Keystone species
A keystone species is a species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 that has a disproportionately large effect on its environment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

 relative to its abundance. Such species play a critical role in maintaining the structure of an ecological community, affecting many other organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s in an ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

 and helping to determine the types and numbers of various other species in the community.

The role that a keystone species plays in its ecosystem is analogous to the role of a keystone
Keystone (architecture)
A keystone is the wedge-shaped stone piece at the apex of a masonry vault or arch, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight. This makes a keystone very important structurally...

 in an arch
An arch is a structure that spans a space and supports a load. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-Technical aspects:The...

. While the keystone is under 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
Biomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...

 or productivity
Productivity (ecology)
In ecology, productivity or production refers to the rate of generation of biomass in an ecosystem. It is usually expressed in units of mass per unit surface per unit time, for instance grams per square metre per day. The mass unit may relate to dry matter or to the mass of carbon generated...

It has become a very popular concept in conservation biology
Conservation biology
Conservation biology is the scientific study of the nature and status of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction...



The keystone species concept was coined, in 1969, by the zoologist Robert T. Paine
Robert T. Paine (zoologist)
Robert T. Paine, PhD, is a zoologist and professor emeritus of the University of Washington,who coined the keystone species concept in order to explain the relationship between Pisaster ochraceus, a species of starfish, and Mytilus californianus, a species of mussel.In his classic 1966 paper, Dr...

, professor emeritus of the University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...

, to explain the relationship between Pisaster ochraceus, a species of starfish, and Mytilus californianus, a species of mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...

In his classic 1966 paper, Dr. Robert Paine described such a system in Makah Bay
Neah Bay, Washington
Neah Bay is a census-designated place on the Makah Indian reservation in Clallam County, Washington, United States. The population was 794 at the 2000 census.-Geography:Neah Bay is located at ....

 in Washington State.
This led to his 1969 paper where he proposed the keystone species concept.


Given that there are many historical definitions of the keystone species concept, and without a consensus on its exact definition, a list of examples best illustrates the concept of keystone species.

A classic keystone species is a small predator that prevents a particular herbivorous species from eliminating dominant plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

 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
A weevil is any beetle from the Curculionoidea superfamily. They are usually small, less than , and herbivorous. There are over 60,000 species in several families, mostly in the family Curculionidae...

 and its suggested keystone effects on aquatic plant species diversity by prey activities on nuisance Eurasian Watermilfoil
Myriophyllum spicatum
Myriophyllum spicatum is a species of Myriophyllum native to Europe, Asia, and north Africa. It is a submerged aquatic plant, and grows in still or slow-moving water.-Description:...



As was described by Dr. Robert Paine in his classic 1966 paper, some sea stars may prey on sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

s, mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...

s, and other shellfish
Shellfish is a culinary and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some kinds are found only in freshwater...

 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.

Similarly, sea otters protect kelp forest
Kelp forest
Kelp forests are underwater areas with a high density of kelp. They are recognized as one of the most productive and dynamic ecosystems on Earth. Smaller areas of anchored kelp are called kelp beds....

s from damage by sea urchins. Kelp "roots", called holdfasts, are merely anchors, and not the vast nutrient gathering networks of land plants. Thus the sea urchins only need to eat the roots of the kelp, a tiny fraction of the plant's biomass, to remove it from the ecosystem.
These creatures need not be apex predator
Apex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...

s. Sea stars are prey for sharks, rays, and sea anemones. Sea otters are prey for orca
The killer whale , commonly referred to as the orca, and less commonly as the blackfish, is a toothed whale belonging to the oceanic dolphin family. Killer whales are found in all oceans, from the frigid Arctic and Antarctic regions to tropical seas...


In the case of the jaguar
The jaguar is a big cat, a feline in the Panthera genus, and is the only Panthera species found in the Americas. The jaguar is the third-largest feline after the tiger and the lion, and the largest in the Western Hemisphere. The jaguar's present range extends from Southern United States and Mexico...

, whose numbers in Central and South America have been classified as Near Threatened
Near Threatened
Near Threatened is a conservation status assigned to species or lower taxa that may be considered threatened with extinction in the near future, although it does not currently qualify for the threatened status...

, 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.


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
Avon Wheatbelt
Avon Wheatbelt is an Interim Biogeographic Regionalisation for Australia region in Western Australia and part of the larger Southwest Australia savanna ecoregion.-Further reading:...

 region of Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...

, there is a period of each year when Banksia prionotes
Banksia prionotes
Banksia prionotes, commonly known as Acorn Banksia or Orange Banksia, is a species of shrub or tree of the genus Banksia in the family Proteaceae. It is native to the southwest of Western Australia and can reach up to 10 m in height. It can be much smaller in more exposed areas or in the...

