is the spread of desert-like conditions in arid and semi-arid areas, due to human influence and/or climatic change. Some of the natural causes of are wind erosion, climatic conditions, and scarce water supply. There is a grave difference between areas where vegetation has been retained and surrounding vegetation. For example, "Nefta in southern Tunisia, the coverage of vegetation inside an area fenced 60 years ago is 85%, in contrast to 5% outside the area." Approximately one-third of the earth's land surface is semi-arid or arid. This is th ....