Pronghorn

Abstract

Pronghorn (Antilocapra americana) are an endemic ungulate in western North America and occupy rangelands concurrently with domestic livestock. When rangelands are in healthy condition, there is little-to-no competition between pronghorn and domestic livestock. When rangeland health deteriorates, direct competition occurs when both compete for limited resources. Pronghorn are a highly mobile species that cope with challenging environmental conditions (both natural and human-imposed) through daily and seasonal movements to more favorable habitats. Maintaining healthy rangelands and rangeland connectivity will allow pronghorn to move freely and adapt to increased human disturbance. In addition, understanding the cumulative effects and identifying mitigation strategies of deleterious anthropogenic effects (i.e., habitat conversion, linear features, energy development, and climate changes) will help to ensure long-term persistence of pronghorn populations. Mitigation will be critical, in conjunction with expanded research efforts, to help gain a greater knowledge of the role of environmental conditions and anthropogenic disturbances on pronghorn fitness, persistence, and their ability to move across the land in response to an ever-changing landscape.

1 General Life History and Population Dynamics

Pronghorn (Antilocapra americana), commonly called antelope, are an endemic western North American (Fig. 19.1) ungulate that are found nowhere else in the world. The unique pelage of pronghorn makes them readily identifiable, with a large white rump, white underbelly, white bands on the neck, and a dark nose (Fig. 19.1). Both males and females possess horns, but when present in females, they tend to be shorter than their ears (O’Gara 2004a). The name pronghorn comes from the front prong on the horns of mature males (Fig. 19.1). Pronghorn are the world’s second fastest land mammal, able to reach speeds between 70 and 100 km hr⁻¹ [40–60 miles hr⁻¹ (O’Gara 2004a)].

Fig. 19.1

The 2000 geographical distribution of pronghorn across North America. Photos are of a female (a) and a male (b) pronghorn in the grasslands of Alberta, Canada. Photos: P. Jones, Alberta Conservation Association. Spatial data source: Jensen et al. (2004)

Pronghorn are the last remaining species of their taxonomic family (Antilocapridae), which roamed North America during the Pleistocene epoch (O’Gara and Janis 2004a). The current form of pronghorn has evolved over the last 20 million years (O’Gara and Janis 2004a). The most common of five subspecies is A. a. americana which is widely distributed from Texas, north into Alberta and Saskatchewan. The Sonoran pronghorn (A. a. sonoriensis) is the smallest subspecies, is classified as endangered, and can be found only in southwestern Arizona and northwestern Sonora, Mexico (USFWS 2015). The Mexican pronghorn (A. a. mexicana) is an endangered subspecies found in Mexico and the Marathon Basin of Texas (O’Gara and Janis 2004b). The peninsular pronghorn (A. a. peninsularis) is also an endangered subspecies found in the Vizcaino Desert, Mexico (O’Gara and Janis 2004b). The Oregon pronghorn (A. a. oregona) is a subspecies found in Oregon, Idaho, California, and Nevada (O’Gara and Janis 2004b). However, Lee Jr (1992) analyzed mitochondrial DNA and concluded that pronghorn in the range of A. a. oregona were not dissimilar to A. a. americana and therefore should not be treated as a subspecies. While 3 subspecies are currently federally listed as endangered, the most recognized subspecies A. a. americana are common on the rangelands of western North America, though local populations vary in their conservation status (Jakes 2021).

2 Distribution and Population Status

2.1 Distribution

The current distribution of pronghorn spans 23 jurisdictions including 17 US states, 2 Canadian provinces, and 4 Mexican states (Yoakum et al. 2014). The range of pronghorn in 2000 is depicted in Fig. 19.1 (Jensen et al. 2004). Missing from the figure are populations in Washington that are the result of recent re-introductions (Jakes 2021). Almost half of all pronghorn are found in Wyoming, and approximately 80% of the population occurs within Colorado, Montana, New Mexico, and Wyoming, with these 4 states being considered the “core area” of suitable pronghorn habitat (Schroeder 2018; Jakes 2021). While pronghorn occupy most of their historic range, their numbers are drastically lower than prior to European settlement (Yoakum 2004a). The 2017 population estimate was just under 1 million pronghorn, compared with historical estimates of 30–40 million (Yoakum 2004a; Schroeder 2018; Jakes 2021).

2.2 Monitoring

Pronghorn are surveyed to determine population estimates and demographic data for setting harvest rates by each jurisdiction or for assessing species status across their range. Population surveys for pronghorn are dependent upon the survey objective(s), local habitat, population density, and the distribution (e.g., evenly distributed, clumped, etc.) of animals across the landscape (Yoakum et al. 2014). Most jurisdictions that survey pronghorn use aerial surveys (via fixed-wing aircraft or helicopter), with a few still using ground surveys (Schroeder 2018). Surveys to detect animals using fixed-wing aircraft disturb pronghorn less than helicopters due to being flown at higher altitudes with lower noise levels (Yoakum et al. 2014). Survey protocol and coverage is often dictated by available financial resources and human safety requirements. Most surveys are conducted between May and August when pronghorn are most widely distributed, in smaller groups, with mobile and detectable fawns allowing for the classification of both sex and age structure (Yoakum et al. 2014).

A variety of survey protocols have been employed to estimate pronghorn population size including: (1) strip transects, (2) line transects with distance sampling, and (3) quadrats or area sampling (Pojar and Guenzel 1999; Pojar 2004). While the ideal survey would produce a population estimate with an associated confidence interval, this is not always achievable. Some jurisdictions have used the strip transect method and relied on trend counts to assess annual differences in relative population estimates. The detection of a change using trend data is contingent upon the assumption that survey conditions (e.g., weather, time of survey, habitat, observer, etc.) are consistent and that the percentage of animals detected is similar between surveys (Nichols 1992). Recent developments in survey methodology and statistical analysis allows for more precise population estimates. For example, the use of line transects with distance sampling allows for the correction of population estimates based on the detection probability of observing animals on the transect (Ward 2016). Whichever survey protocol is used, one should strive to minimize bias (e.g., observer, survey assumptions), produce the most precise estimate possible, and validate visibility bias for the geographic region and survey protocol to which they will be applied (Guenzel 1997; Ward 2016).

In addition to estimating population size, most surveys assess the ratio of fawns and males (bucks) to females (does; i.e., ff:dd and bb:dd ratios). Late summer is the optimal time to conduct classification surveys, especially to estimate ff:dd ratios as postnatal fawn mortality has subsided and fawns are still easily distinguishable from females, which is not the case come fall or winter (Yoakum et al. 2014). The ff:dd ratios can be used to estimate recruitment in population models. Fall surveys are not ideal for estimating bb:dd ratios because fawns can be mistaken for adult females, which inflates the female count and widens the bb:dd ratio (Yoakum et al 2014). Winter surveys are not ideal as males lose their horn sheaths after October which could result in younger males being classified as females and would underestimate the bb:dd ratios. The bb:dd ratios are used as sex ratios in population models. Linking demography data (sex, age) with spatiotemporal variables can help forecast and classify populations based on current structure, as well as current and future landscape conditions (Arnold et al. 2018). Pronghorn are considered to have ecological and economic value across their range and therefore, State and Provincial agencies are responsible for setting pronghorn harvest rates (Jakes 2021; Stoner et al. 2021). Demography data combined with population estimates form the foundation for which decisions on pronghorn harvest levels are made by wildlife managers. Most pronghorn tag numbers are based on limited-quota or limited-entry licences due to the low number of animals in most states and provinces (O’Gara and Morrison 2004), making pronghorn one of the most sought-after harvestable species.

