The thriving population was reduced to a few individuals in 25 years by a series of land use decisions. In the 1790s, humans introduced cattle, sheep, goats, dogs and pigs to the islands. Both domestic and feral animals foraged on the open range or were fed on native plants. By 1851 there were about 3,000 feral sheep and 12,000 feral cattle on Mauna Kea. Feral dogs kept the sheep population in check until 1921, when the dogs were systematically eliminated. The population of Mauna Kea silverswords fell as the sheep population rose after 1921; live silverswords were seen only on cliffs in Puhakuloa, Waikahalulu, and Waipahoehoe Gulches. The sheep population on Mauna Kea eventually exceeded 40,000 in the 1930s, decimating the defenseless native plants including remaining silversword. Feral sheep were then greatly reduced from 1936 to 1950 but protected until 1981 as sport game in the Forest Reserve lands, which overlap the original Mauna Kea silversword habitat.