Bison Management - Theodore Roosevelt National Park (U.S. National Park Service) Twenty bison from the South Unit herd were transported to the North Unit in 1962 to found a herd there. Attendees watched from a distance as the bison exited the truck. Bison are one of this country’s great symbols for wildlife and land conservation. Once numbering in the tens of millions, they were nearly driven extinct by the end of the 19th century due to rampant overhunting. Had it not been for the concern of early conservationists and ranchers, including Theodore Roosevelt, they may have been lost forever. Today, due to protection and reintroduction efforts, their numbers have rebounded from just a few hundred to around 30,000 in conservation herds (around 500,000 others exist on commercial ranches).