The buffalo herds dwindled because they were shot down and because of loss of habitat. This had devastating effects on the Plains Indians.
Step-by-step explanation:
During the 1800's, the buffalo herds went from millions of individuals, to only few tens of them, almost becoming extinct. There were two reasons for this. One reason was that the white settlers started with mass killing off the buffalo herds. The other reason was that on what was buffalo habitat, has quickly turned into farmlands.
The annihilation of the buffalo herds had devastating effect on the Plains Indians. These Indians were heavy dependent on the buffalo herds for their survival, but in only matter of decades they disappeared, leaving the Indians without any resources for survival. This forced them to change their lifestyle and to start adopting the way of life that the white settlers brought with them.