In the early 1800's there were approximately 60,000,000 bison roaming the plains. In the mid-1800's as the west was developed, the bison were slaughtered. Since the bison were central to the lives of the Plains Indians, the army slaughtered the bison to practically extinction in an effort to conquer the Indians. It worked all too effectively. At the end of the 19th century, there were only a few hundred bison left alive at most.
Today there are many herds amounting to several thousand bison in the United States and Canada.