3 Habitat Associations

3.1 Historical/Evolutionary

Pronghorn are largely found in the same habitats that they occupied historically, including Grasslands (i.e., southern mid-grass prairie, northern mid-grass prairie, short grass prairie; hereafter grasslands), Intermountain Valleys and Lower Mountain Slopes (e.g., Great Basin Sagebrush [Artemisia], Sagebrush Steppe [Artemisia-Perennial Bunchgrasses]; hereafter shrub-steppe), and Warm Deserts and Grasslands (i.e., Chihuahuan Desert, including chaparral in Mexico; Sonoran Desert, including chaparral in Arizona; hereafter desert; Yoakum 2004a). Collectively we refer to grasslands, shrub-steppe, and desert as rangelands. Pronghorn, with their excellent long-distance vision and speed are uniquely adapted to these relatively flat, rolling landscapes (O’Gara 2004a). Many of these adaptations are relics of the predator species with which pronghorn coexisted millions of years ago. Their ability to reach tremendous speeds, for example, is attributed to the ancient predation threat of the now-extinct American cheetah (Miracinonyx spp.; Byers 1997). Pronghorn once roamed alongside ungulates including camels (Paracamelus spp.) and tapirs (Tapiris spp.) and faced predation threats from saber-toothed cats (genera Megantereon, Smilodon, and Homotherium), giant short-faced bears (Arctodus simus), and dire wolves (Canis dirus; Byers 1997; McCabe et al. 2004). More recently pronghorn share habitat with bison (Bison bison), elk (Cervus canadensis), deer (Odocoileus spp.), gray wolves (Canis lupus), golden eagles (Aquila chrysaetos), mountain lions (Puma concolor), bobcats (Lynx rufus), and coyotes (Canis latrans) (Byers 1997). While pronghorn still occupy rangelands with other ungulate species, many predators that were previously common in pronghorn habitats are often absent today or occur at lower densities than they did historically (Byers 1997). Therefore, predation is typically not a limiting factor for most pronghorn populations. However, predation of fawns can be significant, and, in some populations, adult predation can be high (O’Gara 2004b; Keller et al. 2013).

Historically, fires were the chief disturbance in the grassland, shrub-steppe, and desert regions that pronghorn occupy (Yoakum 2004b). It has been suggested that reduced shrub density and increased forb availability resulting from periodic burns are likely to benefit pronghorn populations (Greenquist 1983; Augustine and Derner 2015). As Europeans settled in North America in the early 1800s, such natural disturbance regimes were altered, and new sources of habitat changes ensued, resulting in habitat conversion, loss, and fragmentation (Greenquist 1983; O’Gara and McCabe 2004). Across much of the current pronghorn range, vast networks of wire fencing associated with nineteenth century property delineation and livestock production are still present. These fences currently represent a source of direct and indirect pronghorn mortality (Oakley 1973; Harrington and Conover 2006; Jones 2014; Jones et al. 2019) and alter behavior and movement (Jakes et al. 2018a; Seidler et al. 2018; Reinking et al. 2019; Smith et al. 2020). The conversion of rangelands that began in the 1800s, coupled with additional anthropogenic development since, has reduced native habitat availability to pronghorn and caused deterioration of rangelands through erosion, weeds, conifer encroachment, and brush removal (O’Gara and McCabe 2004). This habitat loss and degradation continue to present issues across much of the current pronghorn range and are further described in Sect. 19.7.

3.2 Contemporary

Current pronghorn habitat is characterized by low, rolling hills with limited visual barriers, and ranges in elevation from roughly 0 to 3000 m (0–9850 ft) above sea level (Yoakum 2004b). Vegetation in pronghorn habitats mainly consists of grasses, forbs, and shrubs, with vegetation height typically ranging from 13 to 76 cm (5–30 in), though use at the upper end of this height range is minimal (Yoakum 2004c). The usage of vegetation types for forage varies by location, availability, and season, and is described in Sect. 19.5.1.

Annual precipitation varies widely across pronghorn range, but most animals occur in areas receiving 20–40 cm (8–16 in) annually (Yoakum 2004b). Population persistence depends on both the amount and timing of annual precipitation (Brown et al. 2006; Simpson et al. 2007). Precipitation during late gestation and lactation may be especially important, particularly for animals in the arid southwestern United States (Gedir et al. 2015). During colder seasons, most of the current pronghorn range (70%) typically experiences precipitation in the form of snow (Yoakum 2004b). Pronghorn mainly rely on snow and free water (Yoakum et al. 2014), but succulent forage may also be used as a water resource in drier areas or drought years (Büechner 1950; Beale and Smith 1970; Clemente et al. 1995).

Pronghorn habitat requirements also include topographic and vegetative features (e.g., taller shrubs) that provide protection (i.e., cover) from both the elements and predators. Thermal cover can include shade-providing features (e.g., tall trees and shrubs) to help keep animals cool when air temperature is high (Yoakum 2004b; Wilson and Krausman 2008). However, pronghorn have a high heat tolerance and are typically able to mitigate high temperatures through unique morphological and physiological adaptations. Topographic and vegetative features can provide refuge from high wind speeds by minimizing wind chill in low temperatures, while offering areas of shallower snow (Bruns 1977; Ryder and Irwin 1987). Security cover that provides protection from predators is also required, but mainly as fawn hiding habitat (Barrett 1982; Jacques et al. 2015).

3.3 Seasonal

Suitable pronghorn habitat must provide adequate seasonal ranges, as well as functional landscapes connecting seasonal ranges (see Sect. 19.4). These varied habitats allow pronghorn to maintain access to forage, minimize energetic demands, and maximize fitness (i.e., survival and reproductive success) as resources fluctuate annually (Dalton 2009; Yoakum et al. 2014). In winter, pronghorn seasonal ranges are generally larger than in summer (Sheldon 2005; Reinking et al. 2019). Winter range is largely selected to avoid deep snow and maximize the period of exposure to high quality forage and can be either lower in elevation or latitude than summer range, fawning areas, or migration habitat (Yoakum 2004b). Snow depths < 15 cm (< 6 in) are preferable in pronghorn winter range both to maintain forage accessibility above the snowpack and mitigate the energetic costs of locomotion through snow (Yoakum et al. 2014). Snow depths become particularly detrimental at roughly 30 cm (12 in), limiting access to forage, and when at mid-limb height on an individual, inhibiting their movement (Telfer and Kelsall 1984; Yoakum et al. 2014). The interaction of snow conditions and anthropogenic features like railroads, highways, and fences can also present extreme challenges (Jones et al. 2020a). Deep snow can force animals onto snow-cleared railroads and highways that offer easier movement (O’Gara 2004b) but increase the risk of collision and energy expenditure (Seidler et al. 2018; Jones et al. 2020a). Moreover, deep snows can reduce the open space beneath wire fences, eliminating the ability of pronghorn to pass underneath and move to more suitable habitats during winter when resources are already limited (Bruns 1977; Sheldon 2005; Yoakum et al. 2014; Seidler et al. 2018).

