Zoologist Robert T. Paine, who coined the term "keystone species," had an unorthodox way of doing his work. Instead of just observing the habitat of the Pisaster ochraceus sea star, Paine experimented by changing the habitat. Paine and his students from the University of Washington spent 25 years removing the starfish from a tidal area on the coast of Tatoosh Island, Washington, in order to see what happened when they were gone. He was one of the first scientists in his field to experiment in nature in this manner.
A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether.
All species in an ecosystem, or habitat, rely on each other. The contributions of a keystone species are large compared to the species' prevalence in the habitat. A small number of keystone species can have a huge impact on the environment.
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.
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.
Without the keystone species, new plants or animals could also come into the habitat and push out the native species. Some species of hummingbirds are keystone species in the Sonoran Desert of North America. Hummingbirds pollinate many varieties of native cactus and other plants. In areas of the Sonoran Desert with few hummingbirds, invasive species such as buffelgrass have taken over the ecosystem.
The theory that the balance of ecosystems can rely on one keystone species was first established in 1969 by American zoology professor Robert T. Paine. Paine's research showed that removing one species, the Pisaster ochraceus sea star, from a tidal plain on Tatoosh Island in the U.S. state of Washington, had a huge effect on the surrounding ecosystem. The sea stars are a major predator for mussels on Tatoosh Island. With the sea stars gone, mussels took over the area and crowded out other species. In this ecosystem, the sea star was the keystone species.
The sea otter is another example of a keystone species in the Pacific Northwest. These mammals feed on sea urchins, controlling their population. If the otters didn't eat the urchins, the urchins would eat up the habitat's kelp. Kelp, or giant seaweed, is a major source of food and shelter for the ecosystem. Some species of crabs, snails, and geese depend on kelp for food. Many types of fish use the huge kelp forests to hide from predators. Without sea otters to control the urchin population, the entire ecosystem would collapse.
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. Smaller animals such as mice and shrews are able to burrow in the warm, dry soil of a savanna. Predators such as lions and hyenas depend on the savanna for prey. Elephants are the keystone species that maintain the entire savanna ecosystem.
In addition to keystone species, there are other categories of species that are crucial to their ecosystems' survival.
Foundation species play a major role in creating or maintaining a habitat that supports other species. Corals are one example of a foundation species in many islands in the South Pacific Ocean. Corals produce the reef structures on which countless other organisms, including human beings, live.
An umbrella species is a large animal or other organism on which many other species depend. Umbrella species are very similar to keystone species, but umbrella species are usually migratory and need a large habitat.
Protection of umbrella species is thought to automatically protect a host of other species. Tigers are an example of an umbrella species. Efforts to save wild tigers in forests in the Indian state of Rajasthan also accomplish the goal of saving other species there, such as leopards, boars, hares, antelopes, and monkeys.
An indicator species is a plant or animal that is very sensitive to environmental changes in its ecosystem. This means it is affected almost immediately by damage to the ecosystem and can give early warning that a habitat is suffering. Damage from external influences such as water pollution, air pollution, or climate change first appear in indicator species.
In the United States, the Environmental Protection Agency (EPA) studies the population and health of fish in the Chesapeake Bay to evaluate the quality of water in the ecosystem. The EPA uses the fish as indicator species of the bay.
Term Part of Speech Definition Encyclopedic Entry acacia Noun
tree or shrub that is often thorny.
air pollution Noun
harmful chemicals in the atmosphere.
Encyclopedic Entry: air pollution altogether Adverb
entirely or completely.
organisms that have a well-defined shape and limited growth, can move voluntarily, acquire food and digest it internally, and can respond rapidly to stimuli.
body of water partially surrounded by land, usually with a wide mouth to a larger body of water.
Encyclopedic Entry: bay bird Noun
egg-laying animal with feathers, wings, and a beak.
mammal, related to a pig, native to Europe and Asia.
grass native to Africa and Asia.
small hole or tunnel used for shelter.
type of plant native to dry regions.
to stop or end.
climate change Noun
gradual changes in all the interconnected weather elements on our planet.
Encyclopedic Entry: climate change collapse Verb
to fall apart completely.
tiny ocean animal, some of which secrete calcium carbonate to form reefs.
type of marine animal (crustacean) with a flat body, hard shell, and pincers.
harm that reduces usefulness or value.
mammal whose male members have antlers.
to vanish or leave without a trace.
the way something is spread out over an area.
