer 1978; Ayotte et al. 2008; Poole et al. 2010); use of mineral licks is rarer in more nutritionally productive coastal areas. Use of mineral licks is primarily driven by the need to acquire sodium, although other nutrients (i.e., selenium, calcium, and magnesium) also may be important (Hebert and Cowan 1971; Ayotte et al. 2006). Because mineral licks are uncommon on the landscape, mountain goats may undertake substantial seasonal movements through atypical habitats to access these critical nutritional resources (Rice 2010).

5 Interaction with Livestock

5.1 Bighorn Sheep

Competition for forage and spatial interactions can occur seasonally between livestock and bighorn sheep (Chap. 4). Bighorn sheep and cattle generally eat grass-dominated diets, and dietary overlap can be high, especially during drought or other times of reduced forage abundance (Coughenour 1991; Bailey 2004; Chaikina and Ruckstuhl 2006; Garrison et al. 2016). Also, spatial competition between livestock and bighorn sheep can occur (Risenhoover et al. 1988). Bite rates of forage can decrease, and vigilance rates can increase for bighorn sheep when cattle are near; also vigilance rates were higher for females than for males with cattle nearby (Brown et al. 2010). Bighorn sheep avoided cattle and decreased use of areas when cattle were in proximity (Bissonette and Steinkamp 1996). Grazing of domestic cattle was negatively correlated with rate of population increase for translocated populations of bighorn sheep (Singer et al. 2000). Also, sharing of ranges by domestic cattle and bighorn sheep ostensibly has led to mountain lions switching from bighorn sheep to livestock predation (Rominger 2018).

5.2 Mountain Goats

Interactions between livestock and mountain goats can occur in high-elevation alpine meadows and associated habitats, but most mountain goat habitat is unsuitable for livestock grazing because of its rugged terrain. Most potential for co-occurrence is limited to the southern latitudes of mountain goat range—predominately where mountain goats were introduced. For example, non-native mountain goats in the East Humboldt Mountains, Nevada, may contact domestic livestock on public grazing allotments or on private lands (Wolff et al. 2019).

6 Effects of Disease

Risk of pathogen spillover is a major force shaping rangeland dynamics and management of bighorn sheep and mountain goats. Pathogen spillover is a concern between livestock and bighorn sheep, between livestock and mountain goats, between populations of bighorn sheep, between populations of mountain goats, and between populations of bighorn sheep and mountain goats.

Bighorn sheep and mountain goats are vulnerable to a suite of pathogens, including contagious ecythma (Samuel et al. 1975; Tryland et al. 2018), Johne’s disease (Williams et al. 1979), bovine viral diarrhea (Wolff et al. 2016), and a variety of helminths and ectoparasites. Epizootic hemorrhagic disease and bluetongue also pose threats to bighorn sheep, though their effects on mountain goats are likely more limited (Ruder et al. 2015). Infectious pneumonia often associated with the bacterial pathogen Mycoplasma ovipneumoniae (Besser et al. 2008; Cassirer et al. 2018) can result in extensive, all-age mortality, and place serious constraints on bighorn sheep population growth (Besser et al. 2012), and the same pathogen also may be problematic for mountain goats (Blanchong et al. 2018) and thinhorn sheep (Black et al. 1988). Although M. ovipneumoniae is not detected universally in bighorn sheep disease events, and other bacteria can produce sporadic acute pneumonia—e.g., leukotoxin-positive Pasteurellas (Shanthalingam et al. 2014)—M. ovipneumoniae appears to be a common player in the preponderance of well-documented disease events.

The distribution and demographic structure of bighorn sheep and mountain goat populations has important implications for disease transmission and risk. A disease outbreak in one population may not spread rapidly to nearby populations, despite proximity (Flesch et al. 2020), a somewhat atypical scenario compared with other ungulate species that exhibit more complete mixing patterns. Yet, because of the gregarious nature of bighorn sheep and mountain goats, particularly within female-offspring nursery groups, within-population rates of pathogen transmission can be high. Infected bighorn herds can also pose transmission risks to healthy neighboring herds, emphasizing the fundamental need for separation of bighorn sheep from domestic sheep and from infected bighorn and mountain goat herds as a core component of species conservation.