Summer range requirements are largely synonymous with ideal fawning habitat. These areas provide high quality herbaceous vegetation for does and fawns, offer sufficient vegetative cover to protect fawns and vulnerable birthing females from predators, and usually have higher temperatures with little to no snow (Yoakum 2004b). Unlike other ungulates that largely rely on previously acquired fat stores to fuel reproduction and survival (i.e., capital breeders; Jönsson 1997), pronghorn are thought to be income breeders, meaning that they mainly meet energetic demands as they arise with the immediate intake of resources (Smyser et al. 2005; Reinking et al. 2018). Therefore, fawn survival and the survival of adult females facing the high energetic costs of reproduction are dependent on high forage quality and availability on summer range (Smyser et al. 2005; Reinking et al. 2018; Panting et al. 2020; Bender and Rosas-Rosas 2021).

4 Movement, Migration, and Dispersal

Pronghorn move amongst and between habitats or to completely new suitable habitats for population maintenance (Dingle and Drake 2007). Movements undertaken by pronghorn provide connections between suitable habitats across spatiotemporal scales, which include daily movements amid vegetation patch types, annual migrations between seasonal ranges, or dispersal events to seek out appropriate habitat in new areas, thus providing functional connections between herds and populations (Sawyer et al. 2005; Jacques and Jenks 2007; Kolar et al. 2011; Collins 2016; Jakes et al. 2018b). Because migration is an annually repeated phenomenon, it can be a useful focus for identifying and maintaining landscape connectivity to sustain pronghorn populations. Pronghorn use such movements to maximize access to high-nutrition vegetation, improve physical condition to increase reproductive success, find mates, decrease intraspecific competition, and respond to changing environmental conditions (Hoskinson and Tester 1980; Bolger et al. 2007; Barnowe-Meyer et al. 2017). Across North America, caribou (Rangifer tarandus) and mule deer (Odocoileus hemionus) are the only ungulates reported to have made greater annual long-distance movements than pronghorn (Joly et al. 2019).

Pronghorn populations are often partially migratory (White et al. 2007; Jacques et al. 2009; Kolar et al. 2011; Jakes et al. 2018b), meaning that some individuals migrate, and others do not (Dingle and Drake 2007). At the northern range, pronghorn that migrated were found to have a 7% increase in survival probability, compared to individuals that remained residential (Jones et al. 2020a). Some pronghorn individuals switched movement tactics from one year to the next (Jakes et al. 2018b), suggesting that pronghorn exhibit plasticity in movement decisions. Indeed, factors such as demography and learning through social interactions, may also influence the strategy employed, indicating that migration may not be a fixed behavior (Bauer et al. 2011; Barnowe-Meyer et al. 2013; Jesmer et al. 2018).

Depending on the distance and duration of migration, pronghorn may use stopover sites to energetically recover and amass fat and protein reserves to complete their journey (Bolger et al. 2007; Sawyer et al. 2009). Stopover sites are typically areas of higher forage productivity with lower densities of anthropogenic features relative to migratory pathways (Jakes 2015). However, pronghorn may stopover along suboptimal areas such as roads and fences (Seidler et al. 2015). These human-induced stopovers can delay migration and deplete important energy reserves needed to navigate terrain successfully or detect alternative locations to traverse these features. In some instances, linear features become an impermeable barrier and deter pronghorn crossing opportunities altogether.

Other long-distance movements by pronghorn have been observed at various times of year. Across their range, pronghorn may display unpredictable movements to apparently follow forage maturation and availability (e.g., nomadism) as opposed to exhibiting fidelity to any one area, although this is not well understood (Milligan et al. 2021; Morrison et al. 2021). Alternatively, long-distance movements may occur as a survival tactic in response to stochastic events such as fire, drought, or extreme snowfall. For example, at the northern periphery of pronghorn range, movements from one winter range to another in response to extreme environmental conditions (i.e., facultative winter migration), as well as movements from an initial distinct fawning range during known parturition dates to a separate summer range (i.e., potential post-fawning migration), have been reported (Jakes et al. 2018b). In general, facultative winter migrations made by pronghorn occurred from winter range, where sagebrush and other forage was unavailable, to winter range where sagebrush was accessible (Jakes et al. 2018b).

Pronghorn seasonal and daily movements are influenced by environmental gradients and anthropogenic factors. In general, pronghorn spring migrations follow the ‘green-wave’ of available forage to acquire protein-rich resources while avoiding heavily used or high densities of human development (Mysterud 2013; Jakes et al. 2020). For pronghorn, anthropogenic disturbances include features such as roads, fences, energy infrastructure, and other developments such as houses (Sheldon 2005; Jones et al. 2019; Jakes et al. 2020). During fall migration, pronghorn tend to select for native grasslands and avoid roads, with some populations also following large stream and river systems, to quickly arrive onto winter grounds (Jakes et al. 2020). Unfragmented rangelands offer the best areas for pronghorn to move through during these succinct, yet important migratory periods. Alternatively, examination of daily movement rates can identify spatiotemporal factors that are significant to pronghorn movements, including migration and dispersal (Jones et al. 2017). Increased movement rates were observed following periods where migrations were protracted by linear features such as roads and fences, which may act as semi- or complete barriers to movement (Seidler et al. 2015). While spatiotemporal components are extremely important in understanding pronghorn movements, cognitive learning, as well as individual and group memory, likely influence pronghorn movements, though these are not fully understood (Barnowe-Meyer et al. 2013).

5 Interaction with Livestock Grazing Management

It is estimated that 99% of pronghorn populations share their distribution with domestic or feral livestock (Yoakum 2004d; Stoner et al. 2021) including domestic cattle (Bos taurus), sheep (Ovis aries), and domestic and feral horses (Equus ferus caballus), with low co-occurrence with pigs (Sus domesticus), goats (Capra hircus), and burros (E. asinus). With such a large overlap in distribution, interactions between pronghorn and livestock are inevitable. These interactions may be direct (i.e., diet overlap or competition for forage/water) or indirect (i.e., management practices for livestock affect habitat selection by pronghorn). The following subsections will focus on the direct and indirect interactions between pronghorn with domestic cattle, sheep, and feral horses.

5.1 Forage Competition and Diet Overlap

Pronghorn have physiological traits similar to other concentrate feeders (Van Soest 1994) and intermediate feeders (Hofmann 1989), suggesting they are adapted to feed on diets high in cell solubles, such as forbs and higher quality shrubs. Showing preference for forbs and shrubs during all seasons and having a digestive system engineered to pass food through the system relatively quickly is consistent with the intermediate (Hofmann 1989) or mixed feeder category (Kauffman et al. 2021). Indeed Yoakum (2004c) called pronghorn “forage switchers” because of their ability to switch forage preference to take advantage of succulent vegetation resulting from seasonal phenological changes. To demonstrate, pronghorn forage on grasses that tend to green up before forbs during spring, then switch to predominantly forbs during summer months, then switch to shrubs in fall and winter (Mitchell and Smoliak 1971; Pyrah 1987; Yoakum 2004c). In grassland diet studies, the vegetation composition was predominately grass (74%), followed by forbs (16%), and shrubs (9%) with pronghorn diet selection being predominately forbs (62%), followed by grasses (19%) and shrubs (17%; Yoakum et al. 2014). In contrast, the vegetation composition in shrub-steppe studies was predominately shrubs (46%), followed by grasses (37%), and forbs (15%), with pronghorn diet selection being predominately shrubs (62%), followed by forbs (30%), and grasses (7%; Yoakum et al. 2014). The diet preference between forbs and shrubs in the desert biome is regulated by sporadic precipitation, with forbs being preferred when adequate rainfall provides succulent forbs (Cancino 1994; Yoakum 2004c). The diets of desert-dwelling pronghorn likely include more succulent and cacti species than are consumed by populations in grasslands and shrub-steppe (Yoakum 2004c). Pronghorn in extremely arid environments utilize succulents not only to meet their nutritional requirements, but also as a major water source (Büechner 1950; Beale and Smith 1970; Clemente et al. 1995). In years of particularly severe drought, succulents may be crucial for pronghorn survival.

