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Desertification is the diminution or destruction of the biological potential of land and can ultimately lead to desert-like conditions. It is an aspect of the widespread deterioration of ecosystems under the combined pressure of adverse and fluctuating climate and excessive exploitation. Such pressure has diminished or destroyed the biological potential for multiple purposes at a time when increased productivity is needed to support growing populations in the quest for development. Desertification produces many changes in the ecosystems of a region and is recognized primarily by the physical changes in the environment, including the reduction of plant cover, soil loss, loss of soil organic matter, deposition of sand bodies, and increased run-off. In extreme cases, desertification can result in marked reduction in vegetative cover and loss of soil by water and wind erosion. Not all desertified regions suffer a loss of primary productivity. Human activities that alter the ocean–atmosphere interactions, that change atmospheric and stratospheric chemistry, and that alter land surface–atmosphere interactions directly affect regional climates around the world. One of the major biological challenges of the 21st century is the resolution of the relationship between the diversity of organisms in ecosystems and the long-term behavior of those ecosystems. There are four prevalent hypotheses concerning this relationship: the diversity–stability hypothesis, the rivet hypothesis, the redundancy hypothesis, and the idiosyncratic response hypothesis.