Keywords

1 General Life History

1.1 Feral Equid Species

Feral equids of North America, referred to as free-roaming, free-ranging, or wild equids, include horses (Equus ferus caballus) and burros (E. asinus), and are the only federally-protected feral species in North America. The term feral constitutes “species that have been established from intentional or accidental release of domestic stock that results in a self-sustaining population(s)” and “are generally non-indigenous and often invasive” (The Wildlife Society 2021). Feral animals are wild descendants of a domesticated species. To better understand how feral equids became federally-protected, we must consider the evolutionary and domestication history of these animals and their relationship to humans. The socio-ecological mismatch of protecting a feral species translates into great potential for feral equids to negatively affect the ecosystems they inhabit. Together, these aspects frame the controversy surrounding the contemporary management of feral equids on western North American rangelands (Beever et al. 2018; Scasta et al. 2018). In this chapter, we provide greater content and focus on feral horses because they are more numerous and more widely researched than burros in North America. In contrast, the lack of research on burros has resulted in a general gap in our knowledge of this species.

1.2 Evolutionary and Domestication History

Equidae, the family containing horses and burros, originated in North America approximately 50 million years ago (Hurlbert Jr. 1993). Ancient equids included a diverse assemblage of species possessing a variety of physiological and morphological features. Hypohippus was a three-toed browsing species while Dinohippus was a single-toed grazing species (Fig. 21.1). All equid species in North America ultimately became extinct during the late-Pleistocene epoch due to a combination of environmental change, disease, and the arrival of humans and hunting (Buck and Bard 2007). Prior to their North American extinction, equids crossed the Bering Strait and dispersed into Eurasia 20 million years ago (Kelekna 2009). The horses that radiated across the steppes of Eurasia eventually were domesticated by humans approximately 6,000 years ago (Outram et al. 2009). Burros, meanwhile, originated from African wild asses (E. africanus), and were likely domesticated in Egypt and Mesopotamia over 5,000 years ago.

Fig. 21.1
A painting of the scenery has three horses and a foal at the center. A horse on the left side is grazing the grass.

Equidae evolved in North America and ancient horses were physiologically and morphologically diverse. Depicted here by the American Natural History Museum, as example, are the large single-toed grazing species Dinohippus (left), the small three-toed mixed feeder Nannippus (center), and the three-toed browsing species Hypohippus. Picture provided by the American Natural History Museum (ANHM)

Full size image

The earliest records of horse domestication were from the Botai people of north-central Kazakhstan whose horse-centric cultures were highly influential (Outram et al. 2009). Early cultures hunted horses and likely captured orphaned foals leading to breeding horses and keeping them for milk and meat in an intimate association where horse and human survival were closely intertwined (Levine 1999). Horse domestication was a critical component of human history and provided a valuable utility for many cultures. Domestic horses were transported across the globe and their distribution generally tracks the expansion and distribution of humans. Today, the emotional attachment of humans to horses helps explain the ubiquity of feral equids worldwide. It was the horse, and it’s raw “horse-power”, that enabled cultures to disperse and advance agriculture, transportation, industry, commerce, and warfare (Ransom and Kaczensky 2016). Domestication included artificial selection for certain traits over many years leading to horses that were optimized for particular size, color, and reproduction characteristics. All domesticated and feral horses today differ genetically and phenotypically from their non-domesticated ancestors (Fages et al. 2019) and they are morphologically different from their only extant wild relative, the Przewalski’s horse (Equus ferus przewalskii; Groves 1994).

1.3 Feralization and Protection of Equids in North America

Italian explorer Christopher Columbus first transported domestic horses to North America on his second voyage to the continent in 1493 (Kelekna 2009). The best evidence suggests that burros were brought to North America around the same time (Antonius 1938; McKnight 1958). A half-century later, an estimated 10,000 horses roamed central Mexico with both Pueblo and Apache peoples possessing equestrian skills (Kelekna 2009). In 1680, the Pueblo peoples revolted against Spanish conquistadors, facilitating the release of several thousand horses which served as the “nucleus” of mustang horse herds in North America (Kelekna 2009). Ever since, such horses have become a fundamental aspect of North American human cultural evolution (Berger 1986). Additional escapes along with intentional releases by Native Americans, European settlers, and the military during the 1700s provided more sources of horses that enhanced genetic diversity and boosted population densities (Mitchell 2015). With the advent of the industrial age in the nineteenth and twentieth centuries, demand for horses and burros declined due to a combination of a rapidly urbanizing and mechanized society and high costs of equid care and land (Garrott 2018; Scasta et al. 2018). Consequently, the post-industrial period in the mid-twentieth century saw an increase in intentional horse releases. Feral horses became more abundant across western rangelands, until they were captured by mustangers and others who sold them for slaughter, re-sale, or other economic purposes (Danvir 2018). Spurred by citizens concerned about the dwindling population of horses and burros in the West, the U.S. government enacted a law called the Wild Free-roaming Horses and Burros Act (WFRHBA) in 1971 to protect the remaining populations of feral equids on federally-owned land (Public Law 92-195, see Rangeland Management section).

