Environmentally, the horse-centered way of life was highly unstable. Near their encampments the Indians concentrated horses in numbers greater than the local grass could bear. The strain was greatest in the winter, when the people were least mobile and the grass was less nutritious, but the horses needed more calories to stay warm. Consequently, the horse herds depleted the most fragile, scarce, and important niches on the Great Plains: the river and stream valleys that provided the winter refuges.
