Keystone species have a major influence on the structure of their ecological community. The profound influence of keystone species occurs because of their position and activity within the food chain/web . In the sense meant here, a "major influence" means that removal of a keystone species would result in a large change in the abundance, and even the local extirpation, of one or more species in the community. This would fundamentally change the structure of the overall community in terms of species composition, productivity, and other characteristics. Such changes would have substantial effects on all of the species that are present, and could allow new species to invade the community.
The original use of the word "keystone" was in architecture. An architectural keystone is a wedge-shaped stone that is strategically located at the summit of an arch. The keystone serves to lock all other elements of the arch together, and it thereby gives the entire structure mechanical integrity. Keystone species play an analogous role in giving structure to the "architecture" of their ecological community.
The concept of keystone species was first applied to the role of certain predators (i.e., keystone predators) in their community. More recently, however, the term has been extended to refer to other so-called "strong interactors." This has been particularly true of keystone herbivores that have a relatively great influence on the species composition and relative abundance of plants in their community.
Keystone species directly exert their influence on the populations of species that they feed upon, but they also have indirect effects on species lower in the food web. Consider, for example, a hypothetical case of a keystone predator that regulates the population of a herbivore. This effect will also, of course, indirectly influence the abundance of plant species that the herbivore feeds upon. Moreover, by affecting the competitive relationships among the various species of plants in the community, the abundance of plants that the herbivore does not eat will also be indirectly affected by the keystone predator. Although keystone species exert their greatest influence on species with which they are most closely linked through feeding relationships, their influences can ramify throughout the food web.
Ecologists have documented the presence of keystone species in many types of communities. The phenomenon does not, however, appear to be universal, in that keystone species have not been identified in many ecosystems.
Predators as keystone species
The term "keystone species" was originally used by the American ecologist Robert Paine to refer to the critical influence of certain predators. His original usage of the concept was in reference to rocky intertidal communities of western North America, in which the predatory starfish Pisaster ochraceous prevents the mussel Mytilus californianus from monopolizing the available space on rocky habitats and thereby eliminating other, less-competitive herbivores and even seaweeds from the community. By feeding on mussels, which are the dominant competitor among the herbivores in the community, the starfish prevents these shellfish from achieving the dominance that would otherwise be possible. This permits the development of a community that is much richer in species than would occur in the absence of the predatory starfish. Paine demonstrated the keystone role of the starfish by conducting experiments in which the predator was excluded from small areas using cages. When this was done, the mussels quickly became strongly dominant in the community and eliminated virtually all other species of herbivores. Paine also showed that once mussels reached a certain size they were safe from predation by the starfish. This prevented the predator from eliminating the mussel from the community.
Sea otters (Enhydra lutris ) of the west coast of North America are another example of a keystone predator. This species feeds heavily on sea urchins when these invertebrates are available. By greatly reducing the abundance of sea urchins, the sea otters prevent these herbivores from overgrazing kelps and other seaweeds in subtidal habitats. Therefore, when sea otters are abundant, urchins are not, and this allows luxurious kelp "forests" to develop. In the absence of otters, the high urchin populations can keep the kelp populations low, and the habitat then may develop as a rocky "barren ground." Because sea otters were trapped very intensively for their fur during the eighteenth and nineteenth centuries, they were extirpated over much of their natural range. In fact, the species had been considered extinct until the 1930s, when small populations were "discovered" off the coast of California and in the Aleutian Islands of Alaska. Thanks to effective protection from trapping , and deliberate reintroductions to some areas, populations of sea otters have now recovered over much of their original range. This has resulted in a natural depletion of urchin populations, and a widespread increase in the area of kelp forests.
Herbivores as Keystone Species
Some herbivorous animals have also been demonstrated to have a strong influence on the structure and productivity of their ecological community. One such example is the spruce budworm (Choristoneura fumiferana ), a moth that occasionally irrupts in abundance and becomes an important pest of conifer forests in the northeastern United States and eastern Canada. The habitat of spruce budworm is mature forests dominated by balsam fir (Abies balsamea ), white spruce (Picea glauca ), and red spruce (P. rubens ). This native species of moth is always present in at least small populations, but it sometimes reaches very high populations, which are known as irruptions. When budworm populations are high, many species of forest birds and small mammals occur in relatively large populations that subsist by feeding heavily on larvae of the moth. However, during irruptions of budworm most of the fir and spruce foliage is eaten by the abundant larvae, and after this happens for several years many of the trees die. Because of damages caused to mature trees in the forest the budworm epidemic collapses, and then a successional recovery begins. The plant communities of early succession contain many species of plants that are uncommon in mature conifer forests. Eventually, however, another matures, conifer forest redevelops, and the cycle is primed for the occurrence of another irruption of the budworm. Clearly, spruce budworm is a good example of a keystone herbivore, because it has such a great influence on the populations of plant species in its habitat, and also on the many animal species that are predators of the budworm.
