A keystone species is an organism that helps define an entire ecosystem. Without its keystone species, the ecosystem would be dramatically different or cease to exist altogether. 
 
Keystone species have low functional redundancy. This means that if the species were to disappear from the ecosystem, no other species would be able to fill its ecological niche. The ecosystem would be forced to radically change, allowing new and possibly invasive species to populate the habitat.
 
Any organism, from plants to fungi, may be a keystone species; they are not always the largest or most abundant species in an ecosystem. However, almost all examples of keystone species are animals that have a huge influence on food webs. The way these animals influence food webs varies from habitat to habitat.
 
Carnivores, Herbivores, and Mutualists
 
Predators
A keystone species is often, but not always, a predator. Just a few predators can control the distribution and population of large numbers of prey species. 
 
The entire concept of keystone species was founded on research surrounding the influence of a marine predator on its environment. American zoology professor Robert T. Paine's research showed that removing a single 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 ecosystem. Pisaster ochraceus, commonly known as purple sea stars, are a major predator of mussels and barnacles on Tatoosh Island. With the sea stars gone, mussels took over the area and crowded out other species, including benthic algae that supported communities of sea snails, limpets, and bivalves. Lacking a keystone species, the tidal plain’s biodiversity was cut in half within a year.
 
Another example of a predator acting as a keystone species is the presence of gray wolves in the Greater Yellowstone Ecosystem. The Greater Yellowstone Ecosystem (GYE) is an enormous and diverse temperate ecosystem stretching across the boundaries of the U.S. states of Montana, Wyoming, and Idaho. The GYE includes active geothermal basins, mountains, forests, meadows, and freshwater habitats.
 
The elk, bison, rabbit, and bird species in the Greater Yellowstone Ecosystem are at least partly controlled by the presence of wolves. The feeding behavior of these prey species, as well as where they choose to make their nests and burrows, are largely a reaction to wolf activity. Scavenger species, such as vultures, are also controlled by the wolf activity.
 
When the U.S. government designated land for Yellowstone National Park in the late 19th century, hundreds of wolves roamed the GYE, preying primarily on abundant herds of elk and bison. Fearing the wolves’ impact on those herds, as well as local livestock, governments at the local, state, and federal level worked to eradicate wolves from the GYE. The last remaining wolf pups in Yellowstone were killed in 1924.
 
This started a top-down trophic cascade in the Greater Yellowstone Ecosystem. A trophic cascade describes changes in an ecosystem due to the addition or removal a predator. A top-down trophic cascade describes changes that result from the removal of an ecosystem’s top predator. (A bottom-up trophic cascade describes changes that result from the removal of a producer or primary consumer.)
 
Lacking an apex predator, elk populations in Yellowstone exploded. Elk herds competed for food resources, and plants such as grasses, sedges, and reeds did not have time or space to grow. Overgrazing influenced the populations of other species, such as fish, beaver, and songbirds. These animals rely on plants and their products—roots, flowers, wood, seeds—for survival.
 
The physical geography of the Greater Yellowstone Ecosystem was also impacted by the loss of wolves and subsequent elk overgrazing. Stream banks eroded as wetland plants failed to anchor valuable soil and sediments. Lake and river temperatures increased as trees and shrubs failed to provide shaded areas.  
 
Starting in the 1990s, the U.S. government began reintroducing wolves to the Greater Yellowstone Ecosystem. The results have been noteworthy. Elk populations have shrunk, willow heights have increased, and beaver and songbird populations have recovered.
 
Herbivores
Herbivores can also be keystone species. Their consumption of plants helps control the physical and biological aspects of an ecosystem.
 
In African savannas such as the Serengeti plains in Tanzania, elephants are a keystone species. Elephants eat shrubs and small trees, such as acacia, that grow on the savanna. Even if an acacia tree grows to a height of a meter or more, elephants are able to knock it over 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. 
 
Keystone Mutualists
Keystone mutualists are two or more species that engage in mutually beneficial interactions. A change in one species would impact the other, and change the entire ecosystem. Keystone mutualists are often pollinators, such as bees. Pollinators often maintain gene flow and dispersal throughout widespread ecosystems.
 
