Nomadic pastoralism is a form of pastoralism in which livestock are herded in order to seek for fresh pastures on which to graze. 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—and in historical cases the regularity of movements is often unknown in any case. The herded livestock include cattle, water buffalo, yaks, llamas, 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 North and West Africa, such as Fulani, Tuaregs, and Toubou, with some also in the Middle East, such as traditionally Bedouins, and in other parts of Africa, such as Nigeria and Somalia. 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 for this practice. There is substantive uncertainty over the extent to which the various causes for degradation affect grassland. Different causes have been identified which include overgrazing, mining, agricultural reclamation, pests and rodents, soil properties, tectonic activity, and climate change. Simultaneously, it is maintained that some, such as overgrazing and overstocking, may be overstated while others, such as climate change, mining and agricultural reclamation, may be under reported. In this context, there is also uncertainty as to the long-term effect of human behavior on the grassland as compared to non-biotic factors. Origin and history Nomadic pastoralism was a result of the Neolithic revolution and the rise of agriculture. During that revolution, humans began domesticating animals and plants for food and started forming cities. Nomadism generally has existed in symbiosis with such settled cultures trading animal products (meat, hides, wool, cheese and other animal products) for manufactured items not produced by the nomadic herders. Henri Fleisch tentatively suggested the Shepherd Neolithic industry of Lebanon may date to the Epipaleolithic and that it may have been used by one of the first cultures of nomadic shepherds in the Beqaa valley. Andrew Sherratt demonstrates that "early farming