In 1500, between 60 million and 125 million american bison roamed the plains. these bison were used by native americans as a primary food source, but this did not have much effect on the bison population. a few hundred years later, railroad crews and settlers moving west also depended on the bison for food. by 1892, it is estimated that less than 1,000 bison were left. what were the two main causes of the near-extinction of the bison?