• keystone species
    Prairie dogs are a keystone species in the Great Plains region of the U.S. and Canada.

    Photograph by Donald Kanzler, My Shot

    Keystone Paine
    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

    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.

    Umbrella Species

    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.

    Indicator Species

    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.

    animal Noun

    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.

    antelope Noun

    grazing mammal.

    bay Noun

    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.

    boar Noun

    mammal, related to a pig, native to Europe and Asia.

    buffelgrass Noun

    grass native to Africa and Asia.

    burrow Noun

    small hole or tunnel used for shelter.

    cactus Noun

    type of plant native to dry regions.

    cease Verb

    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.

    coral Noun

    tiny ocean animal, some of which secrete calcium carbonate to form reefs.

    crab Noun

    type of marine animal (crustacean) with a flat body, hard shell, and pincers.

    crucial Adjective

    very important.

    damage Noun

    harm that reduces usefulness or value.

    deer Noun

    mammal whose male members have antlers.

    disappear Verb

    to vanish or leave without a trace.

    distribution Noun

    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.

    dramatic Adjective

    very expressive or emotional.

    ecosystem Noun

    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.

    extinct Adjective

    no longer existing.

    feeding behavior Noun

    methods by which an organism obtains food and eats.

    fish Verb

    to catch or harvest fish.

    foundation species Noun

    species that creates or maintains an ecosystem.

    function Verb

    to work or work correctly.

    goose Noun

    aquatic bird with a long neck.

    grass Noun

    type of plant with narrow leaves.

    grassland Noun

    ecosystem with large, flat areas of grasses.

    grazing animal Noun

    animal that feeds on grasses, trees, and shrubs.

    habitat Noun

    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.

    herbivore Noun

    organism that eats mainly plants.

    Encyclopedic Entry: herbivore
    hummingbird Noun

    type of very small bird.

    hyena Noun

    predatory mammal native to Africa and Asia.

    immediately Adverb

    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.

    lion Noun

    large cat native to sub-Saharan Africa and Gir Forest National Park, India.

    mammal Noun

    animal with hair that gives birth to live offspring. Female mammals produce milk to feed their offspring.

    migratory Adjective

    organisms that travel from one place to another at predictable times of the year.

    monkey Noun

    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.

    mouse Noun

    small mammal, usually with a pointed snout and long, hairless tail.

    mussel Noun

    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.

    nest Noun

    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.

    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.

    pollinate Verb

    to transfer pollen from one part of a flower (the anther) to another (the stigma).

    population Noun

    total number of people or organisms in a particular area.

    predator Noun

    animal that hunts other animals for food.

    prevalent Adjective

    common or widespread.

    prey Noun

    animal that is hunted and eaten by other animals.

    professor Noun

    highest-ranking teacher at a college or university.

    rabbit Noun

    mammal with long ears that hops on strong hind legs.

    reef Noun

    a ridge of rocks, coral, or sand rising from the ocean floor all the way to or near the ocean's surface.

    resource Noun

    available supply of materials, goods, or services. Resources can be natural or human.

    roam Verb

    to wander or travel over a wide area without a specific destination.

    savanna Noun

    type of tropical grassland with scattered trees.

    scavenger Noun

    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.

    seaweed Noun

    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.

    shelter Noun

    structure that protects people or other organisms from weather and other dangers.

    shrew Noun

    type of small mammal resembling a mouse with a long nose.

    snail Noun

    marine or terrestrial animal (mollusk) with a shell and one foot on which it glides.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    sustain Verb

    to support.

    thrive Verb

    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.

    tiger Noun

    large cat native to Asia.

    umbrella species Noun

    large, usually migratory species on which other species in an ecosystem depend.

    unique Adjective

    one of a kind.

    uproot Verb

    to tear or remove a tree or other plant by the roots.

    vulture Noun

    bird that mostly eats dead animals.

    water pollution Noun

    introduction of harmful materials into a body of water.

    wildebeest Noun

    type of antelope native to Africa. Also called a gnu.

    woodland Noun

    land covered with trees, usually less dense than a forest.

    Encyclopedic Entry: woodland
    zebra Noun

    mammal, related to a horse, native to Africa.

    zoology Noun

    the study of animals.

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