Once American Bisons roamed across much of North America before they were slaughtered in thousands and their population shrinked from lakhs to just 325 in the United States. "Buffalo Bill" Cody, who was hired to kill bison, slaughtered more than 4,000 bison in two years. Today they are considered ecologically extinct. Now they live in a few national parks and other small wildlife areas.
Due to the conservation efforts, the bison population in the US has increase to to 500,000 bison in the US, including 5,000 in Yellowstone.