Keystone Species
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 havethe 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 quicklyadapted 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.
Ecosystem; Interspecies Interactions.
Bibliography
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.
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