6.1 Bighorn Sheep

M. ovipneumoniae can be carried at high prevalence (Manlove et al. 2019) and genotypic diversities (Kamath et al. 2019) in large flocks of domestic sheep; accordingly, domestic sheep pose serious disease-mediated risks to bighorn sheep. This pathogen is not particularly troublesome in domestic sheep (Besser et al. 2019; Manlove et al. 2019), but it can persist and cause damage to bighorn herds for many years following exposure (Cassirer et al. 2018).

M. ovipneumoniae is primarily transmitted through respiratory droplets. When the bacteria encounter a new host, the pathogen takes up residence in the upper respiratory tract of the host, where it can proliferate and impede motion of the host’s cilia. This allows a diverse suite of bacteria that are commensal in the upper respiratory tract to gain access to the lower respiratory tract where they can become pathogenic (Besser et al. 2008). The acute phase of an M. ovipneumoniae infection is characterized by symptoms like coughing, which likely facilitates pathogen spread. Animals either resolve their lower respiratory tract infections or succumb to disease. Spillover events vary in their severity—documented die-offs range from 10 to 90% of the infected herd (Cassirer et al. 2018) and have occurred regularly for as long as detailed records exist (Marsh 1938; Buechner 1960).

A small subset of chronic-carrier hosts can continue to harbor M. ovipneumoniae in their nostrils even after acute respiratory symptoms decline (Plowright et al. 2017). Chronically infected animals appear to be less apt to transmit the pathogen, and intense contact may be required to generate new infections in previously unexposed conspecifics. Chronically infected females, however, are thought to transmit M. ovipneumoniae to susceptible offspring, which then develop acute infections and effectively transmit the pathogen to other susceptible young in their nursery groups. In this way, a small number of chronically infected individuals can affect recruitment for the entire herd. Chronic infection may be facilitated by paranasal sinus tumors that have recently been detected in multiple bighorn herds and have been associated with the presence of M. ovipneumoniae and P. multocida (Fox et al. 2011, 2015, 2016).

Some habitat manipulations could limit the risk of contact between host animals, but designing appropriate manipulations requires a strong understanding of factors that motivate bighorn sheep movements, and, in particular, forays—i.e., short-term movements of animals that begin and end within an established home range (Singer et al. 2001; Carpenter et al. 2014). Both sexes go on forays, though the distances and frequencies vary by sex. Some herds exhibit higher rates of such movements than others (Singer et al. 2001), and there are many hypotheses regarding the factors that encourage these events (Lassis et al. 2022). Commonly postulated drivers are herd density and sex ratio, habitat structure and viewshed, location of attractive resources (e.g., mineral licks, water, other bighorn sheep, mountain goats, domestic sheep, or domestic goats), rut, and individual age. Which factors are most important in particular contexts remain an open question that if addressed, may help alleviate some of the conflict associated with pathogen transmission to bighorn sheep occupying North American rangelands.

Understanding movements and migrations of bighorn sheep is critically important, as is the proximity of release areas for translocated bighorn sheep to other bighorn sheep, mountain goats, and domestic sheep or goat grazing allotments (Clifford et al. 2009; Shannon et al. 2014). Also, consideration should be given to the presence of hobby farms and trailing operations of domestic sheep and goats in locations adjacent to areas occupied by bighorn sheep (Shannon et al. 2014). If conservation of bighorn populations is the goal, spatial and temporal separation of bighorn and domestic sheep should occur wherever possible (Schommer and Woolever 2008; Wehausen et al. 2011; Besser et al. 2013).

6.2 Mountain Goats

Current knowledge of mountain goat disease risk and parasitology is limited when compared with bighorn sheep. Among the most documented diseases reported in mountain goats is contagious ecthyma, a viral disease that causes lesions to eyes, nose and mouth that can be severely debilitating, sometimes leading to death (Samuel et al. 1975; Tryland et al. 2018). While M. ovipneumoniae has been documented in mountain goats (Lowrey et al. 2018; Wolff et al. 2019), extreme mortality events commensurate with those observed in bighorn sheep have not been reported. Nonetheless, recent studies of sympatric mountain goat and bighorn sheep populations in Nevada documented extensive M. ovipneumoniae related mortality among mountain goat young leading to significant reductions in population recruitment (Blanchong et al. 2018; Wolff et al. 2019). Whether adult mountain goats are similarly vulnerable and the extent to which they are capable of being sources of disease for bighorn sheep populations is unclear. Again, if management of mountain goat populations is the goal, spatial and temporal separation of these ungulates and domestic sheep should occur. Considering the propensity of mountain goats to occupy steep and rugged habitats, reducing livestock interactions with mountain goats may be easier to accommodate than with bighorn sheep (Bailey et al. 2001).