The documented breadth of forage species selected by pronghorn is tremendous with the use of 124 different species (96 forbs, 14 shrubs and 14 grasses; Mitchell and Smoliak 1971; Pyrah 1987). Of the plants identified as being consumed by pronghorn, 21 were considered poor forage, and 51 were unpalatable to livestock (Büechner 1950). Indeed, pronghorn consume many plants considered toxic or poisonous to livestock, including locoweed (Astragalus spp.), larkspur (Delphinium spp.), lupine (Lupinus spp.), and death camas (Toxicoscordion spp.), to name a few (Einarsen 1948; Büechner 1950; Yoakum 2004c).

Pronghorn propensity and variety of plant species consumed results in little to no competition for forage with cattle and horses, but competition can be extensive with domestic sheep. There is little dietary overlap between pronghorn, which prefer forbs, and domestic cattle and horses, which prefer grasses (Yoakum et al. 2014; Scasta et al. 2016). Yoakum (2004d) determined the annual diet overlap was less than 25% between cattle and pronghorn, and less than 36% between horses and pronghorn. Domestic sheep prefer forbs, which results in intense competition for forage with pronghorn, and diet overlap can range between 33% (moderate overlap) and 66% (high overlap; Yoakum 2004d). Yet, diet overlap with livestock in general is based on rangeland conditions being in good health, and when rangeland conditions deteriorate, competition for remaining forage intensifies (Yoakum 2004d). In addition, indirect competition may occur in areas where habitat quality is decreased through soil compaction and increased erosion (Eldridge et al. 2020). Lastly, there are specific instances when competition can be prevalent. For example, feral horses can compete directly with pronghorn in arid environments for water resources (Gooch et al. 2017). While Hennig et al. (2021) found significant temporal overlap in the use of watering sources between pronghorn and feral horses in Wyoming they could not conclude that interference was occurring between the 2 species. However, they did note the infrequent occurrence of both species being observed together.

5.2 Rangeland Management Practices

Western rangeland management has historically been for the benefit of livestock production, but recently, specific management actions have been completed with wildlife solely in mind. Management actions fall into two categories: (1) livestock grazing management, and (2) rangeland improvements for livestock and/or wildlife. Actions associated with livestock management include type and breed of livestock grazed, grazing intensity (i.e., stocking rate), timing of grazing (e.g., year-long, spring, etc.), and grazing system (e.g., rest-rotation, deferred, etc.). Pronghorn occupy an assortment of rangeland types; therefore, it is not our intent to evaluate and/or recommend prescriptive livestock management actions. However, we provide general livestock grazing management principles that can be practiced across a diversity of vegetation communities to improve or maintain pronghorn habitat. Rangelands that are maintained in good ecological condition and provide ecological resiliency will benefit both pronghorn and livestock (Yoakum 2004d). Livestock managers should consult with local rangeland specialists and wildlife managers when designing their grazing system. The following general recommendations are adapted from Yoakum (2004d) and Yoakum et al. (2014) and are intended as guidelines for livestock managers to enhance pronghorn habitat while maintaining high-quality livestock grazing:

• Livestock grazing systems should be designed around the local ecosystem and vegetation community and should account for the forage needs of pronghorn. Grazing systems that result in seral vegetation conditions and closely resemble the ecological potential of the local area will provide the greatest benefit to pronghorn. Grazing systems that restrict, alter, limit, or deleteriously affect the native vegetation community will negatively impact pronghorn habitat and should therefore include mitigation and alternative procedures for enhancing pronghorn habitat. As part of the grazing system, adequate amounts of preferred forage should be allocated for pronghorn and should include a variety of forbs, shrubs, and grasses identified as key forage species for pronghorn.

• Grazing capacity should be designed around the local ecosystem and vegetation community and should account for the forage needs of pronghorn. Grazing capacity should be modified based on annual precipitation levels (e.g., reduced during drought). Livestock should be restricted from key pronghorn fawning areas during the fawning season to ensure adequate forage and hiding cover.

• Livestock mangers should consider developing a ranch or allotment management plan that accounts for the needs of their livestock as well as local wildlife populations, including pronghorn.

Rangeland improvement and wildlife enhancement projects are used by livestock managers to either improve existing forage or change the utilization of existing forage by redistributing livestock (Yoakum 2004d). Improvements focused on enhancing existing forage include seeding, brush control, and burning. Seeding projects can be beneficial or detrimental to pronghorn, depending on the species used to seed the area. If the seed mixture includes forb and shrub seeds, then the project can enhance pronghorn habitat (Yoakum 2004d) but comes at a higher monetary cost than just seeding a monoculture of grasses (Yoakum et al. 2014; Downey et al. 2013). Historically, seeding projects, in which the goal was to increase forage for livestock or establish permanent vegetative cover, used seed mixtures limited to either a single or a few grass species (Yoakum 2004d). The lack of vegetation diversity established on these seeded sites generally made them poor pronghorn habitat (Yoakum et al. 2014). Recently, seeding projects have begun to use mixtures of native species to re-establish rangelands for both wildlife and livestock utilization (Downey et al. 2013; Espeland 2014). Areas that have entered late successional stages and are dominated by shrubs and shrubby trees provide poor pronghorn habitat and are of limited value to livestock. For pronghorn, once an area becomes composed of 25% or greater shrub cover, with shrubs that are ≥ 76 cm (≥ 30 in) tall, the area provides poor pronghorn habitat because of limited forage availability and the resulting reduction in predator detection capacity (Yoakum 2004e; Yoakum et al. 2014). Areas with high shrub cover and height can be treated, either mechanically, chemically, or through prescribed fire; however, prior to any management the habitat needs of other sensitive species (e.g., sage-grouse, Centrocercus spp.) in the area should be considered. Yoakum (2004e) recommended shrub treatment projects be no larger than 405 ha (1000 ac) and implemented in a mosaic fashion so not all shrubs (especially those palatable to pronghorn) are removed; 5–20% retention of shrubs is ideal to maintain winter forage and fawn hiding habitat (Bayless 1969; Bruns 1977; Barrett 1981). Fire has the potential to benefit pronghorn if it returns climatic vegetation communities back to early successional stages of forbs and grasses (Yoakum 2004e). Pronghorn quickly move into areas following a fire and readily forage on newly sprouted forbs and cacti that have had their spines burned off (Courtney 1989; Van Dyke 1990; Payne and Bryant 1998; Augustine and Derner 2015). In areas with silver sagebrush (A. cana) burning resulted in low plant kill rates and vigorous resprouting (White and Currie 1983). However, other species of sagebrush (e.g., A. tridentata subsp. wyomingensis) when burned can create conditions were invasive species such as cheatgrass (Bromus tectorum) become dominant (Davies et al. 2007; Crist et al. 2021). Therefore, caution should be exercised before using fire in sagebrush habitat as the impacts to pronghorn habitat can be detrimental.

