The main factor is when govt banned hunting people used to select animals and perform the task of domestication of animals for their survival .

Nomadic pastoralism is a form of pastoralismwhen 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.