7 Ecosystem Threats

7.1 Bighorn Sheep

Wild asses (Equus asinus), wild horses (E. caballus), introduced mountain goats, and introduced aoudads (Ammotragus lervia) all present issues for bighorn sheep in one or more ways. Specifically, wild asses are known to compete with bighorn sheep for forage or water and to foul water sources in western North American rangelands (Weaver et al. 1959; Seegmiller and Ohmart 1981; Marshal et al. 2008). Wild horses, although not widely sympatric with bighorn sheep, may influence use of water sources by bighorn sheep through interference competition, by competing directly with bighorn sheep for forage or water, or by altering ecosystem processes through trampling of vegetation (Ostermann-Kelm et al. 2008, 2009). 

Mountain goats and aoudads have been introduced outside of their native distributions and are sympatric with bighorn sheep in some locations. Although naturally sympatric with bighorn sheep in some areas, introduced populations of mountain goats are viewed as potential competitors with bighorn sheep for forage or space, and as possible vectors of disease (Reed 1986; Blanchong et al. 2018; Lowrey et al. 2018). Aoudads are native to North Africa and occur in bighorn sheep habitat in parts of western Texas, New Mexico, and northern Mexico. Aoudads use habitat similar to that occupied by bighorn sheep, compete with bighorn sheep for the same resources, and are agonistic or otherwise behaviorally incompatible with bighorn sheep (Seegmiller and Simpson 1979; Brewer and Hernandez 2011). Recently, concerns have arisen about the potential for pathogen transfer from aoudads to bighorn sheep (Wiedmeier 2021).

Bighorn sheep were categorized as “wilderness game” by Leopold (1933), because they may not thrive in contact with human settlement, but some populations continue to do well near urban areas. Investigators have examined effects of recreational activities (Papouchis et al. 2001; Longshore et al. 2013; Wiedmann and Bleich 2014), mineral extraction (Oehler et al. 2005; Jansen et al. 2006, 2007), and road or highway development (Epps et al. 2005; Bleich et al. 2016) on populations of bighorn sheep. Expansion of renewable energy infrastructure is of increasing concern (Kuvlesky Jr et al. 2007; Lovich and Ennen 2011), especially as it relates to negative influences on desert bighorn sheep.

Ecosystem threats to bighorn sheep have been variable and have expanded substantially in recent years; considerable research has been conducted to ascertain the influence of recreational activities. Responses of bighorn sheep to recreational disturbance have ranged from little response (Hicks and Elder 1979; Bates et al. 2021) to temporary displacement (Papouchis et al. 2001; Longshore et al. 2013; Bates et al. 2021), permanent abandonment of previously occupied habitat (Wiedmann and Bleich 2014), and altered foraging regimes (Sproat et al. 2020). Some forms of recreation affect males differently than females. For example, male bighorn sheep respond differently to shed antler hunting than  did females (Bates et al. 2021). Although mineral extraction has the potential to modify habitat, negative effects on bighorn sheep have been benign aside from the net loss of habitat associated with mine development; despite this outcome, activities associated with mining can have a positive effect in terms of landscape architecture, forage availability at revegetation sites, or deterring predation (Jansen et al. 2007; Bleich et al. 2009; Anderson et al. 2017). Further, mine reclamation can enhance per capita nutrient availability and increase population size following cessation of extraction activities (MacCallum 1992; MacCallum and Geist 1992).

Development of linear features including canals and highways likely has altered metapopulation processes by affecting movement corridors between sub populations, particularly in areas inhabited by desert bighorn sheep (Schwartz et al. 1986; Epps et al. 2005; Bleich et al. 2016). Such linear features have implications for genetic exchange between bighorn sheep populations, even though they may not be impervious barriers to movement by bighorn sheep (Epps et al. 2018). Continued fragmentation of ecosystems occupied by bighorn sheep, whether the result of infrastructure development for transportation or solar energy, will be problematic (Schwartz et al. 1986; Bleich et al. 1996).

Bighorn sheep occupy habitats ranging in elevation from below sea level to nearly 4500 m; as such they are adapted to a wide variety of environmental conditions. Thus, a changing climate has ecosystem-level implications for population persistence and habitat quality for this species. Vegetation changes resulting from a changing climate will affect distribution and habitat use by bighorn sheep (Epps et al. 2004) and will have evolutionary implications (Bleich 2017) and potential physiological challenges. Nevertheless, responses of bighorn sheep to changes in ecosystem structure or function are influenced greatly by the consistency, predictability, and level of threat associated with each disruption rather than the mere presence of people or other perturbations perceived as benign by these large ungulates (Wiedmann and Bleich 2014). Ultimately, the fate of bighorn sheep is tied to the size and needs of the human population (Bowyer et al. 2019).

