Genetic evidence finds that the donkey was domesticated twice based on two distinct mitochondrial DNA haplogroups. It also points to a single ancestor, the Nubian wild ass. Attempts to domesticate zebras were largely unsuccessful, though Walter Rothschild trained some to draw a carriage in England. Humans have had a great impact on the populations of wild equines. Threats to wild equines include habitat destruction and conflicts with local people and livestock. Since the 20th century, wild equines have been decimated over many of their former ranges and their populations scattered. In recent centuries, two subspecies, the quagga and the tarpan, became extinct. The IUCN lists the African wild ass as critically endangered, Grévy's zebra, the mountain zebra, and Przewalski's horse as endangered, the onager as vulnerable, the plains zebra as near threatened, and the kiang as least concern. Przewalski's horse was considered to be extinct in the wild from the 1960s to 1996. However, following successful captive breeding, it has been reintroduced in Mongolia. Feral horses vary in degree of protection and generate considerable controversy. For example, in Australia, they are considered a non-native invasive species, often viewed as pests, though are also considered to have some cultural and economic value. In the United States, feral horses and burros are generally considered an introduced species because they are descendants from domestic horses brought to the Americas from Europe. While they are viewed as pests by many livestock producers, conversely, a view also exists that "E. f. caballus" is a reintroduced once-native species returned to the Americas that should be granted endangered species protection. At present, certain free-roaming horses and burros have federal protection as "living symbols of the historic and pioneer spirit of the West" under the Wild and Free-Roaming Horses and Burros Act of 1971, and in "Kleppe v. New Mexico", the United States Supreme Court ruled that the animals so designated were, as a matter of law, wildlife.