Unintended consequences of the wild fee-roaming horses and burros act

Wild horses and burros (WHB) are currently protected under the Wild Free-Roaming Horses and Burros Act of 1971 (WHB Act; Public Law 92–195). This Act protects unbranded or unclaimed WHB on select public lands from being captured, branded, harassed, or killed. The Act authorizes the Bureau of Land Management (BLM) and Forest Service (USFS) to provide habitat for WHB in areas where they existed on public lands in 1971. Both agencies must manage WHB at a ‘minimum feasible level’ to maintain their wild status. This means that, unlike livestock, it is difficult to control when, where, or how long WHB may graze within (or outside of) a designated management area. Additionally, because federal public lands are managed to sustain multiple uses (What are Public Lands and who are they for? - see below), the BLM and USFS are mandated to manage WHB in a way that maintains a ‘thriving and natural ecological balance’ on the landscape.

To appropriately answer this question, a brief history of western public lands is required. After the U.S. government gained control of western lands, the lands owned by the federal government (e.g., public lands) were governed under a ‘commons’ system where individuals shared use of these lands. Over time, the commons system led to competition among multiple interests (cattle, sheep, local, and transient operators) competing with one another in a “race for feed” which resulted in poor rangeland conditions from overstocking and excessive grazing impacts known as the “tragedy of the commons” (Hardin 1968).

To limit rancher conflict and improve rangeland health, Congress passed the Taylor Grazing Act in 1934 (Public Law 73-482). This Act installed a system of grazing districts and a grazing permit system on public lands that allowed ranchers to obtain leases or permits from the government to graze their livestock based upon priority use (1929-1934), base property production, and public land carrying capacity. With ranchers now having more control over specific grazing allotments, they also had more accountability to sustainably manage land under their lease or permit. The BLM initiated an “adjudication” period in the mid-1950s to mid-1960s to survey and establish the carrying capacity of public lands being grazed under lease. This adjudication period began the determination of our public land's carrying capacity that could sustainably support livestock and wildlife. However, feral horses were not considered livestock or wildlife and became increasingly viewed as pests by land and wildlife management agencies now responsible for the sustainable management of public lands and wildlife habitat, and as competition to the ranchers' dependent upon public lands for grazing. During World War II, free-roaming domestic and feral equids were rounded up for use in the military cavalry and domestic and foreign meat markets. Following the war, feral horses were gathered and processed into commercial pet food (Sowell et al 1983). Post-World War II, feral horses and burros remaining on public lands that were not owned or claimed, were captured and removed in significant numbers through a process known as ‘mustanging’ (Godfrey and Lawson 1986) which involved hunting and/or capturing feral horses and burros, using airplanes, trucks, pit traps, and poisoned water holes.

By the 1950s, feral populations had decreased significantly, and the capture methods employed by mustangers impelled Velma B. Johnston of Reno, Nevada, later known as "Wild Horse Annie," to protect these animals. She organized a letter-writing campaign, known as the “Pencil War,” to bring attention to the inhumane methods implemented to gather feral horses and burros (Burkhardt 2024). In response, Congress passed the Wild Horse Annie Act (Public Law 86- 234) to ban the use of motorized vehicles and the poisoning of water sources to hunt, capture, or kill feral horses and burros on public lands. However, this act did not include Wild Horse Annie’s recommendation that Congress initiate a program to protect, manage, and control feral horse and burro populations. Public interest and concern continued to gain momentum until Congress eventually passed the WHB Act in 1971. Ironically, Mrs. Johnston later expressed how she may have unintentionally “started something that has gotten out of control” and condemned many of the advocacy groups and provisions in the WHB Act. She believed that unmanaged WHB populations would lead to inhumane suffering and death of her beloved WHBs from starvation and thirst (Burkhardt 2024).

This is a tricky question because horses and burros are part of the Equidae family which arose in North America about 4 million years ago. However, likely due to a combination of climate change and hunting, all equid species became extinct on the continent during the last Ice Age roughly 12,000 years ago (Grayson 2007). For several thousand years, between the Ice Age and the European discovery of North America, horses were absent from the continent until Christopher Columbus brought domesticated horses with him on his second voyage in 1493 (Cortés et al. 2016). Soon after, Spanish conquistadors brought more horses and burros to Mexico and what is now the southwestern U.S. Over time, domestic horses and burros escaped, or were intentionally let loose, and feral populations became well-established (Wild, native, free-roaming, or feral - see below). Between the 1600s and early 1900s feral horses and burros were abundant across much of the West. With the onset of the Industrial Revolution in the late 1800s, horses and burros, once integral to modern civilization, became less needed for transportation, agriculture, and other needs. Consequently, more unneeded horses were released into the wild.

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 Locations where WHB existed on public lands when the WHB Act was passed in 1971 are known as Herd Areas. However, Herd Areas are not currently managed for WHB populations as part of the multiple-use concept. Areas where WHB must be explicitly managed according to the 1971 WHB Act are known as Herd Management Areas (HMA) on BLM lands and Wild Horse and Burro Territories (WHBT) on USFS lands. Currently, there are 177 HMAs spread across 10 states with a total acreage of 31.6 million acres (BLM 2024). The USFS manages 53 Wild Horse and Burro Territories in 9 states, but only 34 of these currently have WHB populations (USFS 2014).