2 Distribution and Population Dynamics

2.1 Distribution of Feral Equids in the United States

Feral equids are generally found in areas where they escaped after humans no longer needed them or were released on public lands during stark economic times (to avoid feeding costs, e.g.). The areas where horses have been allowed to remain typically have low human population densities, minimal human use, and are of little economic value for row-crop agriculture or commercial development. Feral equids can be found across the United States with most populations occurring on rangelands in western states (Fig. 21.2). Small populations also exist on barrier islands off the Atlantic coast, along with isolated populations in eastern forests. Feral equids inhabit federally-owned land managed by the Bureau of Land Management (BLM), the U.S. Forest Service (USFS), the U.S. Fish and Wildlife Service (USFWS), the National Park Service (NPS), and the Department of Defense (DOD). Horses and burros can also be found on private, municipal, state, and sovereign tribal lands. The feral equids that occur on BLM and USFS lands are protected by the WFHBA (Public Law 92-195). These populations are managed in the areas where they occurred at the time of the Act’s passing. On BLM land, these areas are called Herd Management Areas (HMA). There are also populations on BLM land where they are not specifically managed for, and these areas are known as Herd Areas (HA). On USFS land, management areas are termed Wild Horse and Burro Territories (WHBT). In total, there are 177 HMAs and 53 WHBTs spread across 10 western U.S. states (BLM 2022; USFS 2022).

Fig. 21.2
2 photos. A. Three standing and one lying feral horse on a snow-covered mountain landscape. B. A grassland with a horse standing on the ground and resting its mouth on the trunk of a tree.

Feral horses in western North America arid and semi-arid regions characterized by complex topography and extreme temperatures. (Top) Southwestern Wyoming, January 2017, photo credit: J. D. Scasta. (Bottom) Southern Nevada, September 2015

Full size image

2.2 Global Distribution of Feral Equids

Feral equids inhabit a wide range of habitats throughout the world, with many populations existing in ecosystems characterized by rugged topography, limited net primary production, and extreme weather patterns (Fig. 21.3). We do not present an exhaustive list of all global feral equid populations here; rather, we list select populations to highlight that they are widespread across the globe. In North America, feral equids also occur in Canada and Mexico, in addition to the U.S. (Schoenecker et al. 2021). In South America, populations occur in Ecuador and Argentina (Scorolli 2018). Australia is thought to have the greatest abundance of feral equids of any country (Schoenecker et al. 2021), and New Zealand also contains feral equids (the Kaimanawas). A small population also occurs in French Polynesia in the South Pacific. In Africa, feral horses and burros are known to inhabit the Namib desert (Cothran et al. 2001). In Europe, some populations have been introduced as part of rewilding efforts (Linnartz and Meissner 2014), while others are managed extensively (i.e., handled annually). Populations are present in France (Camargue), in the United Kingdom (e.g. Dartmoor; Exmoor, New Forest, and Welsh Mountain ponies), in the Danube Delta region of Romania, in the Pyrenees Mountains of France and Spain (Galacia ponies, Pottoka horses), and in Portugal (Sorraia horses and Garrano ponies). In Asia there are some Misaki-uma horses occurring within the designated National Monument on Cape Toi, Japan.

Fig. 21.3
A map of the western United States is categorized into states of Washington, Montana, Oregon, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, New Mexico, North Dakota, and California. It highlights the regions of B L M, U S F S, N P S, D O D, tribal nations, and rangelands boundary.

