<P> 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 . 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 . </P>
why do nomadic herders move their livestock from place to place
to find fresh pastures on which to graze