7.2 Mountain Goats

Landscapes used by mountain goats are subject to a variety of conventional and non-conventional threats. Timber harvest (with its associated roads and infrastructure), mining, and hydroelectric development can have negative effects on mountain goats because of habitat removal or disturbance (Hebert and Turnbull 1977; Foster and Rahs 1985; Joslin 1986). Mining activity at a site in coastal Alaska resulted in a 42% reduction in carrying capacity of winter range habitat for a local population because of apparent displacement effects (White and Gregovich 2017). In other areas, logging of forested winter range resulted in direct removal of important winter habitat, or indirect effects because of disturbance or increased access and subsequent harvest (Hebert and Turnbull 1977).

Mountain goats are obligates of steep terrain and thus sensitive to climate-induced changes in high-elevation environments, particularly heat stress during summer (Sarmento et al. 2019) or severe snow conditions during winter (White et al. 2011; Richard et al. 2014). Climate change may have negative effects on mountain goat populations because of shrinkage of alpine habitats and through indirect effects associated with thermal stress or deleterious change in nutritional characteristics of summer foraging ranges (White et al. 2018). Although changes in climate may negatively influence population dynamics of mountain goats in some regions, further study is needed to assess how dynamics vary across the broad distributional range of the species and whether populations respond more strongly in some areas, as compared with others (White et al. 2018).

8 Conservation and Management Actions

8.1 Bighorn Sheep

Regulated hunting has played an important role in the conservation and reintroduction of bighorn sheep into rangelands of western North America (Monteith et al. 2013; Hurley et al. 2015). Economic considerations, largely in response to demand for hunting opportunities, have been an important force driving the restoration of bighorn sheep (Lee 2011; Gonzalez-Rebeles Islas et al. 2019). Much of the money garnered through the sale of bighorn sheep hunting tags is used for restoring populations of bighorn sheep to rangelands in western North America (Krausman 2000).

Active restoration of bighorn sheep to their historical distribution has been ongoing for about 100 years. Reintroductions and translocations remain an essential component of bighorn sheep management and conservation (Krausman 2000; Whiting et al. 2012; Sandoval et al. 2019). Recovery of populations of bighorn sheep largely has been a function of successful programs to return these mountain ungulates to their historical ranges, and translocations have contributed to the restoration or maintenance of ecosystem function in alpine or desert regions in much of western North America (Kie et al. 2003; Flesch et al. 2020). Past efforts to restore bighorn sheep to historical habitat have involved extensive efforts by resource-management agencies and conservation organizations, and tremendous financial commitments (Hurley et al. 2015; Donovan et al. 2020). Although translocation has been the primary tool used to reestablish bighorn sheep in rangelands across western North America, use of that method may become more limited in the foreseeable future, in part because of growing recognition that moving animals always includes risk of potentially moving diseases or exposing individuals to disease at the release site.

Management efforts surrounding infectious disease fall into one of two broad categories: actions to limit risk of pathogen spillover, and actions to limit pathogen burden following its introduction. Bighorn sheep often are culled by state wildlife agencies when they are discovered wandering outside of their established ranges to keep them from carrying pathogens back to their herd. At the same time, domestic sheep producers have experienced increasing restrictions on public land grazing allotments near bighorn habitat, leaving federal land-management agencies caught between maintaining healthy bighorn herds and maintaining grazing permits. Formal risk assessment tools exist (O’Brien et al. 2014), but an ongoing evolution in wildlife tracking technology means that the precise methods on which the tools rely are subject to regular revision and updating. Both culling and loss or modification of grazing permits engender frustration within their respective communities, but in the absence of effective treatments, limiting spillover risk through species separation remains the most effective strategy for protecting bighorn sheep (Brewer et al. 2014; Jex et al. 2016).

