A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Such species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community.
One problem with the concept of keystone species is that it is difficult to measure all the direct and indirect impact of a keystone species on other organisms in a community. Consequently, most evidence for the existence of keystone species is based on indirect observations rather than on experimental manipulations.
In a classic experiment, Robert Paine of the University of Washington demonstrated the disproportionate influence of ochre sea stars on species richness by removing them from selected sites repeatedly over a 5-year period. Two major changes occurred in the areas where sea stars were absent. First, the lower edge of the mussel bed extended farther down into the intertidal zone, showing that sea stars are able to eliminate mussels completely in areas that are submerged most of the time. Second, and more dramatically, 28 species of animals and algae disappeared from the sea star removal sites.
Various shark species help to keep the population of some species in check, while supporting the populations of other species. in 2007, scientists reported that the decline in certain shark populations along the U.S. Atlantic coast led to an explosion in populations of rays and skates, which sharks normally feed on. Now the rays and skates are feasting on bay scallops, resulting in a sharp decline in their population and in the area's bay scallop fishing business.
The effects that keystone species have in their ecosystems is often much larger than their numbers would suggest. Because of their relatively limited numbers, some keystone species are more vulnerable to extinction than others are, and eliminating a keystone species can dramatically alter the structure and function of a community.
Herbivores can also be keystone species. In African savannas such as the Serengeti plains in Tanzania, elephants are a keystone species. Elephants eat small trees, such as acacia, that grow on the savanna. Even if an acacia tree grows to a height of several feet, elephants are able to knock over the tree and uproot it. This feeding behavior keeps the savanna a grassland and not a forest or woodland. With elephants to control the tree population, grasses thrive and sustain grazing animals such as antelopes, wildebeests, and zebras.
A keystone species' disappearance would start a domino effect. Other species in the habitat would also disappear and become extinct. The keystone species' disappearance could affect other species that rely on it for survival. For example, the population of deer or rabbits would explode without the presence of a predator. The ecosystem cannot support an unlimited number of animals, and the deer soon compete with each other for food and water resources. Their population usually declines without a predator such as a mountain lion.
A keystone species is often, but not always, a predator. A few predators can control the distribution and population of large numbers of prey species. A single mountain lion near the Mackenzie Mountains in Canada, for example, can roam an area of hundreds of kilometers. The deer, rabbits, and bird species in the ecosystem are at least partly controlled by the presence of the mountain lion. Their feeding behavior, or where they choose to make their nests and burrows, are largely a reaction to the mountain lion's activity. Scavenger species, such as vultures, are also controlled by the activity of the mountain lion.
Paine (1969) first used the term “keystone species" for particularly strong interactors: species whose activities maintain species and habitat diversity and whose effects are disproportionate to their abundance. Where the density of a keystone species falls below some threshold, the species diversity in the area may decrease, triggering ecological chain reactions, ending with degraded or simplified ecosystems. Among such are large predators.Keystone species can be thought of as having the highest per capita interaction strengths.
Early work demonstrated that sea otters, by limiting populations of herbivorous invertebrates such as sea urchins and large gastropods, played a keystone role in preventing kelp forests from being overgrazed. Subsequent research has focused on the broader influences of this otter-herbivore-kelp trophic cascade, including both the indirect effects on other coastal species and ecosystem processes, and the evolutionary consequences over longer periods of time.
A keystone species is one that's vital to a particular habitat, and if it goes extinct, other species will have a tough time adapting to the loss of this crucial member of their ecosystem. A keystone species can hold a vitally important role in habitat creation, seed dispersal, soil aeration and nutrient replenishment.