Mustangs are free-roaming horses of the American west that first descended from horses brought to the Americas by the Spanish. Mustangs are often referred to as wild horses, but because they are descended from once-domesticated horses, they are properly defined as feral horses. The original mustangs were Colonial Spanish horses, but many other breeds and types of horses contributed to the modern mustang, resulting in varying phenotypes. Most contain a greater genetic mixture of ranch stock and more recent breed releases, while a few are relatively unchanged from the original Iberian stock, most strongly represented in the most isolated populations. In 1971, the United States Congress recognized that "wild free-roaming horses and burros are 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." The free-roaming mustang population is managed and protected by the Bureau of Land Management (BLM). Controversy surrounds the sharing of land and resources by the free-ranging mustangs with the livestock of the ranching industry, and also with the methods with which the federal government manages the wild population numbers. The most common method of population management used is rounding up excess population then offering them to adoption by private individuals. There are inadequate numbers of adopters, so many animals now live in temporary and long-term holding areas with concerns that the animals may be sold for horse meat. Additional debate centers on the question of whether mustangs—and horses in general—are a native species or an introduced invasive species.