Improvement practices associated with livestock distribution are frequently employed on western rangelands. These practices, such as fencing, water development, salting/mineral supplementation, and in the case of domestic sheep and goats, herders, are implemented to enhance livestock distribution to maximize the use of available forage. Fencing has historically impacted pronghorn negatively and is discussed in Sect. 19.5.3. Pronghorn will readily use natural and artificial water sources (Einarsen 1948; Beale and Smith 1970; Gooch et al. 2017). Water developments allow greater pronghorn distribution, particularly during dry seasons or periods of drought (Beale and Smith 1970). However, Yoakum (2004e) noted that water developments have the potential to cause competition by allowing livestock to move to previously under-utilized areas; therefore, new water developments need to be assessed in terms of their benefit or disadvantages for pronghorn. Pronghorn will utilize salt and mineral blocks placed on the landscape to improve the distribution of livestock, but the nutritional benefits to pronghorn are poorly understood (Yoakum et al. 2014).

5.3 Fencing and Pronghorn

Fencing is a ubiquitous feature on rangeland landscapes (Jakes et al. 2018a; Mcinturff et al. 2020), and as far back as 1877 has been documented as a detriment to pronghorn (Caton 1877: 48 in Yoakum et al. 2014). Having evolved on treeless landscapes, pronghorn have not developed an instinct to jump over vertical obstacles, including fences (O’Gara 2004c), although they can physically jump (Harrington and Conover 2006; Jones et al. 2018, 2020b). Fences can cause mortality when pronghorn get caught in the wires (Harrington and Conover 2006). In addition, fences indirectly impact pronghorn when wounds are inflicted by barbs when crawling underneath the bottom wire or between wires, when the fence reduces access to resources (e.g., prime habitat, water, etc.), or when fences alter a pronghorn’s ability to freely move across the landscape, trapping them in inhospitable habitat during environmental extremes (Jones 2014; Jones et al. 2019; Reinking et al. 2019; Xu et al. 2020).

The primary purposes of fencing on the landscape are to delineate ownership boundaries, control the distribution of livestock, and keep livestock and wildlife off roads to reduce wildlife-vehicle collisions (Jakes et al. 2018a). While there are a variety of fence designs used on western rangelands (e.g., 4-strand barbed-wire, woven wire, etc.), it is the height of the bottom wire that determines if pronghorn are able to cross the fence successfully. The predominant recommendation (Fig. 19.2) is to raise or set the bottom wire height to a minimum of 46 cm (18 in) above the ground to allow ample room for pronghorn to crawl under (Jones et al. 2018, 2020b). In addition, it is recommended that a double stranded smooth wire be used on the bottom to reduce potential injuries to pronghorn from crawling under a fence with a barbed bottom wire (Jones 2014). Enhancements to existing sheep fences (i.e., woven wire) are more problematic for livestock producers because of the requirement for a low bottom wire to contain sheep and goats. Woven wire fences can be replaced with a 4-strand barbed-wire fence with a bottom wire 25 cm (10 in) above the ground (Paige 2020). While not an ideal bottom wire height, using a barbed-wire fence (with a smooth wire on bottom), as opposed to a woven wire fence, does create some opportunity for pronghorn to pass underneath. Ideally sections of woven wire fence could be dropped when small livestock are not present. Other mitigations include leaving gates open, virtual fencing, or using lay down fence designs when livestock are not present (Paige 2020).

Fig. 19.2

Adopted from Paige 2020. Illustration by K. Rumbolt Miller

A pronghorn friendly alternative fence design with a double stranded bottom wire at 18ʺ (46 cm) from the ground.

6 Impacts of Disease

Infectious diseases can cause locally extensive mortality, but seldom produce the population level impacts that are associated with severe weather, habitat degradation, and barriers to movement. Diseases affecting pronghorn caused by viruses, bacteria, or parasites are typically shared with other wild or domestic ruminants, and frequently occur in partnership with other stressors.

Respiratory diseases of pronghorn are a frequent cause of death and are typically present as adhesions of the lung to the surface of the chest cavity, pneumonia, and fluid or hemorrhages in the lungs. Bacterial pathogens that are identical or related to those of cattle and sheep are often found. Viruses are infrequently identified, but transient infections are thought to make individuals susceptible to secondary bacterial pneumonia.

The list of bacterial pathogens which impact pronghorn is extensive, and many of these bacterial diseases are exacerbated by poor rangeland quality and overcrowding, both among pronghorn and with domestic livestock (O’Gara 2004d). Significant bacterial diseases that infect pronghorn include Anaplasmosis, Campylobacter, leptospirosis, Mycoplasma bovis, and necrobacillosis. Additional bacterial diseases which impact pronghorn to varying degrees include Actinobacillosis, Actinomycosis, Escherichia coli infections, Pasteurellosis, and Vibriosis (Jaworski et al. 1998; Kreeger et al. 2011). Mycoplasma bovis is a bacterial disease of cattle causing pneumonia, mastitis, and arthritis. Mycoplasma pneumonia has recently been identified as the cause-of-death for hundreds of pronghorn in northern Wyoming (Malmberg et al. 2020). These mortality events occurred in the late winter to early spring. The pronghorn died quickly, even though they were in good body condition, and upon necropsy, results indicated massive pneumonia with yellow fibrin covering the surface of the lungs. At this point, it is unclear whether mycoplasma pneumonia in pronghorn is a localized problem, or if the organism is established in pronghorn, but only infrequently causes disease.

The most significant viral pathogens of pronghorn are those causing hemorrhagic disease, including epizootic hemorrhagic disease virus (EHDV) and bluetongue virus (BTV). Hemorrhagic disease outbreaks in pronghorn can produce significant die-offs but occur in four- to seven-year cycles. Typically, there is minimal or no mortality between large-scale hemorrhagic disease events. Outbreak years often correspond with exceptionally hot and dry summers, which favor large vector populations and increased animal density around limited water sources. These seasonal variations, combined with waning population immunity, contribute to the risk for outbreaks. Bluetongue virus and EHDV also affect deer, elk, cattle, and domestic sheep.

Scours, or diarrhea, can be caused by a rapid change in diet, particularly during the spring green up, but it also occurs during the summer months. Animals are seen with a soiled hind end and may be listless and appear unkempt. Scours is more prevalent in young animals, but all ages and both sexes can be affected, and pronghorn mortality can be locally extensive. Scours frequently occurs in animals feeding on alfalfa (Medicago sativa), but a variety of bacteria, viruses, and parasites have also been identified as potential causes. Extensive research has failed to identify a definitive origin for this condition, which is frequently a cause of concern for livestock producers whose animals share rangeland with affected pronghorn.

Pronghorn harbor a number of parasites also present in domestic ruminants, but health impacts are usually restricted to crowded situations or overgrazed rangelands shared with livestock. Increased transmission occurs near water sources such as stock ponds and water tanks that are heavily used by livestock and wildlife. The large stomach worm or barber pole worm (Haemonchus contortus), is the most significant parasite of pronghorn (Kreeger et al. 2011). This parasite attaches to the mucosa of the fourth stomach and feeds on blood. Heavy infection results in anemia and may contribute to mortality in animals already in poor nutritional condition. This parasite is well recognized in domestic livestock, especially sheep, goats, and cattle, and parasite burdens may increase on rangelands shared by susceptible ruminant species. Although infrequently found, round worms (e.g., Ostertagia sp., Nematodirus sp., and Cooperia sp.), lung worm (Protostrongylus macrotis), and tapeworms (Monezia sp.) infect pronghorn, cattle, and sheep (Goldsby and Eveleth 1954; Greiner et al. 1974).