Approximate range of known feral horse and burro populations on western United States rangelands. Note not all areas within each polygon are occupied by horses or burros, and there are likely feral equid populations not represented here

Full size image

2.3 Population Estimates of Feral Equids in the United States

The nationwide estimate of feral free-ranging equids across all land jurisdictions is approximately 275,000 (Table 21.1). The majority of feral equids are thought to occur on tribal nations, with 75,000 horses estimated on the Navajo nation alone (Schoenecker et al. 2021; Wallace et al. 2021). There were roughly 72,000 horses and 14,500 burros on BLM land in 2021 (BLM 2022), and approximately 9,000 feral equids on USFS land (T. Drotar, pers. comm.). These estimates far exceed maximum appropriate management levels (AML) which are population ranges set to balance equid populations with the other uses of public rangelands (see Rangeland Management for more details). The nationwide AML for feral equids is 26,785 on BLM land and 2,253 on USFS land (BLM 2022; USFS 2014). Feral equid population growth rates range from 11% to over 25% (Roelle et al. 2010), but the protected status of feral equids on BLM and USFS lands makes them a challenge for management (Messmer et al. 2021). In addition, there were an estimated 59,749 horses and 862 burros in 2021 living in ‘off-range’ BLM facilities consisting of corrals and pastures (BLM 2022).

Table 21.1 Population estimates of feral horses (Equus ferus caballus) on different land jurisdictions in the United States
Full size table

2.4 Population Monitoring

The BLM, USFS, and NPS conduct regular population surveys for feral equids following established methods (Lubow and Ransom 2016, 2009; Griffin et al. 2020). Feral equid populations on other land jurisdictions are surveyed less regularly. Survey methods differ among populations but include simultaneous double-observer aerial surveys (Lubow and Ransom 2016; Griffin et al. 2020; Hennig et al. 2022), photo mark-resight surveys (Lubow and Ransom 2009), genetic capture-recapture models using fecal DNA (Schoenecker et al. 2021), employing distance sampling within aerial infrared surveys (Schoenecker et al. 2018) and direct visual counts by ground observers (Friends of a Legacy, Little Book Cliffs HMA, Colorado).

3 Habitat Associations and Impacts

3.1 Habitat Selection, Home Range Sizes, and Movement Patterns

Because feral equids did not co-evolve within the areas they reside in, generalizing habitat selection across populations is inherently difficult. While habitat selection is context dependent, there are a few patterns that are common across studies. Terrain strongly influences the habitat selection of feral horses, and they are much more likely to utilize relatively flat topography or gently sloping ridgetops (Ganskopp and Vavra 1986; Henning 2022; Schoenecker et al. 2022a, b) than steep slopes. Habitat selection by feral horses is also strongly linked to forage availability (Schoenecker et al. 2016, 2022a, b). Horses are large-bodied grazers (Van Soest 1994) that consume large quantities of graminoids (King 2002; King and Gurnell 2005; Girard et al. 2013); therefore they tend to select for grassland or shrubland landcover types (Smith 1986; Crane et al. 1997; King 2002; King and Gurnell 2005; Schoenecker et al. 2022a, b). Horses that inhabit heavily forested environments select for disturbed areas, such as roadside edges, where grass production is higher (Irving 2001; Girard et al. 2013). Equids are relatively inefficient in water retention, compared to ruminants, owing to their cecal digestion (Janis 1976). Consequently, equids select for closer proximity to water sources during the growing season and foaling season (Arandhara et al. 2020; Esmaeili et al. 2021; Schoenecker et al. 2022a, b; Girard et al. 2013). Horses can eat snow for hydration, and are therefore less reliant on open water during the winter (Mejdell and Boe 2005; Kaczensky et al. 2008; Salter and Hudson 1979). The social status of individuals can also affect habitat selection. Different male social classes vary in their use of the landscape: harem-holding stallions are constrained by the habitat selection of their mares who need to remain closer to surface water during foaling and lactation, whereas bachelors are free to travel longer distances to access prime forage (Schoenecker et al. 2022a, b).

