Nomadic pastoralism is a form of pastoralism when livestock are herded in order to find fresh pastures on which to graze. Strictly speaking, true nomads follow an irregular pattern of movement, in contrast with transhumance where seasonal pastures are fixed. However this distinction is often not observed and the term nomad used for both—in historical cases the regularity of movements is often unknown in any case. The herded livestock include cattle, 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, especially in the steppe lands north of the agricultural zone of Eurasia. 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.