Foot rot is the common term used to describe the disease caused by the bacterium Fusobacterium necrophorum. Animals often show signs of lameness, with swollen feet and fetlocks, but may also have ulcers in their mouths. Mortality occurs during the spring when snowmelt produces muddy conditions, or during the summer when pronghorn congregate around ponds or stock tanks where the muddy substrate has been contaminated with feces containing the bacteria. Mortality events are usually localized with most animals recovering from infection.

Management practices for both pronghorn and livestock influence the transmission of diseases and parasites between individuals and among species. Good nutrition and maintaining animals at or below the carrying capacity of their summer range are the basic tenets of healthy populations, both wild and domestic.

7 Ecosystem Threats

During the nineteenth century, pronghorn populations range-wide were decimated from market hunting by European settlers, and by the 1920s, the species was nearly extinct across their range (Grinnell 1929; Greenquist 1983; O’Gara and McCabe 2004; McCabe et al. 2004). As twentieth century regulation of pronghorn hunting was implemented, initiating the species’ recovery (Greenquist 1983; O’Gara and McCabe 2004), multiple factors impacting pronghorn sustainability on the landscape began to shift.

Today, long-term pronghorn population persistence is chiefly threatened by human-caused habitat conversion, fragmentation, and loss (O’Gara and McCabe 2004). Additionally, anthropogenic development and activity have impacted pronghorn populations by producing behavioral changes (Sawyer et al. 2002; Beckmann et al. 2012; Seidler et al. 2015; Reinking et al. 2019; Jones et al. 2019) and dramatically altered weather and climate regimes related to global climate change (Christie et al. 2015; Gedir et al. 2015; McKelvey and Buotte 2018). The variety of ecosystem threats facing pronghorn populations today and into the future are explored in greater detail below.

7.1 Farming and Ranching

Habitat alteration associated with farming and ranching began in the nineteenth century with the arrival of European settlers. Agricultural production equated to the conversion of native pronghorn range, particularly in grassland habitats, where nutrient-rich soils are ideal for the growth of staple crops like corn and wheat (O’Gara and McCabe 2004). In the 1930s, the dust-filled winds and economic recession of the Great Depression frequently caused farming families to abandon property located on marginal prairie lands. Despite the drought conditions of that time, many of these uninhabited areas reverted to native vegetation, creating short-term benefits for pronghorn populations (O’Gara and McCabe 2004). However, reprieve was temporary; as rampant drought abated, farming expanded and became increasingly mechanized, and practices that caused rapid deterioration of rangelands were further employed. Clearing of native vegetation on highly erodible grasslands (i.e., sodbusting), was common into the 1990s, despite legislation designed to discourage the practice (e.g., the Sodbuster Provision of the 1985 Food Security Act; O’Gara and McCabe 2004). Although federal, state, and non-government organization’s programs and partnerships look to curb habitat conversion, the threat of losing additional native habitats to agricultural land still exists (Smith et al. 2016).

Improper livestock management represents a source of pronghorn habitat alteration. Heavy livestock stocking rates can negatively impact pronghorn habitat quality through overgrazing and trampling of native vegetation, compaction of soil, and damage to riparian areas (O’Gara and McCabe 2004). Such imprudent management of rangelands, exacerbated by the historical prioritization of livestock grazing over wildlife management, resulted in degraded landscapes and was particularly problematic in the arid, desert portions of pronghorn range (O’Gara and McCabe 2004). Legislation, including the Public Rangelands Improvement Act of 1978, and interventions like the removal of cattle from crucial Sonoran pronghorn habitat in the 1980s, helped to bring livestock production into greater equilibrium with pronghorn conservation and management (O’Gara and McCabe 2004). While improper grazing still occurs, but to a lesser extent than the last several decades, today the largest impact of ranching practices on pronghorn populations is the fragmentation caused by fences erected to exclude wildlife or contain livestock (see Sect. 19.5.3).

7.2 Habitat Alteration

One prevalent threat to pronghorn persistence is the alteration of their habitat through increased invasion of rangelands by non-desirable species (i.e., non-native grasses and shrubs or trees) and the associated changes in fire regimes. Many non-native grasses (e.g., cheatgrass, smooth brome [B. inermis], and crested wheatgrass [Agropyron cristatum]) are capable of out-competing native vegetation, resulting in critical habitat changes (Boyd et al. 2021; Gaskin et al. 2021). Many invasive grasses can dominate native species because they quickly colonize disturbed areas, mature early, have short root systems for absorbing water quickly in soil, and are prolific seed producers (Boyd et al. 2021). In addition, many invasive species, and especially cheatgrass, respond positively to and can alter fire regimes, catalyzing a detrimental invasive plant-fire regime cycle (Brooks et al. 2004). These characteristics result in pronghorn habitat being altered from diverse mosaics of grasses, forbs, and shrubs to monocultures of invasive grasses (Boyd et al. 2021; Gaskin et al. 2021). Moreover, climate change has the potential to exacerbate these changes (Adler et al. 2021).

Fire suppression on western rangelands has caused the transition of vegetative communities from early to late succession. Associated with this change is the encroachment and expansion of coniferous trees, especially pinyon pine (Pinus sp.) and juniper (Juniperus sp.), into western rangelands (Maestas et al. 2021). This increase in coniferous trees results in declines in perennial grasses, forbs, and more generally, productivity (Maestas et al. 2021). Such encroachment has also resulted in changes to sagebrush communities as they become increasingly susceptible to invasive species (e.g., cheatgrass) because of increases in overstory crown fires (Chambers et al. 2014; Maestas et al. 2021). With these changes, the availability of forage and cover provided by native rangeland species declines considerably, resulting in a dramatic shift to rangelands composed of more coniferous species (Maestas et al. 2021).

7.3 Residential and Urban Development

In addition to the expansion of agriculture and livestock production, European settlement of the North American West spurred residential and urban growth (O’Gara and McCabe 2004). As was common in farming and ranching practices, fences were frequently used to delineate property boundaries, posing risks to pronghorn survival, and presenting direct barriers to movement (Harrington and Conover 2006; Jones et al. 2018). For example, in the 1960s in Arizona, the sectioning of large swathes of native rangeland for residential plots (i.e., ranchettes), allowed people to feel closer to wildlife in relatively rural settings but ultimately fragmented large portions of the landscape at a detriment to pronghorn (O’Gara and McCabe 2004). In addition to habitat fragmentation, residential expansion can result in direct and indirect habitat loss and mortality for pronghorn because of greater traffic levels and higher road/fence densities. In rare instances, residential and urban development can also cause direct mortality for pronghorn; for example, animals have been known to forage on toxic, ornamental vegetation in landscaped yards or even trash at city waste facilities, ultimately dying from inflamed stomachs and toxicity (O’Gara 2004b).

7.4 Energy Development

Much of pronghorn range is conducive to energy development (wind, solar, oil and natural gas, mining) due to the impressive wind speeds, high incoming solar radiation, and sizable underlying fossil fuel and mineral depositions found in these rangelands (Yoakum 2004b; Copeland et al. 2009). A growing body of literature indicates that the infrastructure and activity associated with these land uses can directly eliminate portions of pronghorn habitat and indirectly cause habitat loss by altering pronghorn behavior (Sawyer et al. 2002; Beckmann et al. 2012; Christie et al. 2015; Reinking et al. 2019; Jakes et al. 2020; Smith et al. 2020). Notably, the infrastructure of these developments also typically includes high densities of roads and fencing, which influence pronghorn behavior (Gavin and Komers 2006; Seidler et al. 2015; Jones et al. 2018) and present direct and indirect mortality risks (Harrington and Conover 2006; Jones 2014).