Few studies have evaluated the movement patterns of feral horses, but variation in resources across space and time seem to drive their movements. Berger (1986) found that a horse population in the Great Basin exhibited altitudinal migration to enhance their access to forage availability, while a population in the Red Desert of Wyoming, where spatiotemporal variation was less extreme, exhibited relatively stable, year-long home ranges (Hennig 2021). Movements of equids are strongly influenced by seasonal vegetation biomass and availability (Salter and Hudson 1982; Kaczensky et al. 2008), which subsequently influences home range size (McLoughlin and Ferguson 2000). Older studies in North America that relied on visual observations reported wide variation in horse home range size, between 2.6 and 48 km2 (Pellegrini 1971; Feist and McCullough 1976; Berger 1977, 1986; Salter and Hudson 1982; Miller 1983). Home range size from these earlier studies are smaller than what has been found in studies using global positioning system (GPS) telemetry data. Home ranges sizes reported for feral horses living in forested areas in Alberta and open shrublands in Wyoming were 48.4 km2 and 40.4 km2, respectively (Girard et al. 2013; Hennig et al. 2018). In Utah, average home range size for mares was 110.3 km2 (Schoenecker et al. 2022a, b). Mares in Alberta and Wyoming inhabited areas with abundant water sources; whereas mares in Utah had larger home range sizes most likely to accommodate larger distances to water (Schoenecker et al. 2022a, b).

3.2 Feral Equid Effects on Rangeland Ecosystems

Equids are cecal digestors with agile lips and upper sets of canines and incisors (Janis 1976; Scasta et al. 2016). Cecal digestion is comparatively less efficient at nutrient extraction than rumination, meaning that equids need to consume more plant biomass relative to a comparatively-sized ruminant (Hanley 1982; Menard et al. 2002). Their agile lips and upper teeth allow equids to crop plants closer to the ground, compared to cattle, when grazing (Menard et al. 2002). Together, and along with their relatively large body size, poorly-managed feral equid populations can have severe negative effects on the rangeland systems they inhabit (Boyd et al. 2017; Eldridge et al. 2020). Studies have linked feral horse grazing with decreased vegetation biomass, lower plant height, decreased plant species richness, increased cover of exotic and invasive species, reduced seed banks, increased soil penetration resistance, and increased bare ground cover (Baur et al. 2018; Beever 2003; Beever and Brussard 2004; Beever and Herrick 2006; Beever et al. 2008; Beever and Aldridge 2011; Boyd et al. 2017; Davies and Boyd 2019; King et al. 2019; Loydi et al. 2012; Stoppelaire et al. 2004; Zeigenfuss et al. 2014; Hennig 2021). These effects contribute to decreased overall rangeland health, less forage for livestock and native herbivores, and degraded wildlife habitat (Jones 2000; Beever 2003; Scasta et al. 2018). Indeed, research has documented lower small mammal, reptile, and invertebrate densities in horse-occupied versus un-occupied sites (Beever and Brussard 2004; Beever and Herrick 2006). Moreover, increasing populations of feral horses was correlated with population declines of the greater sage-grouse (Centrocercus urophasianus; Coates et al. 2021).

In arid rangelands, feral equid effects extend to interference competition at limited water sources. Feral horses are large and often aggressive, which can translate into subordinate species altering their behavior at water. Bighorn sheep (Ovis canadensis) have been shown to avoid water sites when horses are present (Osterman-Kelm et al. 2008), and pronghorn (Antilocapra americana) show increased vigilant activity around horses (Gooch et al. 2017). Both pronghorn and mule deer (Odocoileus hemionus) have been documented to shift their temporal or spatial watering activity in response to horses, and watering sites with horses tend to have fewer vertebrate species richness (Hall et al. 2016, 2018). Equid grazing and trampling at watering sites influences plant communities, particularly during the critical growing period. Impacts can include reduced vegetation cover, greater percent bare ground, and less litter (Boyd et al. 2017). In combination with other grazers, forage species and soils become highly vulnerable to grazing impacts when they are in close proximity to these water sources. Agencies and land owners that limit equid access to riparian areas experience increased vegetation cover and greater soil protection from compaction and erosion. For example, following 3 years of exclusion, Boyd et al. (2017) found that plant cover and litter increased by as much as 40% and the extent of bare ground decreased by 30%. Higher vegetation cover and reduced bare ground can reduce erosion potential and decrease the vulnerability of these sites to invasive species.

