Eastern South Dakota is home to most of the state's population, and the area's fertile soil is used to grow a variety of crops. West of the Missouri River, ranching is the predominant agricultural activity, and the economy is more dependent on tourism and defense spending. Most of the Native American reservations are in West River. The Black Hills, a group of low pine-covered mountains sacred to the Sioux, are in the southwest part of the state. Mount Rushmore, a major tourist destination, is there. South Dakota has a temperate continental climate, with four distinct seasons and precipitation ranging from moderate in the east to semi-arid in the west. The state's ecology features species typical of a North American grassland biome.