The risk-avoidance hypothesis suggests that animals can perceive human-induced landscape disturbance similarly to predators, and that similar risk-avoidance responses may result when they are exposed to anthropogenic activity and infrastructure (Frid and Dill 2002; Gavin and Komers 2006). These responses include behavioral alterations, such as spending an increased proportion of time in a vigilant state and less time foraging (Gavin and Komers 2006; Seidler et al. 2015, 2018), and avoidance of developed areas (Beckmann et al. 2012; Reinking et al. 2019; Smith et al. 2020). For pronghorn, these responses may be amplified in winter, when individuals are generally in reduced physical condition and already stressed by limited forage and increased energetic requirements (Yoakum et al. 2014). For example, pronghorn in the Shirley Basin, Wyoming were found to avoid wind turbines after installment in their winter home range, and this effect was stronger in more severe winters (Taylor et al. 2016; Smith et al. 2020). Within the same study area, Milligan et al. (2021) found pronghorn were displaced when selecting a home range by existing turbines in both summer and winter, but there was little evidence of avoidance behaviour within the home range at the population level. A similar trend has been observed in multiple studies of the impacts of oil and natural gas development and associated infrastructure (Beckmann et al. 2012; Reinking et al. 2019). Moreover, energy development within winter range can cause cumulative changes in pronghorn habitat use over time including initial and continued avoidance that can ultimately result in increasing abandonment of these seasonally crucial areas (Sawyer et al. 2019). While studies have yet to make the mechanistic connection between energy development and its potential influence on pronghorn survival and reproductive success, it is likely that the habitat loss and behavioral changes these disturbances produce negatively impact pronghorn fitness (Sawyer et al. 2002; Beckmann et al. 2016). This is supported by research linking long-term pronghorn population declines to the density of energy development on the landscape (Christie et al. 2015).

Given its relatively low cost (e.g., seeding to non-natives) and simple implementation, reclamation of rangelands altered by energy extraction efforts is often more preferable for industry stakeholders than modifying their procedures, such as directional drilling of oil and gas wells to minimize the number of required well pads (O’Gara and McCabe 2004). However, reclamation has largely been proven to be inadequate in the biomes pronghorn inhabit, as much of the landscape damage that results from energy development is irreversible (Rottler et al. 2018). Reclamation efforts frequently fail to restore habitat to its former condition, and can result in the establishment of invasive, noxious weeds (Padgett 2020). Additionally, the mitigation requirements for energy development stakeholders are often vague, with little to no post-reclamation monitoring or land use management required (Zimmerman 1983; O’Gara and McCabe 2004).

7.5 Climate Change

Given the spatially expansive nature of pronghorn range, and the variety of habitats it includes, the expected alterations to weather and climate regimes resulting from global climate change are myriad. In general, pronghorn range-wide are likely to experience increased air temperatures year-round, causing warmer winters, more extreme summer heat, and greater frequency of drought conditions (McKelvey and Buotte 2018; Adler et al. 2021). Other climate alterations, such as changes in precipitation patterns, are likely to be more influential at the periphery of pronghorn range and will vary both latitudinally and longitudinally. Overall, high quality pronghorn seasonal and connectivity habitat is likely to be reduced because of climate change (Zeller et al. 2021).

In the northern portion of their range, pronghorn will likely experience more stochastic winter conditions, which will include some years with reduced winter precipitation as well as years with dramatically increased winter precipitation (McKelvey and Buotte 2018; Adler et al. 2021). This will result in winters with severely reduced snowpack, and some with extremely deep snow accumulations. Years of limited winter precipitation will produce drier conditions in the following summer, which in-turn, can lead to increased frequency and severity of wildfire (Halofsky et al. 2018). Increased frequency and severity of wildfires can reduce overall shrub density and allow non-native, noxious weeds to flourish (Yoakum et al. 2014; Adler et al. 2021; Boyd et al. 2021). Years with greater winter precipitation and lower temperatures could result in population declines (Barrett 1982; Christie et al. 2015; Jones et al. 2020a). The physical properties of future snowpacks, such as their ability to support pronghorn on the surface or the wetness of the snow, are also likely to be altered (Berteaux et al. 2017; Boelman et al. 2019). These properties can influence the energetic expense of moving through snow (Parker et al. 1984) and therefore have implications for pronghorn fitness.

The southern portions of pronghorn range are expected to receive less moisture and average higher air temperatures (Gedir et al. 2015; Adler et al. 2021). Studies that project the impacts of such reduced precipitation in southwestern regions anticipate decreased pronghorn abundance and local extirpations resulting from these hotter, drier conditions (Gedir et al. 2015). In arid and semi-arid areas, precipitation is crucial for maintaining adequate forage and water resources on the landscape (Beale and Smith 1970; Yoakum 2004b). Deficiencies in these resources resulting from drought conditions have been linked to reduced pronghorn reproductive success, lower survival, and ultimately, population declines (Brown et al. 2006; Simpson et al. 2007; McKinney et al. 2008).

8 Conservation and Management Actions

Across the extent of pronghorn range, the landscape is a matrix of habitats (i.e., grasslands, sagebrush, agricultural crops) and ownership (public and private). In addition, pronghorn individuals and populations currently move between jurisdictions (e.g., between Colorado—Wyoming, Montana—Idaho, and Alberta—Saskatchewan) and even countries (e.g., between Saskatchewan, Canada and Montana, USA). Continued prospects to travel throughout and between habitats, independent of jurisdictional boundaries, is particularly important to pronghorn as movement is one of their key adaptations to maintain populations and genetic diversity. Landscape connectivity for pronghorn allows them to track spatiotemporal shifts in vegetation condition and availability, adapt to anthropogenic influences, and move to landscapes that may become more suitable for pronghorn over time (e.g., as a result of climate change) while maintaining genetic diversity (Hilty et al. 2006).

8.1 Barriers to Movement and Functional Connectivity

In general, natural landscapes are more connected, functioning, and resilient ecosystems than those inundated by human-made features and development. Subsequently, pronghorn need specified areas and/or identified locales to navigate anthropogenic impediments and sustain movements across fragmented landscapes (Beier and Noss 1998; Hilty et al. 2006). Simple and cost-effective measures can be taken to allow for continued daily and seasonal use by pronghorn. Solutions exist for providing pronghorn safe passage across linear anthropogenic features, such as roads and fences that fragment the landscape.

Roads (paved and unpaved and with or without fences) typically have a major influence on pronghorn, presenting barriers to movement and in many cases causing avoidance behaviors, increased vigilance, and reduced foraging opportunities (Gavin and Komers 2006; Dodd et al. 2011; Jones et al. 2019; Jakes et al. 2020). In concert with roadside fencing, direct mortalities along roads occur to pronghorn by being caught in fencing, fawns being separated from does and predated upon, or individuals being trapped within the road right-of-way and struck by vehicles (Sawyer and Rudd 2005; Harrington and Conover 2006; Seidler et al. 2018). While mitigation opportunities do exist, the risk of wildlife-vehicle collisions, both in terms of the safety of vehicular passengers and risk of property damage, must be considered (Dodd et al. 2011; Lee et al. 2021). One mitigation measure is wildlife crossing structures. In Wyoming, pronghorn have been observed to use highway underpasses (Plumb et al. 2003) to navigate roads, but given a choice, pronghorn preferred to use highway overpasses 93% of the time, rather than underpasses (Sawyer et al. 2016). The construction of wildlife crossings, particularly overpasses, has been effective in allowing for continued seasonal migrations of pronghorn and provides an additional option to communities and jurisdictions to allow for wildlife movement in a safe manner for both people and wildlife (Seidler et al. 2018). While the up-front costs of planning and constructing these features can be significant, they are offset by the long-term savings in costs associated with insurance claims and the value of increased human safety (Huijser et al. 2009).