Another example of a keystone herbivore concerns snow geese (Chen caerulescens ) in salt marshes of western Hudson Bay. In the absence of grazing by flocks of snow geese this ecosystem would become extensively dominated by several competitively superior species, such as the salt-marsh grass Puccinellia phryganodes and the sedge Carex subspathacea. However, vigorous feeding by the geese creates bare patches of up to several square meters in area, which can then be colonized by other species of plants. The patchy disturbance regime associated with goose grazing results in the development of a relatively complex community, which supports more species of plants than would otherwise be possible. In addition, by manuring the community with their droppings, the geese help to maintain higher rates of plant productivity than might otherwise occur. In recent years, however, large populations of snow goose have caused severe damages to the salt-marsh habitat by over-grazing. This has resulted in the development of salt-marsh "barrens" in some places, which may take years to recover.
Plants as keystone species
Some ecologists have also extended the idea of keystone species to refer to plant species that are extremely influential in their community. For example, sugar maple (Acer saccharum )isa competitively superior species that often strongly dominates stands of forest in eastern North America. Under these conditions most of the community-level productivity is contributed by sugar-maple trees. In addition, most of the seedlings and saplings are of sugar maple. This is because few seedlings of other species of trees are able to tolerate the stressful conditions beneath a closed sugar-maple canopy.
Other ecologists prefer to not use the idea of keystone species to refer to plants that, because of their competitive abilities, are strongly dominant in their community. Instead, these are sometimes referred to as "foundationstone species." This term reflects the facts that strongly dominant plants contribute the great bulk of the biomass and productivity of their community, and that they support almost all herbivores, predators, and detritivores that are present.
[Bill Freedman Ph.D. ]
Begon, M., J. L. Harper, and C. R. Townsend. Ecology. Individuals, Populations and Communities. 3rd ed. London: Blackwell Sci. Pub., 1996.
Krebs, C. J. Ecology. The Experimental Analysis of Distribution and Abundance. San Francisco: Harper and Row, 1985.
Ricklefs, R. E. Ecology. New York: W. H. Freeman and Co., 1990.
Paine, R. T. "Intertidal Community Structure: Experimental Studies of the Relationship Between A Dominant Competitor and Its Principal Predator." Oecologia 15 (1974): 93–120.
All ecosystems on Earth are formed of a delicate balance of species. When an ecosystem is at equilibrium , the relative numbers of organisms within each species remain stable. A food web is a graphical representation of the trophic (food-based) interactions between species. Arrows are drawn between every species and its prey, and the sum of these interconnections forms a complicated tangle of lines. In 1966 Robert Paine, an American ecologist, conceived of the idea that not every interaction in the food web is equally important in maintaining the equilibrium of the ecosystem.
Some species can dramatically increase or decrease in population and have little effect on the gradual return of a static-state ecosystem. This means that if this species propagates or dies off in large numbers, the relative population sizes of other species in the community will be skewed, but the community will eventually return to its original state.
Alternatively, some species have much greater importance to the community. A sizable change in the population of this type of species causes a cascade of direct and indirect effects leading to the collapse of the food web and possible loss of habitat . The few web links that hold such a critical importance are called strong interactions, and the species responsible for this effect are keystone species.
For example, one species of shore crab in the tropical dry forest of Costa Rica feeds primarily on tree saplings. The saplings that are distasteful to crabs grow into mature trees and eventually dominate the landscape. The environment provided by a forest of these trees is relatively open compared to the denser off-coastal forest, and this environment attracts particular animal species that like open forests, such as howler monkeys, coatis, and tapirs.
If the crab colony were to suddenly become extinct, the forest would recover its dense heterogeneous character because the saplings of invasive trees would no longer be cut back. Those animals that depend on the open forest ecosystem would languish, and could undergo local extinction.
Keystone species were considered by Paine to be top predators. He based this definition on the observation of a tidal ecosystem at Mukkaw Bay on the coast of Washington, in which the diet of a particular species of starfish included several secondary predators (when a carnivore feeds on other carnivores, these prey are called secondary predators). In Paine's example, the secondary predators were in direct competition. Paine observed that removing the dominant starfish from an experimental plot increased competition among these secondary predators. Their populations increased because the top predator was no longer killing them off, and thus they were able to kill off more of their own prey, which were lower on the food chain.
As a result, the populations of the remaining species fluctuated rapidly. After two years, the species diversity, defined as the number of species per area of land, had decreased from fifteen to eight. This effect showed how the top predator's existence was keeping the other species in check. By feeding on the competing secondary predators, the keystone starfish had prevented them from devastating populations of species lower down the food chain.
When the starfish was removed, the food web was thrown into chaos. Note that the keystone species is not the dominant species—it does not have the largest number of individuals in the ecosystem. By definition its influence must be far larger than its population can account for.