A suite of new tools is emerging to manage populations struggling to rebound from pathogen introductions. Wildlife management agencies have employed strategies ranging from complete depopulation followed by reintroduction to selective culling of individuals. Although efficacy of these actions has varied, test-and-remove (Garwood et al. 2020) and range expansion (Lula et al. 2020) strategies appear to have promise (Almberg et al. 2021). Under test-and-remove, managers trap and test as many (typically female) individuals as possible within a population, identify chronic carriers, and remove these animals. This option has yielded encouraging results, but it is labor intensive, and of variable efficacy (Paterson et al. 2020). Range expansion involves splitting an infected herd into multiple subunits to reduce densities and sequester the pathogen into unique subunits of the herd. The premise is that sequestration will facilitate local fade-out of the disease. Range expansion has been associated with improved demographic responses in at least one well-studied herd and is currently being tested in several other settings.

8.2 Mountain Goats

Mountain goats are widely appreciated as big game for their consumptive and non-consumptive value. Indigenous peoples valued this species for subsistence purposes including the use of wool, horns, and hooves in culturally significant ways (Rofkar 2014). The viewing and hunting of mountain goats generate substantial economic returns and re-investment into species conservation. Native populations of mountain goats exhibit low population growth rates and are sensitive to overharvest, especially if females are removed (Hamel et al. 2006; Rice and Gay 2010; White et al. 2021a); in some instances, introduced populations may be more productive, resilient, and able to sustain higher harvest rates, particularly during initial phases of establishment and expansion (Williams 1999; DeCesare and Smith 2018), but contrary results exist (Côté et al. 2001).

Relative to other ungulates, mountain goats are particularly sensitive to mechanized disturbance associated with commercial and recreational activities (Côté 1996; NWSGC 2020). For example, helicopter overflights or other forms of mechanized disturbance (energy development, blasting, and all-terrain vehicle use) can negatively affect mountain goat foraging behavior, movement patterns, and population dynamics, and mountain goats do not typically habituate to human disturbance (Joslin 1986; Côté et al. 2013; St‐Louis et al. 2013). In places where industrial-scale mechanized disturbance occurs, mitigation to lessen or avoid negative effects is important to ensure population sustainability and persistence (NWSGC 2020).

9 Research and Management Needs

9.1 Bighorn Sheep

Historically, translocations and reintroduction of bighorns sheep to rangelands has been somewhat problematic. These problems have stemmed from issues related to habitat suitability, lack of migration opportunities, genetic issues, lack of understanding of ecotypic or phenotypic adaptation, predation, and disease transmission (Risenhoover et al. 1988; Rominger et al. 2004; Whiting et al. 2011b; Bleich et al. 2018). During recent years, disease concerns have been at the forefront of investigations or concern, and likely will remain so. Although there is general concurrence that fires enhance quality of bighorn sheep habitat through increased visibility or forage quality, responses of bighorn sheep to various fire-management strategies (e.g., suppression, wildfire, and prescribed fire) is a meaningful field in need of further inquiry. The utility of natural or artificial barriers that could provide a hedge against pathogen transfer among populations separated by those barriers is worthy of investigation, particularly from a cost–benefit perspective. For example, “What are the evolutionary consequences of maintaining artificial separation using barriers relative to the costs of pathogen spillover and its potential to affect, or perhaps even to decimate, nearby populations, and over what period of time would such costs accrue?” Related to this issue are questions about which bighorn sheep are most apt to make exploratory movements, or to pioneer unoccupied areas. The traditional thinking has been that young males are most apt to do so, but mature males and females also make such moves. The sex and age of the animals involved in such forays has important implications for demography and formulation of hunting regulations.

9.2 Mountain Goats

Mountain goats are among the least-studied large mammals in North America because of the difficulty, expense, and inherent danger of studying a species in remote and rugged landscapes. Although long-term and detailed studies have been conducted in specific areas resulting in substantial advancement of our knowledge of mountain goat ecology (Festa-Bianchet and Côté 2008), key knowledge gaps continue to limit our understanding about how population biology varies across the range of ecological settings inhabited by the species, including neonate survival, density-dependent effects, proportional causes of mortality, predator–prey relationships (including apparent competition), and small population-size effects. For example, recent mountain goat demographic studies have demonstrated reduced resilience and increased risk of extirpation among small populations, as compared with large populations (Hamel et al. 2006; White et al. 2021a). Improved understanding of the relative importance of underlying mechanisms, however, would aid in refining fine-scale conservation strategies. More broadly, detailed understanding of the mechanistic effects of weather and climate, specifically heat stress, represents an important need. Additionally, further study is needed to better understand how industrial or recreational disturbance influences behavior, vital rates, and resultant population productivity (NWSGC 2020).