Answer:
American Indians' way of life changed because buffalo herds were destroyed for commercial markets
Step-by-step explanation:
Most American Indians were dependent on the wild Buffalo for survival, at least in the plains and the West.
When the American settlers arrived in these regions, they quickly drove the Buffalo to the brink of exctintion, and replaced it with domestic cattle in order to supply the commercial markets.
The American Indians were obviously excluded from this new economy, and had very few alternatives for survival, having to resort to looting, raiding, and outright conflict with the Settlers. In the end, most of them were sent to the reservations that still exist to this day.