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At the end of the 15th century, the Spanish reintroduced horses to the Americas.  Escaped horses soon resumed to a wild state and proliferated on the plains of their homeland.  By the time of Anglo exploration in the 1800s, vast herds of wild horses roamed North America.  Their habitat gradually shrank, along with the habitat of other large grazers, such as bison and elk, as settlement spread onto the plains.  Herd size was controlled by ranchers and also by mustangers who hunted the horses or gathered them for commerical purposes.


Starting in the 1950s, public concern about the well-being of wild horses and burros grew.  With the mounting interest and concern came the realization that a federal management, protection, and control program was essential.  After witnessing a livestock truck filled with horses headed to slaughter, Velma Johnston, later known as “Wild Horse Annie”, began a grassroots letter writing campaign involving mostly schoolchildren to prevent wild horses from ending up in the slaughterhouse.  This campaign became known as the “Pencil War” and led to the passage of the “Wild Horse Annie Act” in 1959 to protect these iconic animals.  The exposure of how wild horses were being treated outraged and fully engaged the public in this issue.  A 1959 article in the Associated Press stated “seldom has an issue touched such a responsive chord.”

The follow up efforts of the massive letter writing campaign resulted in the development and enactment of the Wild Free-Roaming Horse and Burro Act of 1971, which gave birth to the Wild Horse and Burro Program in the Forest Service and Bureau of Land Management. 

The Wild Free-Roaming Horses and Burros Act was passed, unanimously, through Congress and signed by former President Nixon on December 15, 1971.  It became Public Law 92-195, which protects wild horses and burros within designated territories on both Forest Service and Bureau of Land Management lands.  The Act declares wild horses and burros to be “living symbols of the historic and pioneer spirit of the West” and mandates that these horses and burros are managed in a thriving ecological balance with the land and as part of the natural landscape.

• Bureau of Land Management Wild Horse and Burro Program
• The Wild Free-Roaming Horses and Burros Act of 1971

Controversy surrounds the presence of feral horse and burro herds, particularly on public lands.  Supporters argue that these animals are a part of the rich heritage of the American West, whose history predates modern land use practices, and thus the animals have an inherent right of habitation.  However, others remain opposed by their presence, arguing that the animals degrade the rangeland, compete with livestock and wild species for forage, and a waste of taxpayer dollars.  Therefore, tensions between the two groups has lead to an emotionally charged social and political debate exists on how to manage for feral horses and burros on public lands.

• Wild Horses.  Article describes the controversy of managing wild horses in Nevada.
• Cruel or necessary? The true cost of wild horse roundups.  Article and video by NBC News Investigators regarding the controversy and taxpayer cost of conducting wild horse roundups.

The Equus species are part of North America's natural ecology, as they evolved on this continent along with the grasslands.  Equus includes horses, donkeys, and zebras.  Fossil history clearly documents that equids developed in North America.  The first equid, Eohippus, appeared in the Eocene Epoch 54-34 million years ago.  This species was a small forest animal suited to the marshy environment of the time.  Thousands of complete, fossilized skeletons of these animals have been found in the Eocene layers in North America, primarily in the Wind River basin of Wyoming.

In the Oligocene Epoch (34-24 million years ago), the climate of North America started changing to a drier climate, and the forests gave way to grasslands.  Mesohippus and Miohippus appeared during this time, and these fossils were also prevalent in Wyoming.  Parahippus and Merychippus arose during the Miocene Epoch (24-5.3 million years ago) as the large grasslands evolved.  Merychippus was distinctly recognizable as a horse.  Equus arrived about 4 million years ago during the Pliocene Epoch.  Equus is the genus of all modern equines.  The first Equus were 13.2 hands tall with a classic "horsey" body.

During the first major glaciations of the late Pliocene (2.6 million years ago), some Equus species crossed to the other continents by way of the Isthmus of Panama into South America and the Bering Strait into Asia and Europe.  Until about 1 million years ago, there were Equus species all over Africa, Asia, Europe, North America, and South America in large migrating herds.  During the late Pleistocene (~10,000 years ago), there was a rash of extinctions that wiped out most of the large mammals in North and South America.  All the horses of North and South America died out, along with the mammoths and saber-tooth tigers.  These extinctions seem to have been caused by a combination of climatic changes and overhunting by humans, who had just reached these continents.  For the first time in tens of millions of years, there were no equids in the Americas.

At the end of the 15th century, the Spanish reintroduced horses and burros to the Americas.  Horses that escaped, soon resumed to a wild state and proliferated on the plains of their homeland.  By the time of Anglo exploration in the 1800s, vast herds of wild horses roamed North America.

The Bureau of Land Management (BLM) estimates that 40,605 wild horses and burros (approximately 33,780 horses and 6,825 burros) are roaming on BLM managed rangelands in 10 western states based on the latest available data, compiled as of February 28, 2013. Wild horses and burros have virtually no natural predators and their herd sizes can double every four years.  As a result, the agency must remove excess animals from the range each year to control herd sizes.  The estimated current free-roaming populations exceeds by nearly 14,000 the number that the BLM has determined can exist in balance with other public rangeland resources and uses.  The maximum appropriate management level (AML) is approximately 26,677 (DOI BLM website). 

• National Academy of Sciences Committee Review:  Using Science to Improve the BLM Wild Horse and Burro Program, A Way Forward.  A 2013 review by the National Academy of Sciences of the Wild Horse and Burro Program detailing the effects these animals have on the landscape and constructive changes that could be implemented into the program.

"Biodiversity is a multifaceted phenomenon involving the variety of organisms present, the genetic differences among them, and the communities, ecosystems, and landscape patterns in which they occur. Society will increasingly value biodiversity and influenced by passage of laws and writing of regulations involving biodiversity which rangeland managers will have to abide by over the coming decades. Even private and developing world rangelands will be affected." - Neil E. West. 1993. "Biodiversity of Rangelands" Journal of Range Management 46(2):2-13.

• Biodiversity of Rangelands: Journal of Range Management 46(1):2-13.
   
• Livestock Grazing and Biodiversity: A brief paper on the positive and negative effects of livestock grazing on biodiversity from Canada Prairie Farm Rehabilitation Administration.
   
• NatureServe: Knowledge to Protect the Diversity of Life: Information from a network of government, academic, and non-profit organizations.
   
• Saving Biodiversity: A Status Report on State Laws, Policies, and Programs: Report from Defenders of Wildlife, a non-profit organization that focuses on preserving biological diversity.