Arid lands are dry areas or deserts where a shortage of rainfall prevents permanent rain-fed agriculture. They are bordered by, and interact with, marginal or semi-arid lands where the annual rainfall (still only 10–15 in; 25–38 cm) allows limited agriculture and light grazing. However, in many parts of the world human mismanagement, driven by increasing populations, has degraded these areas into unusable arid lands. For example, clearing natural vegetation and overgrazing have led to soil erosion, reduced land productivity, and ultimately reduced water availability in the region. This degradation of semi-arid lands to deserts, which can also occur naturally, is known as desertification.