A keystone species is a species that has a disproportionately large influence on its habitat relative to its population size, thus greatly affecting the habitat's general form and composition. Many ecosystems would be dramatically altered with the extinction of their keystone species.
An example is elephants, which uproot and knock down trees and consume their leaves, fruit, branches and bark. This grazing activity allows savannas to remain grasslands rather than to become woodlands or forests. Savannas provide habitat for numerous species, including predators such as lions and hyenas, grazing species such as antelopes, zebras, and wildebeests , and small mammals that dig into the dry, warm soil, such as mice and shrews.
Another example is sea otters, which consume sea urchins and thereby prevent the degradation of kelp forests by the sea urchins. Kelp forests are home to a greater variety and higher diversity of organisms than almost any other ocean ecosystem. Many organisms use the thick kelp blades as a safe shelter for their offspring from predators and from rough storms. Among the numerous mammals and birds that use kelp forests for feeding or protection are whales, seals, sea lions, sea otters, terns, gulls, great blue herons, snowy egrets, and cormorants.