Intolerance is the greatest threat to bison. Though bison are no longer threatened with extinction, there are currently very few places where wild bison are allowed to live as wildlife. Almost all of their historic stomping grounds are off limits, due primarily to opposition from livestock interests. Yellowstone – the one place where wild bison were not completely wiped out – is home to the largest free-roaming bison herd in the United States. But even here bison are not allowed to roam much beyond the park boundary and are kept to fewer than about 3,500 in number. When the herd grows above this level, the “surplus” bison are often shipped to slaughter. For all intents and purposes, bison are still ecologically extinct.