Encyclopedic Entry: distribution domino effect Noun
situation in which one event causes another, which causes yet another, until an entire system is changed.
very expressive or emotional.
community and interactions of living and nonliving things in an area.
Encyclopedic Entry: ecosystem elephant Noun
large mammal with a long trunk, native to Africa and Asia.
no longer existing.
feeding behavior Noun
methods by which an organism obtains food and eats.
to catch or harvest fish.
foundation species Noun
species that creates or maintains an ecosystem.
to work or work correctly.
aquatic bird with a long neck.
type of plant with narrow leaves.
ecosystem with large, flat areas of grasses.
grazing animal Noun
animal that feeds on grasses, trees, and shrubs.
environment where an organism lives throughout the year or for shorter periods of time.
Encyclopedic Entry: habitat hare Noun
mammal, related to rabbits, with long ears and strong legs for hopping.
organism that eats mainly plants.
Encyclopedic Entry: herbivore hummingbird Noun
type of very small bird.
predatory mammal native to Africa and Asia.
at once or quickly.
indicator species Noun
any species that determines a characteristic of its environment, such as range or ecological health.
invasive species Noun
type of plant or animal that is not indigenous to a particular area and causes economic or environmental harm.
Encyclopedic Entry: invasive species island Noun
body of land surrounded by water.
Encyclopedic Entry: island kelp Noun
type of seaweed.
keystone species Noun
a species that has a major influence on the way an ecosystem works.
Encyclopedic Entry: keystone species leopard Noun
large, spotted cat native to Africa and Asia.
large cat native to sub-Saharan Africa and Gir Forest National Park, India.
animal with hair that gives birth to live offspring. Female mammals produce milk to feed their offspring.
organisms that travel from one place to another at predictable times of the year.
mammal considered to be highly intelligent, with four limbs and, usually, a tail.
mountain lion Noun
large cat native to North and South America. Also called a cougar, puma, catamount, and panther.
small mammal, usually with a pointed snout and long, hairless tail.
aquatic animal with two shells that can open and close for food or defense.
native species Noun
species that occur naturally in an area or habitat. Also called indigenous species.
protected area built by birds to hatch their eggs and raise their young.
Pacific Northwest Noun
the area made of the U.S. states of Oregon and Washington, and the Canadian province of British Columbia.
organism that produces its own food through photosynthesis and whose cells have walls.
to transfer pollen from one part of a flower (the anther) to another (the stigma).
total number of people or organisms in a particular area.
animal that hunts other animals for food.
common or widespread.
animal that is hunted and eaten by other animals.
highest-ranking teacher at a college or university.
mammal with long ears that hops on strong hind legs.
a ridge of rocks, coral, or sand rising from the ocean floor all the way to or near the ocean's surface.
available supply of materials, goods, or services. Resources can be natural or human.
to wander or travel over a wide area without a specific destination.
type of tropical grassland with scattered trees.
organism that eats dead or rotting biomass, such as animal flesh or plant material.
Encyclopedic Entry: scavenger sea otter Noun
marine mammal with thick fur native to the Pacific Ocean.
sea star Noun
marine animal (echinoderm) with many arms radiating from its body. Also called a starfish.
sea urchin Noun
marine animal (echinoderm) with a circular, spiny shell.
marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.
Serengeti plains Noun
grassland of the Serengeti ecosystem of Kenya and Tanzania.
structure that protects people or other organisms from weather and other dangers.
type of small mammal resembling a mouse with a long nose.
marine or terrestrial animal (mollusk) with a shell and one foot on which it glides.
top layer of the Earth's surface where plants can grow.
to develop and be successful.
tidal plain Noun
large, flat area where mud and sediment are deposited by ocean tides. Also called tidal flat or mudflat.
large cat native to Asia.
umbrella species Noun
large, usually migratory species on which other species in an ecosystem depend.
one of a kind.
to tear or remove a tree or other plant by the roots.
bird that mostly eats dead animals.
water pollution Noun
introduction of harmful materials into a body of water.
type of antelope native to Africa. Also called a gnu.
land covered with trees, usually less dense than a forest.
Encyclopedic Entry: woodland zebra Noun
mammal, related to a horse, native to Africa.
the study of animals.