Fencing can similarly be modified to allow for continued pronghorn daily and seasonal movements while also addressing human needs. Fences along roadways can be modified to create an opportunity for pronghorn to cross at a specific location (accounting for pronghorn use, traffic levels, and proper fence design), or can provide a funneling mechanism to direct animals towards a crossing structure (i.e., underpass, overpass; O’Gara and McCabe 2004; Sawyer and Rudd 2005; Yoakum et al. 2014). Paired right-of-way fencing gates and lay-down fences have been installed more across the West in the last ten years and are considered important conservation measures benefiting pronghorn movement and landscape connectivity, in general (Paige 2020). In addition, several sportsman and conservation groups (e.g., Alberta Fish and Game Association, Arizona Antelope Foundation, Jackson Hole Wildlife Foundation, etc.) hold volunteer events that modify fences for the benefit of pronghorn and other wildlife species. Pasture fence design and modifications are discussed in detail in Sect. 19.5.3.

8.2 Managing Pronghorn on the Private–Public Landscape Matrix

Habitat management and enhancement within the private–public landscape matrix is important for maintaining pronghorn populations. For example, pronghorn in the Northern Great Plains were found to migrate through a greater percentage of private lands than public lands (Tack et al. 2019). Therefore, listening to, understanding, and accounting for private landowner perspectives is essential to properly manage wildlife populations. For example, landowners require fences that contain livestock in appropriate pastures and at the same time, they spend time and money on fixing fences that are damaged by the wildlife navigating them. The solution is to find ‘win–win’ approaches for both the landowner and wildlife to minimize fence damage and keep livestock contained. Installation of wildlife-friendlier fencing, the use of fence modifications on existing fences, or the installment of gates will result in win–win opportunities (Jones et al. 2018, 2020b). Similarly, water is in limited supply across pronghorn range, and water development, design, and placement will influence its use by pronghorn and domestic livestock (Larsen et al. 2011). Wildlife managers can work with landowners to design stock tanks that most effectively facilitate pronghorn use. Finally, working with landowners to identify priority native habitat for wildlife is of utmost importance. For example, conservation easements have been used as an effective tool to conserve greater sage-grouse habitat that also protected pronghorn habitat (Tack et al. 2019). Many state, provincial, federal, and non-government organizations’ programs provide funding for conservation easements, fence modifications, vegetation treatments, management of annual invasive grass, and water developments which provide benefits to a suite of rangeland wildlife species.

Public land across the range of pronghorn is managed for multi-use including livestock grazing, energy development, recreational use (e.g., hunting and viewing), and wildlife habitat. Within the USA most of the public land falls within the jurisdiction of the Bureau of Land Management (BLM) and the Forest Service and represents 50% of total pronghorn habitat (Yoakum 2004f). Historically public land has been managed with livestock in mind. This fact is exemplified with 95% of the BLM expenditures for rangeland improvements being for the benefit of livestock (Donahue 1999). More recently there has been a decrease in priority for grazing as a balance been livestock and wildlife needs has been struck. For example, at the Hart Mountain National Antelope Refuge sheep, cattle, and feral horses have been removed resulting in improved habitat and pronghorn numbers on the refuge (Yoakum 2004f). In addition, rangeland improvement projects are now completed with wildlife in mind, such as modifying fences to wildlife friendly designs and installation of water developments.

8.3 Genetic Diversity

One area lacking research is the analysis of pronghorn genetics (see Yoakum et al 2014 for a review). Initial genetic work has focused on endangered subspecies, with more recent work focused on the use of genetics to estimate populations and determine if natural and anthropogenetic landscape features are barriers to movement. Both Sonoran and Peninsular subspecies of pronghorn are endangered and continue to severely lose genetic diversity, and if they are to persist, careful genetic management is required through continued captive breeding (Stephen et al. 2005; Klimova et al. 2014). Recently, the use of noninvasively collected fecal DNA and capture-recapture designs at watering holes have been evaluated to determine if the use of genetics can improve population estimates (Woodruff et al. 2016). Except the work of Lee Jr et al. (1994), few genetic studies have been conducted on A. americana that characterize genetic variation between populations. Using mitochondrial DNA, Lee Jr et al. (1994) found differences in allozyme variation in 29 populations across the West. More recently, pronghorn populations in Wyoming were found to be genetically connected throughout the core of their range (LaCava et al. 2020), which is encouraging, given naturally occurring landscape barriers (e.g., mountain ranges) and anthropogenic fragmentation across the state (Copeland et al. 2009).

9 Research and Management Needs

The rangelands of western North America are under increasing pressure from anthropogenic disturbances (e.g., roads, fences, houses/residential development, agricultural conversion, energy development), and climate change. Understanding the effects of a changing landscape will be key to conserving pronghorn populations across their range. We suggest addressing the following research and management needs to ensure healthy, sustainable pronghorn populations, though we do not consider this list exhaustive:

• While continued understanding of each impact is warranted (e.g., wind and solar energy), the real need is to understand the cumulative effects of these factors on pronghorn population persistence (i.e., fitness). To understand the cumulative effects requires long-term datasets and intrinsic information (e.g., body condition, reproductive status, recruitment, and survival). Lastly, understanding cumulative effects of impacts should identify threshold levels that result in population declines or local extirpation.

• The potential effects of climate change on pronghorn population persistence need further exploration, particularly as climate regime alterations continue. Large-scale connectivity modelling that accounts for future climate scenarios should occur. Long-distance movements may be a vital adaptation for pronghorn at the periphery of their range, because these movements offer escape from extreme environmental conditions, stochastic weather and disturbance events, and habitat alterations. Future research is required to identify long-distance movements more clearly and understand the mechanisms driving them.

• A greater understanding of the effects of linear features (i.e., fences and transportation infrastructure) on pronghorn movement and fitness is required. For example, in the management of fences the first step is to develop tools and designate resources to map fences including design specifications across broad spatial scales. For transportation infrastructure, citizen science programs (e.g., Wildlife Xing (www.pronghornxing.org)) can be implemented and promoted. These programs will allow us to better understand where pronghorn interact (e.g., killed, cross, stage) within transportation corridors to assist in identifying key areas for mitigation (e.g., overpasses). Then, these datasets should be coupled with long-term movement datasets and intrinsic fitness information.

• Associated with gaining a greater understanding of the effects of linear features is the evaluation of whether these features, as well as natural features, are acting as barriers to gene flow. In addition, a genetic analysis of the populations across the range of pronghorn is warranted to determine relatedness and to confirm the number of distinct subspecies.

• The development of integrated population models is needed to account for the influence of spatiotemporal factors (e.g., seasonally variable environmental conditions) on pronghorn

Competing Interest

All coauthors do not have any competing interest associated with this chapter.