The Serengeti is semi-desert grassland that predominantly occurs in the arid and semi-arid zones of eastern Africa and has been grazed by wildlife and livestock for millennia. The area is characterized by rich volcanic soils called vertisols and a variety of palatable grass species belonging to two types of plains: long grass and short grass. A multitude of parks and conservation areas exist in this region including Serengeti National Park in Tanzania, the Ngorongoro Conservation Area, Maswa Game Reserve, the Loliondo, Grumeti and Ikorongo Controlled Areas and the Maasai Mara National Reserve in Kenya. The Serengeti plains comprise an area of approximately 17,871 square kilometers (6,900 square miles) and are renowned for having one of the largest mass game migrations in the world. The United Nations Educational, Scientific and Cultural Organization (UNESCO) have listed the Serengeti National Park as a World Heritage Site. Large efforts have been put forth to preserve the integrity of the region, but illegal hunting of wild game continues to be a threat. Recurrent droughts are also a risk for local pastoralists who have historically managed drought through living nomadic lifestyles.