Since Paine's landmark study, species of other trophic levels have been described as keystone, and many of them are not top predators. Animals such as beavers are considered to be keystone because they engineer the environment. Beavers build dams in rivers and streams that create large bodies of still water. Pond-dwelling animals and plants may then colonize the new environment. If beavers were removed, the environment they created would collapse. The dam would eventually break apart and the entire pond food web would be disrupted.
Another type of keystone species is an exotic, or introduced, species. This is a foreign organism that enters a new habitat and disrupts the existing food web. One example of this is the spread of introduced kudzu in the Atlantic region of the United States. Kudzu is a vine native to desert habitats that is known to decrease erosion of sand dunes. After being imported into the United States in 1876 for ornamental gardens, kudzu was adapted to control erosion during the Great Depression of the 1930s. Kudzu quickly adapted to the plentiful water and rich soil of the southern United States, in the process choking out native shrubs, flowers, and trees. Because trophic interactions with other plants and animals in the area were so greatly affected, kudzu is considered to be keystone.
Alternatively, some consider certain pathogens to have a keystone effect, such as the canine distemper virus's effect on lion populations in Serengeti National Park in Africa. This occurred in 1994, when the domesticated stray dogs living along the park boundaries in Tanzania and Kenya transmitted canine distemper virus to the wildlife. This resulted in thousands of deaths within the lion population. The disease also affects leopards, cheetahs, tigers, raccoons, coyotes, wolves, foxes, ferrets, skunks, weasels, mink, badgers, hyenas, and jackals.
Paine's narrow definition has since been broadened to include mutualists (animals that provide benefits for and receive benefits from another species), pathogens (disease-causing microorganisms), parasites, and many more feeding strategies than merely top predator. Also in the 1960s, researchers defined "functional group," a collection of many species that collectively perform the role of a keystone species.
Human beings cannot be called a keystone species because our influence on nature is not disproportionately large compared to our abundance (population size). Our cities, roads, and technologies, however, have altered nearly every ecosystem on Earth. Because species diversity is a strong signifier for a healthy habitat, it is important for humans to understand how to preserve the greatest species diversity within remaining natural habitats. This requires knowledge of how to stabilize a high species diversity in parklands and nature preserves.
The largest grouping of endangered species in the United States consists of primary predators, such as large predatory cats, bears, and eagles. If, as Paine suggested, many of these top predators are keystone, then their extinction will create turmoil in the relative numbers of remaining species. The overabundance of deer and raccoons in urban areas of the United States reflects this destructive trend.
see also Ecosystem; Interspecies Interactions.
Rebecca M. Steinberg
Diamond, Jared, and Ted J. Case, eds. Community Ecology. New York: Harper and Row, 1986.
Nabhan, Gary Paul, and John L. Carr. Ironwood: An Ecological and Cultural Keystone of the Sonoran Desert. Washington, D.C.: Conservation International, 1994.
Power, Mary E. et al. "Challenges in the Quest for Keystones." Bioscience 46 (1996):610-620.
Roughgarden, Jonathan. Primer of Ecological Theory. Upper Saddle River, NJ: Prentice Hall, 1998.
A keystone species is a particular species that has a great influence on the structure or functioning of its ecological community. This influence is far 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 species that have a similarly critical influence on the functioning or structure of their community.
The importance of particular keystone species can often be deduced by careful examination of their interactions with other species or by measuring their functional attributes, especially those important in nutrient cycles. The role of keystone species can also be demonstrated by conducting experiments in which these organisms are removed from their community, and the resulting ecological changes are monitored.
Keystone predators and herbivores
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.
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 species. Good examples of these sorts of keystone species are those that play unique roles in the nitrogen cycle , in particular in the ecologically important process known as nitrification .
Nitrification is a process during which highly specialized bacteria oxidize the positively charged ion ammonium (NH4+) to the negatively charged ion nitrite (NO2-) and then to nitrate (NO3-). Nitrification is a very important component of the larger nitrogen cycle because most plants prefer to utilize nitrate as the chemical form by which nitrogen, an essential nutrient, is absorbed from soil or water . Because this preference for nitrate is true of most agricultural species of plants, nitrification is also an ecological function that is important for human welfare.
Nitrification occurs in two discrete steps. The first stage is the oxidation of ammonium to nitrite, a process that is only carried out by specialized bacteria in the genus Nitrosomonas. The nitrite formed is then quickly oxidized to nitrate by other specialized bacteria in the genus Nitrobacter. Neither Nitrosomonas or Nitrobacter are abundant in soils or water. These microorganisms can, however, be viewed as keystone organisms because nitrification is such an important aspect of the nitrogen cycle in ecosystems, and it is only carried out by these bacteria.
Krebs, C.J. Ecology. The Experimental Analysis of Distribution and Abundance. Harper and Row, New York: 1985.
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—In ecology, a community is an assemblage of populations of different species that occur together in the same place and at the same time.
—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 functioning of its ecosystem, or that has a disproportionate influence on the structure of its ecosystem.
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.
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
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.
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.
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.