Keystone Species
Keystone Species
Keystone predators and herbivores
Keystone species in nutrient cycling
A keystone species is a species that has a great influence on the structure or function of an ecological community. This influence is out of proportion to the relative biomass or productivity of the keystone species within its community. Most keystone species are top predators, although a few are influential because they play a critical role as herbivores or in nutrient cycling.
In engineering, the keystone is a wedge-shaped stone that occurs at the top of a self-supporting stone arch or dome. The keystone is extremely important in the physical integrity of the structure, and if it is removed, the arch or dome will collapse. The keystone metaphor in ecology is used to refer to a species that has a similarly critical influence on the function or structure of a community. A keystone species is often identified when the species is from a community and the resulting changes lead to ecological collapse. Such a removal usually occurs incidentally as a result of human action. Ecologists also develop ecological models of community interactions to test the roles of keystone species within different ecosystems.
Keystone predators and herbivores
The first use of the keystone-species metaphor in ecological literature was in reference to temperate intertidal communities on the west coast of North America. In this ecosystem, experimental removal of a predacious sea star (Pisaster ochraceous ) resulted in a rapid increase in the growth of a filter-feeding mussel (Mytilus californianus ), which then crowded out other species dominating the community. In this case, the sea star was a keystone predator controlling the mussel population. Interestingly, the sea star could not entirely eliminate the mussel from the community because it was not able to prey upon the largest mussels. However, predation on mussels by the sea star allowed other intertidal species to grow and the community maintained great species richness and
KEY TERMS
Community —In ecology, a community is an assemblage of populations of different species that occur together in the same place and at the same time.
Competition —An interaction between organisms of the same or different species associated with their need for a shared resource that is present in a supply that is smaller than the potential, biological demand.
Keystone species —A species that plays a crucial role in the function of an ecosystem, or that has a disproportionate influence on the structure of its ecosystem.
complexity. In the absence of the sea star a monoculture of mussels resulted.
Another case example of a keystone species is the sea otter (Enhydra lutris ) of western North America. These marine mammals mostly feed on sea urchins. The sea urchins, in turn, consume giant kelp, which grows along the Pacific coast. By keeping urchin densities relatively low, the sea otters kelps the kelp develops into dense forests that serve has habitat for diverse species of marine plants and animals. In the absence of the sea otters, the urchin population grows large enough to overgraze the kelp. The absence of habitat for the many organisms that live in the kelp forest decreases the biomass, productivity and diversity of the entire ecosystem. The effect of the urchins on the kelp forest has been demonstrated in experiments in which the urchins were removed by ecologists with the result that kelps flourished. The role of the otters as keystone species was demonstrated indirectly through the ecological changes associated with the widespread extirpation of these animals from a significant portion of their range as a result of overharvesting for their rich, lustrous fur during the eighteenth and nineteenth centuries. With the removal of the otters, the kelp forests declined severely in many places along the Pacific coast. Fortunately, sea otters have recolonized 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 ), a herbivorous species that eats a wide range of herbaceous and woody plants. As it feeds, an elephants commonly knocks shrubs and woody plants over, often killing them. This allows for the growth of herbaceous plant species and increases species diversity. This behavior also benefits the elephants because they require a mixture of herbaceous and woody plants for a balanced diet.
Keystone species in nutrient cycling
Some keystone species are important because they play a crucial role in nutrient cycling, particularly if that function cannot be carried out by other members of a community. Nitrification is a process in which the positively charged ion ammonium (NH4<->+) is oxidized to the negatively charged ion nitrite (NO2<->-). This process is only carried out by specialized bacteria in the genus Nitrosomonas. The nitrite is then quickly oxidized then to nitrate (NO3<->-) by other specialized bacteria in the genus Nitrobacter. Nitrification is a very important component of the larger nitrogen cycle because nitrate is the chemical form of nitrogen that nearly all plants require. Both Nitrosomonas and Nitrobacter are keystone species because nitrification is such an important aspect of the nitrogen cycle in ecosystems.
Resources
BOOKS
MacKay, Richard. The Penguin Atlas of Endangered Species: A Worldwide Guide to Plants and Animals. New York: The Penguin Group, 2002.
Sax, Dov F., John J. Stachowicz, Steven D. Gaines eds. Species Invasions: Insights into Ecology, Evolution, and Biogeography. Sunderland, MA: Sinaur Associates Inc., 2005.
Bill Freedman