Moorland or moor is a type of habitat found in upland areas in temperate grasslands, savannas, and shrublands and montane grasslands and shrublands biomes, characterised by low-growing vegetation on acidic soils. Moorland, nowadays, generally means uncultivated hill land, but also includes low-lying wetlands. It is closely related to heath, although experts disagree on what precisely distinguishes these types of vegetation. Generally, moor refers to highland and high rainfall zones, whereas heath refers to lowland zones which are more likely to be the result of human activity. Moorland habitats mostly occur in tropical Africa, northern and western Europe, and neotropical South America. Most of the world's moorlands are diverse ecosystems. In the extensive moorlands of the tropics, biodiversity can be extremely high. Moorland also bears a relationship to tundra, appearing as the tundra and the natural tree zone. The boundary between tundra and moorland constantly shifts with climatic change.