(Acorn Banksia) is the sole source of nectar for honeyeaters, which play an important role in pollination
Pollination is the process by which pollen is transferred in plants, thereby enabling fertilisation and sexual reproduction. Pollen grains transport the male gametes to where the female gamete are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself...

 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. Another example is frugivores such as the cassowary
The cassowaries are ratites, very large flightless birds in the genus Casuarius native to the tropical forests of New Guinea, nearby islands and northeastern Australia. There are three extant species recognized today...

, which spreads the seeds of many different trees, and some will not grow unless they have been through a cassowary.


In North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, the grizzly bear
Grizzly Bear
The grizzly bear , also known as the silvertip bear, the grizzly, or the North American brown bear, is a subspecies of brown bear that generally lives in the uplands of western North America...

 is a keystone species—not as a predator but as ecosystem engineer
Ecosystem engineer
An ecosystem engineer is any organism that creates or modifies habitats. Jones et al. identified two different types of ecosystem engineers:...

s. They transfer nutrients from the ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

ic ecosystem to the forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...

 ecosystem. The first stage of the transfer is performed by salmon
Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...

, 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
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...

 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
Prairie dog
Prairie dogs are burrowing rodents native to the grasslands of North America. There are five different species of prairie dogs: black-tailed, white-tailed, Gunnison's, Utah and Mexican prairie dogs. They are a type of ground squirrel, found in the United States, Canada and Mexico...

 is also an ecosystem engineer. Prairie dog burrows provide the nesting areas for Mountain Plover
Mountain Plover
The Mountain Plover is a medium-sized ground bird in the plover family . It is misnamed, as it lives on level land...

s and Burrowing Owl
Burrowing Owl
The Burrowing Owl is a tiny but long-legged owl found throughout open landscapes of North and South America. Burrowing Owls can be found in grasslands, rangelands, agricultural areas, deserts, or any other open dry area with low vegetation. They nest and roost in burrows, such as those excavated...

s. Prairie dog tunnel systems also help channel rainwater into the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

 to prevent runoff
Surface runoff
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

 and erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

, 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
Plains Bison
The Plains Bison or Common bison is one of two subspecies/ecotypes of the American Bison, the other being the Wood Bison . Furthermore, it has been suggested that the Plains Bison consists of a northern and a southern subspecies, bringing the total to three...

, pronghorn
The pronghorn is a species of artiodactyl mammal endemic to interior western and central North America. Though not an antelope, it is often known colloquially in North America as the prong buck, pronghorn antelope, or simply antelope, as it closely resembles the true antelopes of the Old World and...

, and Mule deer
Mule Deer
The mule deer is a deer indigenous to western North America. The Mule Deer gets its name from its large mule-like ears. There are believed to be several subspecies, including the black-tailed 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
The beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, North American Beaver and Eurasian Beaver . Beavers are known for building dams, canals, and lodges . They are the second-largest rodent in the world...

, which transforms its territory from a stream to a pond or swamp.

In the African savanna
A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...

, 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 savannah would turn into woodland
Ecologically, a woodland is a low-density forest forming open habitats with plenty of sunlight and limited shade. Woodlands may support an understory of shrubs and herbaceous plants including grasses. Woodland may form a transition to shrubland under drier conditions or during early stages of...


See also

  • Foundation species
    Foundation species
    In ecology, a foundation species is a dominant primary producer in an ecosystem both in terms of abundance and influence. Examples include kelp in kelp forests and corals in coral reefs.-Similar concepts:...

  • Ecosystem engineer
    Ecosystem engineer
    An ecosystem engineer is any organism that creates or modifies habitats. Jones et al. identified two different types of ecosystem engineers:...

  • Indicator species
    Indicator species
    An indicator species is any biological species that defines a trait or characteristic of the environment. For example, a species may delineate an ecoregion or indicate an environmental condition such as a disease outbreak, pollution, species competition or climate change...

  • Flagship species
    Flagship species
    The concept of flagship species is a surrogate species concept with its genesis in the field of conservation biology. The flagship species concept holds that by raising the profile of a particular species, it can successfully leverage more support for biodiversity conservation at large in a...

  • Indigenous
    Indigenous (ecology)
    In biogeography, a species is defined as native to a given region or ecosystem if its presence in that region is the result of only natural processes, with no human intervention. Every natural organism has its own natural range of distribution in which it is regarded as native...

  • Introduced species
    Introduced species
    An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...

  • Umbrella species
    Umbrella species
    Umbrella species are species selected for making conservation related decisions, typically because protecting these species indirectly protects the many other species that make up the ecological community of its habitat. Species conservation can be subjective because it is hard to determine the...

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