4 Rangeland Management

4.1 Guiding Federal Policies

The complexity of rangeland management of feral equids on federally-owned public land in the United States is better understood when considering the laws that govern feral equid protection and public land use. The first law dealing with protection and management of horses and burros was the Wild Horse Protection Act of 1959 (WHPA; Public Law 86-234). This act prevents the use of aircraft or motor vehicles to hunt and capture unbranded horses or burros on public lands. It also prohibits the pollution or poisoning of water holes on public land for the purpose of trapping or killing horses or burros. Congress next implemented the Wild Free-Roaming Horses and Burros Act in 1971 (WFRHBA; Public Law 92-195), which is the sentinel law concerning horse and burro protection and management. This act protects any unbranded or unclaimed horse or burro on public lands from capture, branding, harassment, or death (Public Law 92-195). It also mandates that the BLM and USFS provide habitat for horses and burros in areas where they existed at the time of enactment. These agencies were granted permission to conduct management actions to maintain a natural ecological balance between equid populations and the capacity for public lands to offer other ecosystem services, including livestock grazing, wildlife habitat, and recreation. The WFRHBA gives authority to the BLM and USFS to remove excess horses and burros for private adoption or to humanely destroy individuals if it was deemed necessary to preserve rangeland condition for multiple uses.

The Federal Land Policy and Management Act of 1976 (FLPMA; Public Law 94-579) amended the WFRHBA by authorizing the BLM and USFS to use helicopters for transporting captured horses and burros and the Omnibus Parks and Public Lands Management Act of 1996 (Public Law 104-333) extended the use of helicopters for gathering. FLPMA further defines the concept of multiple uses as the managing of public lands so that they best meet the present and future needs of citizens. This means protecting the ecological, scenic, and historical values and preserving habitat for wildlife and livestock. The WFRHBA was additionally amended through the Public Rangelands Improvement Act in 1978 (PRIA; Public Law 95-514). This act required inventories of horse and burro populations on federal lands and directed the BLM and USFS to determine appropriate management levels (AML) within horse and burro herd management areas (HMA). PRIA gave BLM or USFS the authority to determine whether AML should be achieved by removal or destruction of excess animals, or through non-lethal methods such as sterilization.

When equid populations in HMAs are found to be above the maximum AML, PRIA directs the BLM or USFS to decide which population control method (removal, destruction, sterilization, or other) is most appropriate to implement. Their decisions must be approved by the general public and are often legally challenged and successfully overturned (see Scasta et al. 2018). When removals do occur, excess healthy animals are put up for adoption, but the WFRHBA states that if excess animals are not adopted after three attempts, then they shall be humanely destroyed; however, due to annual riders (amendments) attached to federal appropriations bills, destruction of healthy animals is currently prohibited (Garrott and Oli 2013).

4.2 Livestock Grazing Management in the Feral Equid Context

Administration of livestock grazing on public lands in the western U.S. was prompted by the Taylor Grazing Act of 1934 (Public Law 73-482). This Act ended open grazing on public rangelands and created the Division of Grazing in the Department of Interior (DOI), which has been used to regulate the entry and practice of grazing on approximately 80 million acres of unreserved federal lands (excluding Alaska). This resulted in a highly regulated process that includes permitting, fees, and multi-year leasing. In addition, livestock numbers (i.e., animal unit months or AUMs) and timing of grazing are explicitly stipulated within a permit that is reviewed by specialists from the BLM and USFS in the context of rangeland monitoring data. Adjustments over time are made through collaborative dialogue with permittees. Violations of livestock grazing stipulations, deteriorating rangeland condition concerns, or weather patterns such as drought can manifest in a reduction of AUMs and grazing duration.

Compared to livestock grazing on public lands, feral equid use is much less regulated. In addition to controlling the numbers of livestock and timing of grazing, the areas that livestock can graze are often managed using fencing, deferred grazing rotation, herding, and salt and water distribution (Beever 2003). Contrastingly, feral equids graze year-round in largely unfenced areas that permit free movement across the landscape. Livestock grazing is annually assessed in the context of rangeland monitoring data and adaptively managed to alleviate problems, as compared to equid grazing which is managed with gathers and removals (Fig. 21.4) to move numbers closer to AML (Hurwitt 2017).

Fig. 21.4
2 photos of the mountain landscapes with a herd of horses running in the foreground while a helicopter flies overhead.

Helicopter gathering of horses in southern Wyoming (above) and in Utah (below). Note the handler and Judas horse in the foreground in the Utah roundup. The Judas horse is a trained horse that is released as horses are led into the corrals, subsequently leading the group of feral horses into the trap (not shown). Photo credit USGS

Full size image

4.3 Feral Equid Population Management Tools

Management of feral equid populations involves different approaches to reduce total population on western rangelands and/or growth rates (Scasta et al. 2018; Hendrickson 2018). Non-lethal approaches are the primary strategy, particularly in the most recent report to Congress (BLM 2018) and include several options:

  1. (1)

    Reproduction management where animals are gathered, chemical immunocontraceptive or surgical sterilization are administered, and animals released back ‘on-range’. Some immunocontraceptives can be delivered through darting in the field and do not require gathering animals (Kirkpatrick and Turner 2008; Kane 2018; Bechert et al. 2021).