In Arizona, the BLM manages 7 HMAs while the USFS has jurisdiction over 2 WHBTs (Fig. 2). Arizona is home to the largest population of burros of any state (BLM 2024) where 5 of the 7 Arizona HMAs are solely managed for burros (Table 1). The USFS territories are the Heber (Apache National Forest) and Double A (Kaibab National Forest). The Heber is managed for horses while the Double A is managed for burros.

WHB populations quickly rebounded after the passage of the WHB Act in 1971, so much so that the act was further amended by the Public Rangelands Improvement Act (PRIA) of 1978 (Public Law 95–514). This act was established to improve rangeland conditions, but it was also implemented to manage the high cost of administering the WHB Act. This legislation recognizes that overpopulated WHB ‘pose a threat to their own habitat, fish, wildlife, recreation, water and soil conservation, domestic livestock grazing, and other rangeland values.’ This Act requires the BLM and USFS to determine appropriate management levels (AML) within horse and burro management areas. AML is a population size range of WHB set to achieve healthy WHB herds while simultaneously allowing sufficient forage for livestock and wild animals. Currently, WHB numbers exceed AML in every state except Idaho (Table 2). Across most states population sizes are 2-5 times the maximum AML (Table 2). Nationally, the maximum AML is 26,785 (BLM 2024), but national population size estimates are nearing 59,000 horses and over 14,000 burros, or approximately 275% of the maximum AML (Table 2). The current Arizona population sizes of WHB on BLM land are about 366% of the maximum AML. Recent estimates of populations are not currently available for WHB on USFS lands.

All rangelands have a carrying capacity, which is the average number of wildlife and/or livestock that may be supported within a management unit compatible with the management objectives for the unit (Bedell and SRM, 1998). When there are too many hooves on the ground in the same general location for too long of a time, regardless of species, damage to vegetation communities and related rangeland resources can occur. For example, feral horses and domestic cattle are both implicated in degrading riparian areas. However, horses may have a greater impact on riparian areas than cattle because horses often have year-round access to these areas, while cattle are generally removed for part of the year and oftentimes during growing seasons (Kaweck et al. 2018). Wild horses and burros differ from livestock and wildlife in a few other key ways that make them more likely to negatively impact natural resources.

Cattle, sheep, and most native ungulates (hooved mammals including deer, elk, bighorn sheep, and pronghorn) are ruminants (Janis 1976). This means they have a 4-chambered stomach containing a fermentation chamber where bacteria break down forage into energy for the animal. This chamber is located before the small intestines, which helps a ruminant obtain as much energy from their forage as possible (Fig. 3). Horses and burros are ungulates but are not ruminants; instead, they are considered hindgut fermenters because their fermentation chamber, called a cecum, lies after the small intestine (Fig. 3). This means that WHBs are less efficient in obtaining energy from their forage than a ruminant (Janis 1976). Hindgut fermenters are also less efficient in water retention (Janis 1976). These two factors mean that a WHB must eat and drink more to sustain themselves than a comparatively sized cow or elk. Second, WHB possess upper and lower incisors while ruminants only have lower incisors and an upper dental pad (Fig. 4; Scasta 2014). This allows WHB to clip vegetation closer to the ground than livestock and native ungulates. Thus, WHB grazing methods can limit the regrowth of vegetation because the growth points (i.e., meristems) of grasses occur close to the ground.

A wide body of research shows that when the grazing impacts of WHB are not properly managed they adversely impact western rangelands. For instance, areas in the Great Basin that were exposed to unmanaged horse grazing had lower forage quantity, plant heights, and native plant diversity than similar sites where horses were removed or excluded (Beever et al. 2003, 2008, Davies et al. 2014, Boyd et al. 2017). Additionally, overgrazing by horses may lead to an increase in invasive species, more exposed bare soil, and increased soil compaction (Ostermann-Kelm et al. 2009, King et al. 2019, Hennig et al. 2021). These effects combine to increase erosion rates, degrade rangeland quality, and reduce carrying capacity.

In addition to their effects on vegetation and soil, WHB can also affect the animals they share the land with, especially at watering sites. Water is a precious resource in semi-arid western rangelands, and horses have been shown to actively exclude wild and domestic ungulates from watering holes (Perry et al. 2015). In California, bighorn sheep avoided preferred watering sites when horses were nearby (Ostermann-Kelm et al. 2008). Pronghorn in Nevada spent more time being vigilant and less time eating or drinking near water sources when horses were present (Gooch et al. 2017). In Utah, both pronghorn and mule deer altered which watering sites they used and when they visited them to limit interaction with horses (Hall et al. 2018). Also in Utah, the presence of horses at watering sites resulted in less native plant diversity compared to water holes where horses were excluded (Hall et al. 2016). A recent study showed that in HMAs where horse numbers were consistently above AML, the population sizes of greater sage-grouse declined over time compared to sites that were within AML (Coates et al. 2021).