The Denver Zoo also has a storied history with the American bison as it was one of the first zoos to assist in nationwide efforts to save the species from extinction. A breeding herd of five bison arrived at the zoo in 1899. The zoo no longer houses the species but manages a herd on the Rio Mora National Wildlife Refuge in New Mexico and also contributed animals to both the Genesee herd near Denver and the Heartland Ranch herd near Lamar. The latter herd, behaving naturally on thousands of acres and with the help of the Southern Plains Land Trust, almost entirely returned the ecosystem to its natural state as of 2019.