  2. (2)

    Removal and Adoption where animals are gathered and then adopted to private individuals (Bender and Stowe 2020; Fig. 21.5).

    Fig. 21.5
    2 combined plots of line and scatter graphs represent the population and B L M adoptions of horses and burros versus the years from 1970 to 2020, depicting a fluctuating trend. The estimated population of horses is higher than the Burros.

    (Left) On-range population estimates of feral horses and burros within Bureau of Land Management (BLM) Herd Management Areas from 1970 to 2020. The dotted line signifies the nationwide maximum Appropriate Management Level (AML). In areas where equid populations are above maximum AML, the BLM may conduct gathers to remove excess individuals. These individuals are either put up for adoption or housed in long-term holding facilities. (Right) The number of adopted feral horses and burros by private citizens between 1970 and 2020. All data were acquired via the BLM Wild Horse and Burro Program website (https://www.blm.gov/whb)

    Full size image
  3. (3)

    Relocation to off-range facilities where unadopted animals are transferred to long-term pastures in the central U.S. that are privately owned and a per head payment is provided by the BLM (Elizondo et al. 2016).

Lethal strategies are not currently allowed but do need mention here and include:

  1. (1)

    Capture and euthanasia where an animal is in stress and/or pain due to age, injury, or other condition inhibiting horse welfare. This is in adherence to Instruction Memorandum (IM) 2015-070 for BLM Animal Health, Maintenance, Evaluation, and Response and established the policy and procedures for proactive and preventative medical care (BLM 2015).

  2. (2)

    Slaughter where animals are gathered and killed off-site and the meat is utilized (either human or non-human purposes). While WFRHBA (Public Law 92-195) does provide the authority for “destroying” either excess horses for which there is no adoption demand [see §1333. Powers and Duties of Secretary (a)(2)(C)]; this is not used in the United States currently because the U.S. Congress has prohibited slaughter since 2007 with the Agriculture, Rural Development, Food and Drug Administration, and Related Agencies Appropriations Act (Public Law 109-97) that prohibits use of federal funds for horse inspection, followed by subsequent amendments and ultimately a 2014 federal budget which explicitly prohibited horse slaughter (Norris 2018).

5 Threats to Feral Equid Populations

5.1 Disease

Domestic and feral equids are affected by a variety of maladies (Table 21.2). There is the potential for wild populations to act as a disease reservoir (Gilchrist and Sergeant 2011), with a difference in potential for spread depending on whether they are on-range, or in holding facilities. Additionally, disease is more likely to be expressed and spread in holding facilities due to high density of horses from various HMAs and high stress levels in captive equids. Gastrointestinal parasites can be common among feral equids, which can impair gastrointestinal function, reduce body condition, lower reproductive success, and decrease overall health and longevity (Debaffe et al. 2016; Pihl et al. 2018). In south-east Australia, Harvey et al. (2019) found that the parasite Strongylus vulgaris had infection rates as high as 97%, with symptoms that included fever, elevated heart rate, pain, and gastric reflux. This parasite was transmissible to domestic herds through direct contact with wild horse herds.

Table 21.2 A non-exhaustive list of diseases, infections, and disorders that may affect both domestic and feral equids
Full size table

Blindness, lameness and hoof disorders or damage (i.e. laminitis) all occur to feral equids. Blindness may result from trauma (fighting), impact trauma from branches or grass stems, or disease (i.e. Equine recurrent uveitis, also known as moon blindness, which is the most common cause of blindness in horses). Common causes of lameness include trauma, infection, acquired disorders, metabolic disorders, and nervous and circulatory system disease (Adams 2015). Horses evolved and were artificially selected to travel long distances with repeated low-load concussive conditions, typical of hard terrain. However, they are subsequently predisposed to hoof and leg abnormalities (Hampson et al. 2013). These can also lead to issues such as osteoarthritis, joint pain, foot irregularities, and laminitis. Laminitis is a hoof ailment that has been commonly observed in Australian feral horses than can cause severe pain and difficulty during travel (Hampson et al. 2010a, b).

