Some of the other factors that were involved in this shrinking of the bison herds by the middle of the 19th century include things like this: Horses had been absent from North America for about 10,000 years. They had become extinct during the Pleistocene extinction. They had survived in Asia and Africa, astonishingly enough, but horses had all become extinct in the Americas. When the Europeans arrived, though, they returned horses. And in one of the interesting ecological stories of the American West, horses are able to go feral in the American West extraordinarily easily, probably because they are returning to a former . . . an ecology that they had evolved to.
And so, not only are Indians beginning to acquire horses in large numbers after the Pueblo revolt against the Spaniards in New Mexico in 1680—the Pueblo Indians captured the Spanish horse herds and began trading them to other groups, so that within fifty years after that revolt, most of the Indians in the West had horses. But horses also began going feral in the West about that time, so that by 1800 it's been estimated that south of the Arkansas River alone there were probably two million wild horses.
In other words, by the time of Lewis and Clark, there may have been as many as three million wild horses in the West, and by that time perhaps a quarter to a half million Indian-owned horses. The reason this is significant is because horses have, in some ecologies, a dietary overlap with buffalo that runs as high as 80 percent. So buffalo suddenly, by the 17th, 18th, and 19th centuries, are beginning to compete with one of their old grazing competitors from 10,000 years before—with horses. And horses, then, simply by their very presence, began to reduce the available forage for buffalo.
So that's yet another factor that most traditional histories have not tried to figure into the mix.