'The Yellowstone herd is both genetically and behaviorally unique, being the only herd with continuously wild ancestry from the days when 50 million migrated across the Great Plains.
At the end of the 19th century, after tens of millions had been slaughtered, only 23 wild Bison survived. Taking refuge in Yellowstone, this remnant herd insured the survival of the species. Today there are little more than 3,000 Bison living in and around Yellowstone.'
...courtesy of the Buffalo Field Campaign