Keystone Species

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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.

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.

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.

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.

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.

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