In the woody grasslands of Patagonia (at the southern tip of South America) a species of hummingbird and indigenous plants act together as keystone mutualists. Local trees, shrubs, and flowering plants have evolved to only be pollinated by Sephanoides sephanoides, a hummingbird known as the green-backed firecrown. Green-backed firecrowns pollinate 20% of local plant species. In turn, these plants provide the sugary nectar that makes up most of the hummingbird’s diet
 
Pockets of the existing Patagonian habitat would collapse without green-backed firecrowns, because their functional redundancy is nearly zero—no other pollinator has adapted to pollinate these plants.
 
Other Organisms Crucial to Ecosystems
 
In addition to keystone species, there are other categories of organisms crucial to their ecosystems' survival. 
 
Umbrella Species
Umbrella species are often conflated with keystone species. Both terms describe a single species on which many other species depend. The key distinction between umbrella species and keystone species is that the value of an umbrella species is tied to its geographic species range.
 
Umbrella species have large habitat needs, and the requirements of that habitat impact many other species living there. Most umbrella species are migratory, and their range may include different habitat types.
 
The identification of an umbrella species can be an important aspect for conservation. The minimum species range of an umbrella species is often the basis for establishing the size of a protected area.
 
The Siberian tiger, an endangered species, is an umbrella species with a range of more than 1,000 kilometers (620 miles) in Russia’s far east, with territory stretching into China and North Korea. The species range includes heavily forested ecosystems in both temperate and boreal (subarctic) biomes. Populations of deer, boar, and moose are under the snowy “umbrella” of the Siberian tiger range.
 
Foundation Species
Foundation species play a major role in creating or maintaining a habitat. 
 
Corals are a key example of a foundation species across many islands in the South Pacific Ocean. These tiny animals grow as a colony of thousands and even millions of individual polyps. The rocky exoskeletons of these polyps create enormous structures around islands: coral reefs.
 
Coral reefs are one of the most vibrant and biologically diverse ecosystems on the planet. Microscopic plankton, as well as crustaceans, mollusks, sponges, fish, and marine reptiles and mammals are all part of healthy coral reef ecosystems. 
 
Coral reef ecosystems also contribute to the human geography of a region. Pummeled by waves and ocean currents, coral exoskeletons can experience bioerosion. These eroded fragments of coral (along with bony fragments of organisms such as foraminifera, mollusks, and crustaceans) create a soft sand known as coral sand. Coral sand beaches are among the most popular tourist destinations in the world.  
 
Ecosystem Engineers
Like foundation species, ecosystem engineers contribute to the physical geography of their habitat. Ecosystem engineers modify, create, and maintain habitats.
 
Ecosystem engineers modify their habitats through their own biology or by physically changing biotic and abiotic factors in the environment. 
 
Autogenic engineers modify their environment by modifying their own biology. Corals and trees are autogenic engineers. As they grow, they are a living part of the environment, providing food and shelter to other organisms. (The hard exoskeletons left as corals die continue to define and modify the ecosystem.)
 
Allogenic engineers physically change their environment from one state to another. Beavers are a classic example of allogenic engineers. Beavers help maintain woodland ecosystems by thinning out older trees and allowing young saplings to grow. Converting these trees into timber for dams radically alters woodland meadows and streams, changing them into wetland habitats. 
 
Invasive species are often ecosystem engineers. Lacking natural predators or abiotic factors to constrain them, these introduced species modify the existing environment in ways that inhibit the growth of the indigenous ecosystem. 
 
Kudzu, the so-called “vine that ate the South,” is an invasive species of plant that modified the environment of the southeastern United States. Kudzu regularly outcompetes native species for space and nutrients. As it crowds out native species, kudzu limits the pollinators, insects, and bird species that inhabit an area.
 
Indicator Species
An indicator species describes an organism that is very sensitive to environmental changes in its ecosystem. Indicator species are almost immediately affected by changes to the ecosystem and can give early warning that a habitat is suffering. 
 
Changes associated with external influences such as water pollution, air pollution, or climate change first appear in indicator species. For this reason, indicator species are sometimes known as “sentinel species.”
 
