The Nearctic realm is one of the eight biogeographic realms constituting the Earth's land surface. The Nearctic realm covers most of North America, including Greenland, Central Florida, and the highlands of Mexico. The parts of North America that are not in the Nearctic realm include most of coastal Mexico, southern Mexico, southern Florida, coastal central Florida, Central America, and the Caribbean islands. Together with South America, these regions are part of the Neotropical realm. Major ecological regions The World Wildlife Fund (WWF) divides the Nearctic into four bioregions, defined as "geographic clusters of ecoregions that may span several habitat types, but have strong biogeographic affinities, particularly at taxonomic levels higher than the species level (genus, family)." Canadian Shield The Canadian Shield bioregion extends across the northern portion of the continent, from the Aleutian Islands to Newfoundland. It includes the Nearctic's arctic tundra and boreal forest ecoregions. In terms of floristic provinces, it is represented by part of the Canadian Province of the Circumboreal Region. Eastern North America The Eastern North America bioregion includes the temperate broadleaf and mixed forests of the Eastern United States and southeastern Canada, the Great Plains temperate grasslands of the central United States and south-central Canada, the temperate coniferous forests of the southeastern United States, including central Florida. In terms of floristic provinces, it is represented by the North American Atlantic Region and part of the Canadian Province of the Circumboreal Region. Western North America The Western North America bioregion includes the temperate coniferous forests of the coastal and mountain regions of southern Alaska, western Canada, and the western United States from the Pacific Coast and Northern California to the Rocky Mountains (known as the Cascadian bioregion), as well as the cold-winter intermountain deserts and xeric shrublands and temperate grasslands and shrublands of the Western United States. In terms of floristic provinces, it is represented by the Rocky Mountain region. Northern Mexico and Southwestern North America The Northern Mexico and Southwestern North America bioregion includes the mild-winter to cold-winter deserts and xeric shrublands, warm temperate and subtropical pine and pine-oak forests, and Mediterranean climate ecoregions of the Mexican Plateau, Baja California peninsula, and the southwestern United States, bordered to the south by the Neotropical Trans-Mexican Volcanic Belt. This region also includes the only subtropical dry broadleaf forest in the Nearctic realm, the Sonoran–Sinaloan transition subtropical dry forest. In terms of floristic provinces, it is represented by