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A keystone species is a species that has a disproportionate effect on its environment relative to its abundance. Such species affect many other organisms in an ecosystem and help to determine the types and numbers of various others species in a community.

Such an organism plays a role in its ecosystem that is analogous to the role of a keystone in an arch. While the keystone feels the least pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity. It has become a very popular concept in conservation biology.

A keystone species is a species that plays a critical role in maintaining the structure of an ecological community and whose impact on the community is greater than would be expected based on its relative abundance or total biomass. The ecologist Dr. Robert T. Paine coined the phrase to explain the relationship between Pisaster ochraceus, a species of starfish, and Mytilus californianus, a species of mussel.

Examples

There are many historical definitions of the keystone species concept since it was coined by Robert Paine, as reviewed here http://www.ecologyandsociety.org/vol7/iss1/resp11/.
Without a consensus on its exact definition, we are left to illustrate the concept of keystone species with a list of examples.

A classic keystone species is a small predator that prevents a particular herbivorous species from eliminating dominant plant species. Since the prey numbers are low, the keystone predator numbers can be even lower and still be effective. Yet without the predators, the herbivorous prey would explode in numbers, wipe out the dominant plants, and dramatically alter the character of the ecosystem. The exact scenario changes in each example, but the central idea remains that through a chain of interactions, a non-abundant species has an out-sized impact on ecosystem functions. One example is the weevil and its suggested keystone effects on aquatic plant species diversity by prey activities on nuisance Eurasian Watermilfoil.

Predators

Some sea stars may perform this function by preying on sea urchins, mussels, and other shellfish that have no other natural predators. If the sea star is removed from the ecosystem, the mussel population explodes uncontrollably, driving out most other species, while the urchin population annihilates coral reefs. In his classic 1966 paper, Dr. Robert Paine described such a system in Mukkaw Bay in Washington State.This led to his 1969 paper where he proposed the keystone species concept.

Similarly, sea otters in kelp forests keep sea urchins in check. Kelp "roots", called holdfasts, are merely anchors, and not the vast nutrient gathering networks of land plants. Thus the urchins only need to eat the roots of the kelp, a tiny fraction of the plant's biomass, to remove it from the ecosystem. Wolves are also an example of a keystone species.

These creatures need not be apex predators. Sea stars are prey for sharks, rays, and sea anemones. Sea otters are prey for orca.

In the case of the jaguar, whose numbers in Central and South America have been classified as Near Threatened, it acts as a keystone predator by its widely varied diet, helping to balance the mammalian jungle ecosystem with its consumption of 87 different species of prey.

Mutualists

Keystone mutualists are organisms that participate in mutually beneficial interactions, the loss of which would have a profound impact upon the ecosystem as a whole. For example, in the Avon Wheatbelt region of Western Australiamarker, there is a period of each year when Banksia prionotes (Acorn Banksia) is the sole source of nectar for honeyeaters, which play an important role in pollination of numerous plant species. Therefore the loss of this one species of tree would probably cause the honeyeater population to collapse, with profound implications for the entire ecosystem.

Engineers

In North America, the grizzly bear is a keystone species—not as a predator but as ecosystem engineers. They transfer nutrients from the oceanic ecosystem to the forest ecosystem. The first stage of the transfer is performed by salmon, rich in nitrogen, sulfur, carbon, and phosphorus, who swim up rivers, sometimes for hundreds of miles. The bears then capture the salmon and carry them onto dry land, dispersing nutrient-rich feces and partially-eaten carcasses. It has been estimated that the bears leave up to half of the salmon they harvest on the forest floor.

The prairie dog is also an ecosystem engineer. Prairie dog burrows provide the nesting areas for Mountain Plover and Burrowing Owl. Prairie dog tunnel systems also help channel rainwater into the water table to prevent runoff and erosion, and can also serve to change the composition of the soil in a region by increasing aeration and reversing soil compaction that can be a result of cattle grazing. Prairie dogs also trim the vegetation around their colonies, perhaps to remove any cover for predators. Even grazing species such as Plains bison, pronghorn, and Mule deer have shown a proclivity for grazing on the same land used by prairie dogs. It is believed that they prefer the vegetative conditions after prairie dogs have foraged through the area.

Another ecosystem engineering keystone species is the beaver, which transforms its territory from a stream to a pond or swamp.

In the African savanna, the larger herbivores, especially the elephants, shape their environment. The elephants destroy trees, making room for the grass species. Without these animals, much of the savanna would turn into woodland.

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