Keystone Species

The first use of the keystone-species metaphor in ecological literature was in reference to certain temperate intertidal communities on the west coast of North America. In this ecosystem, experimental removal of a predacious starfish (Pisaster ochraceous) was found to result in a rapid increase in the growth and biomass of a filter-feeding mussel (Mytilus californianus), which then managed to crowd out other species and strongly dominate the community. In this case, the starfish was described as a keystone predator that prevented the mussel from achieving the full degree of community dominance that it was capable of developing on the basis of its competitive superiority over other species. Interestingly, the starfish could not eliminate the mussel from the community because it was not able to predate upon the largest mussels. Therefore, predation on mussels by the starfish allowed other species to occur in the intertidal zone, so the community could maintain a greater richness of species and was more complex in structure because the development of a monoculture of mussels was prevented.

Another case of a predator having a crucial influence on the structure of its community involves the sea otter (Enhydra lutris) of western North America. These marine mammals mostly feed on sea urchins, which are herbivores of the large algae known as kelps. By keeping urchin densities relatively small, the seaotters allow the kelps to maintain a large biomass, and the community develops into a so-called "kelp forest." In the absence of the seaotters, the urchins are capable of developing populations large enough to overgraze the kelps. The ecosystem would then maintain a much smaller biomass and productivity of these seaweeds and would become much more open in structure. This capability of the urchins has been demonstrated in experiments in which these herbivores were removed by ecologists with the result that kelps flourished. The role of the otters was demonstrated indirectly through the ecological changes associated with the widespread extirpation of these animals from almost all of their range as a result of overharvesting for their rich, lustrous fur during the eighteenth and nineteenth centuries. With the otters gone, the kelp forests declined badly in many places. Fortunately, seaotters have been colonizing many of their former habitats since about the 1930s, and this recovery has led to a return of the kelp forests in many of those places.

Another example of a keystone species is the African elephant (Loxodonta africana), an herbivorous species that eats a wide range of herbaceous and woody plants. During its feeding on the foliage of shrubs and trees, elephants commonly knock these woody plants over, which often kills the plants. By feeding in a manner that is destructive to shrubs and trees, elephants shift the balance of the savanna ecosystem toward a greater dominance of herbaceous species. This keeps the habitat in a relatively open condition. This ecological change is not, however, necessarily to the benefit of the elephants because they require a mixture of herbaceous and woody plants for a balanced nutrition.

The beaver (Castor canadensis) also has an enormous influence on the structure of its habitat. Beavers create extensive wetlands by damming streams, causing them to flood low-lying areas. By doing so, beavers create fertile open-water wetlands for their own use as well as for many other species that otherwise might not be able to utilize the local habitats.