When we acquired the ability to hold animals on land that could no longer meet their needs for feed and water, we acquired the ability to destroy our grasslands.
Prior to human intervention, when growing conditions deteriorated on an area, the grazing animals either left or died; they could not remain to pick at the struggling vegetation and permanently degrade the weakened local environment. Grazing lands are composed of four components, all essential: animals, vegetation, soil, and water. When drought, or other disaster, strikes permanent (life ending) damage occurs first to the grazing animals. They can exist for only short periods without feed and water. Vegetation has some ability to go dormant (or retreat into seeds) and can thus survive poor conditions for longer periods of time. The soil will be damaged (mainly loss of soil life) but if it is not physically washed or blown away, can survive periods of poor conditions.