An alvar or pavement barren is a biological environment base on a limestone plain with thin or no soil and, as a result, sparse vegetation. In the United Kingdom this landform is called a limestone pavement. This challenging habitat supports a community of rare plants and animals, including species more commonly found on prairie grasslands. Lichen and mosses are common species. Trees and bushes are absent or severely stunted. Alvars can be found in southern Sweden, northwest Estonia and around the Great Lakes in Michigan, New York and Ohio in the United States and in Ontario and Quebec in Canada. Alvars comprise a small percentage of the Earth's ecosystems by land extent; although, some 120 exist in the Great Lakes region, they comprise only 0.2% of the land area there.