A Mustang is a free-roaming feral horse of the North American West that first descended from the horses brough to the Americas by the Spanish. In 1971, the United States Congress recognized Mustangs as "living symbols of the historic and pioneer spirit of the West, which continue to contribute to the diversity of life forms within the nation and enrich the lives of the American people." Today, the Mustang population is managed and protected by the Bureau of Land Management (BLM). Mustangs have disappeared from 6 states, and according to the BLM their population is fewer than 25,000, with more than half of them in Nevada, and with other significant populations in Montana and Oregon. By 1900, North America had an estimated two million free-roaming horses. Since 1900, the mustang population has been reduced drastically. Mustangs were viewed as a resource that could be captured and used or sold (especially for military use or slaughtered for food). The BLM is tasked with protecting, managing, and controlling wild horses and burros under the authority of the 1971 Wild Free-Roaming Horse and Burro Act, to ensure that healthy herds thrive on healthy rangelands and as multiple-use mission under the 1976 Federal Land Policy and Management Act. Under the Act, shooting or poisoning Mustangs in the wild is illegal, and doing so can be prosecuted as a criminal felony. Mustangs have few natural predators aside from mountain lions, and their herd sizes can multiply rapidly. To counter this, one of the BLM’s key responsibilities under the 1971 law is to determine the “appropriate management level” (AML) of wild horses and burros in areas of public rangelands dedicated specifically for them. The Bureau of Land Management controls the mustang population to within the AML through a capture program. The BLM offers the captured animals for adoption or sale to individuals and groups willing and able to provide humane, long-term care. The adoption fees vary from $25 to $125. In order to prevent the later sale of mustangs as horse meat, adopted mustangs are still protected under the Act, and cannot be sold in the first year except when certain very specific criteria are met. A regular surplus of captured Mustangs over adoptions gave rise to a controverisal January 2005 amendment known as the "Burns rider", after Senator Conrad Burns. This rider modified the management program to allow the sale of captured horses that are "more than 10 years of age" or have been "offered unsuccessfully for adoption at least 3 times." Controversy surrounds the presence of Mustangs on rangeland. Supporters point out the Mustang is part of the natural heritage of the American West, whose history predates modern land use practices, and thus the Mustang has an inherent right of inhabitation. Various groups interested in the retention of the Mustang have formed an advocacy campaign, the American Wild Horse Preservation Campaign. Opponents of the Mustang contend that the Mustang competes with the needs of livestock for the resources of public land forage, notable cattle and sheep.
An aspect of the debate surrounds the role of the Mustang within the North American ecosystem, regarding its potential classification as either an introduced species like cattle, or as a reintroduced species due to the past presence of horses in North America, albeit with a gap of thousands of years.
There is also debate as to what degree Mustangs and cattle actually compete for forage and water, as the evolutionary biology of the Mustang makes it able to forage in areas and on vegetation that would not be grazed by livestock.