Contrary to popular belief, the Native American reliance on the bison was not very old. Before the arrival of Europeans, most tribes typically thought of as Plains nomads lived in agricultural societies on the edges of the Plains. The bison was an important part of their culture, yes, but they had, as Andrew Isenberg, author of The Destruction of the American Bison terms it "ecological safety nets". Bison populations flunctuated widely according to the volatile prairie environment. However, in a bad bison year, Native Americans fell back on their crops.
This all changed when the Pueblo Indians revolted against the Spaniards in the 1580s. The tribes took the Spanish horses and an intertribal trade sprang up almost immediately. The horses spread over the Great Plains, reaching the Northeastern corner in the early 1700s and the hands of tribes, such as the Sioux, that lived there. They quickly became the wealth of the Native American world.
Horses were a revelation for the Plains tribes. Instead of time consuming, inaccurate dangerous pedestrian hunts that involved the relocation and cooperation of numerous villages to drive herds off precipices or into corrals, small bands of hunters could choose the individual animals and number of animals with great ease and accuracy. The complex, organized governments began to fracture, as they were no longer needed for the organization of great hunts. Nomadic tribes relied less and less on their crops, instead trading for them with agricultural villages along rivers. Bison skins, meat, and other products became valuable with great rapidity after the genesis of the nomads and lively trade routes were rising. The bison acquired a special emphasis in Native American religions. Plains societies were transformed.
Nomadic reliance on the bison and trade was reinforced by the arrival of smallpox and other European diseases in the late 1700s. The densely populated agricultural villages made it easy for disease to spread, whereas nomadic bands were isolated, far apart because of their bison hunting and experienced far fewer deaths. The agricultural tribes, such as the Mandan, often lost upwards of 80% of their population in this time period, but the nomadic ones only about 40%.
Thus, the bison spared nomads from the ravages of smallpox and infectious disease. However, they had abandoned the ecological safety nets entirely and depended on the bison for everything. This left them vulnerable to European attacks. The Southern Herd Hide Rush of 1873, the railroads, the Red River Hunting expedition, and campaigns by the US government to kill Native Americans caused the bison population to plummet from perhaps 30,000,000 to less than 1500. The bison brought the downfall of Plains culture in the end.