The Sandhills is a region of mixed-grass prairie on grass-stabilized sand dunes in north-central Nebraska, covering just over one quarter of the state. The dunes were designated a National Natural Landmark in 1984. The World Wide Fund for Nature (WWF) designated the Sandhills as an ecoregion, distinct from other grasslands of the Great Plains. As much as 85% of the ecoregion is intact natural habitat, the highest level in the Great Plains. The Sandhills are home to 314 vertebrate species, 720 different species of plants and are part of the Central Flyway for many species of migratory birds, including sandhill cranes.