Nomadic pastoralism is a form of pastoralism where livestock are herded in order to find fresh pastures on which to graze following an irregular pattern of movement. This is in contrast with transhumance where seasonal pastures are fixed.  The herded livestock include caattle, yaks, sheep, goats, reindeer, horses, donkeys or  camels, or mixtures of species. Nomadic pastoralism is commonly practised in regions with little arable land, typically in the developing world. Of the estimated 30–40 million nomadic pastoralists worldwide, most are found in central Asia and the Sahel region of West Africa. Increasing numbers of stock may lead to overgrazing of the area and desertification if lands are not allowed to fully recover between one grazing period and the next. Increased enclosure and fencing of land has reduced the amount of land available for this practice.