And through unsustainable ways of life depleted all of the resources in the surrounding areas like an algal bloom and had to abandon said settlements. The best studied of large native settlements, Cahokia, failed due to deforestation of the surrounding lands. This caused massive erosion issues to arise, denied them of wood for fuel and building purposes, and erased the habitat of game animals needed to supplement their diets. It always kills me when natives are held up as paragons of living in harmony with nature. The only thing that kept them from even less responsible land usage than that of the white man was the lack of technology with which to do so. 