Pastoralism is the branch of agriculture concerned with the raising of livestock. It is animal husbandry: the care, tending and use of animals such as camels, goats, cattle, yaks, llamas, and sheep. “Pastoralism” generally has a mobile aspect, moving the herds in search of fresh pasture and water (in contrast to pastoral farming, in which non-nomadic farmers grow crops and improve pastures for their livestock). Pastoralism is a successful strategy to support a population on less productive land, and adapts well to the environment. For example, in savannas, pastoralists and their animals gather when rain water is abundant and the pasture is rich, then scatter during the drying of the savanna. Pastoralists often use their herds to affect their environment. Grazing herds on savannas can ensure the biodiversity of the savannas and prevent them from evolving into scrubland. Pastoralists may also use fire to make ecosystems more suitable for their food animals. For instance, the Turkana people of northwest Kenya use fire to prevent the invasion of the savanna by woody plant species. Biomass of the domesticated and wild animals was increased by a higher quality of grass. Pastoralism is found in many variations throughout the world. Composition of herds, management practices, social organization and all other aspects of pastoralism vary between areas and between social groups. Many traditional practices have also had to adapt to the changing circumstance of the modern world, including climatic conditions affecting the availability of grasses. Ranches of the United States and sheep stations and cattle stations of Australia are seen by some as modern variations.