The massive herds of bison had been an ecological anomaly for millennia because it is unusual for one animal so thoroughly to dominate an area as large as the Great Plains. Bison had been a kind of weed species, its numbers and range expanding and contracting along with climatic cycles. Only with the introduction of the horse in the seventeenth century had the nomadic Indian cultures of the Great Plains begun to evolve into their classic form and more fully exploit the herds.