5.2 Climate Change

Effective management of feral equids will require an understanding of the current and future threats from a changing climate (Tietjen and Jeltsch 2007). Forecasted global climate change suggests western North America will be warmer and experience greater variability of extreme events including droughts (Pokhrel et al. 2021). The effects of climate change could be exacerbated in xeric climates. Data suggests that impacts can include high variability in precipitation levels, with xeric areas becoming dryer (Dore 2005). These changes may subsequently impact vegetation and forage production as intensity in precipitation increases but total quantity remains the same, creating more variable soil moisture conditions. If forage production decreases, carrying capacity will also decrease leading to potential overgrazing by herbivores (Tietjen and Jeltsch 2007). Impacts to feral equids may include death and sickness caused by starvation, greater conflicts in urban areas, and increased intraspecific competition. The use of wildlands for grazing are at risk because of unpredictable trends in climate and vegetation dynamics and therefore require careful monitoring and planning to prevent overgrazing and negative impacts by feral equid and other ungulate grazers.

6 Conservation and Management Challenges

6.1 Social Challenges

The management of feral equids is a contentious issue to say the least. While federal protection is stipulated by the WFRHBA, so is the proper management of the broader suite of natural resources (Public Law 92-195). The federal government’s role has been characterized as “a national injustice” and “systematic removal and eradication of an American icon”. Generally, the situation has pitted those who advocate for horses against those who advocate for multiple use and healthy rangelands. Yet, these two groups may not be mutually exclusive because as the population of feral equids increases, there may be negative consequences for horses due to degraded rangelands. In other words, an overabundance of horses and burros leads to overgrazing and potentially health issues for horses and burros as well as a cascade of other issues for soils, water, plants, wildlife, and other user groups. Increasing equid populations, especially in arid landscapes, may lead to decreased body condition, reduced access to forage and water, and an increase in emergency gathers conducted by BLM (Fuller et al. 2016). Further exacerbating the problem is the financial cost of gathering, removing, and maintaining horses in off-range facilities. Off-range care and feeding that are primary costs covered by the BLM Wild Horse and Burro program and these costs exceeded $65.5 million in FY 2020. These off-range costs are projected to be approximately $360 million annually in the next 15–18 years if on-range populations are reduced to AML (BLM 2020b). Future progress on the issue will require finding common ground among different stakeholder groups that enhances the health of the land and the horses and burros.

6.2 Antithetical Litigation

Aside from financial constraints, a major impediment to feral equid management is the prevalence of litigation. Scasta et al. (2018) provided examples of cases filed against the BLM for both managing and not managing equid populations. For example, one lawsuit attempted to bar the BLM from implementing a plan to gather approximately 2,700 wild horses in western Nevada. In a contrasting case, the BLM was sued for allowing too many free-ranging horses in Nevada. This antithetical litigation dynamic creates a very difficult situation for the federal government to effectively manage horse populations, ultimately leading to instances of management stasis while horse populations continue to grow and ecological problems continue to intensify.

7 Research and Management Needs

Feral equids inhabit a vast area of the western North American landscape but their ecology is less understood compared to native ungulates. Only a handful of recent studies have characterized habitat use of feral equids (Edouard et al. 2009; Girard et al. 2013; van Beest et al. 2014; Leverkus et al. 2018; Hennig 2021; Schoenecker et al. 2022a, b). There is a dearth of information regarding feral equids for several reasons. Little funding has been available to study feral equids since the inception of the WFRHBA. Further, feral species ecology was of little interest to basic science (Boyce et al. 2021). Feral equids are both domesticated and introduced; thus their ecology isn’t studied within the context of prevailing evolutionary theory. Instead, their abundances and distributions are a product of human introductions and land use decisions. Consequently, there is a critical need for research examining topics including resource selection, niche overlap and interspecific competition, and density-dependence to better understand the role of how feral species interact with novel environments. In a management context, specific questions that require further research attention include understanding the comparative effects of feral equids versus livestock on rangelands, quantifying competition between equids and both wild and domestic herbivores, assessing if feral equids decrease the fitness or survival of sympatric wildlife species, and better understanding of social issues such as how the general public perceives the feral equid issue. More information on all of these topics will help natural resources managers with sustaining healthy lands and healthy herds into the future.