In the “nation’s estuary” of the Chesapeake Bay, oysters are an indicator species. Oysters and other bivalves are filter feeders, meaning they filter water as they strain it for food particles. Oysters filter nutrients, sediments, and pollutants that enter the bay through natural or anthropogenic sources. Oyster beds help protect fisheries, coastal habitats, and even benthic ecosystems. The health of oyster populations in the Chesapeake, therefore, is used to indicate the health of the entire ecosystem.
 
Flagship Species
A flagship species acts as a symbol for an environmental habitat, movement, campaign, or issue. They can be mascots for entire ecosystems. 
 
The identification of a flagship species relies heavily on the social, cultural, and economic value of a species. They are often “charismatic megafauna,”—large animals with popular appeal due to their appearance or cultural significance. Flagship species may or may not be keystone or indicator species. 
 
Flagship species can sometimes be symbols of general ideas about conservation, not representatives of specific ecosystems. However, specific issues are often associated with a specific animal. The movement to end seal hunting in the Arctic found its flagship species in the juvenile harp seal. Polar bears are the unchallenged flagship species associated with climate change.
 
The giant panda is perhaps the most familiar flagship species. Pandas are the global symbol of endangered species and the value of captive breeding
keystone species
The ochre sea star (Pisaster ochraceus) is the keystone of keystone species.
Keystone Trophics
Keystone species are often predators, but not always apex predators. Instead, they are usually secondary consumers. Sea stars, while voracious predators of mussels and barnacles, for example, are a prey species for sea anemones and fishes.
Keystone Hosts
Plants and other producers that provide food and shelter for keystone species are sometimes called keystone hosts. Kelp is a keystone host. Kelp forests provide stabilizing shelter for sea otters, and nutrient-rich food for their prey, such as fish and sea urchins.
Keystone Prey
Keystone prey are species that can maintain healthy populations despite being preyed upon. Wildebeests, prey for predators from lions to crocodiles of the African savanna, are an example of keystone prey.
Nutrient Vectors
Keystone species can sometimes be “nutrient vectors,” transferring nutrients from one habitat to another. Grizzly bears, for instance, prey on salmon. They can deposit salmon carcasses miles from rivers and streams. Salmon carcasses decompose and fertilize the soil with nutrients that may not be available from local terrestrial ecosystems.

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 actually changing the habitat. Paine and his students from the University of Washington spent 25 years removing the sea stars 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.

abiotic
Adjective

characterized by the absence of life or living organisms

abundant
Adjective

in large amounts.

acacia
Noun

tree or shrub that is often thorny.

Noun

harmful chemicals in the atmosphere.

algae
Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

allogenic engineer
Noun

organism that changes biotic and/or abiotic resources from one physical state to another.

anthropogenic source
Noun

caused by people.

apex predator
Noun

species at the top of the food chain, with no predators of its own. Also called an alpha predator or top predator.

Noun

region at Earth's extreme north, encompassed by the Arctic Circle.

autogenic engineer
Noun

organism that modifies its environment by modifying its own biology.

bank
Noun

a slope of land adjoining a body of water, or a large elevated area of the sea floor.

Noun

body of water partially surrounded by land, usually with a wide mouth to a larger body of water.

benthic
Adjective

having to do with the bottom of a deep body of water.

Noun

all the different kinds of living organisms within a given area.

bioerosion
Noun

the process in which a living organism wears away at rock or another hard substance.

biology
Noun

study of living things.

Noun

area of the planet which can be classified according to the plant and animal life in it.

biotic
Adjective

having to do with living or once-living organisms.

bivalve
Noun

type of animal (mollusk) with two shells hinged together, such as a clam or mussel.

boreal forest
Noun

land covered by evergreen trees in cool, northern latitudes. Also called taiga.

bottom-up trophic cascade
Noun

ecological phenomenon in which a producer or primary consumer is removed from the environment.

burrow
Noun

small hole or tunnel used for shelter.

captive breeding
Noun

reproduction of rare species controlled by humans in a closed environment, such as a zoo.

cease
Verb

to stop or end.

charismatic megafauna
Noun

large animals with popular appeal due to their appearance or cultural significance.

Noun

gradual changes in all the interconnected weather elements on our planet.

collapse
Verb

to fall apart completely.

colony
Noun

group of one species of organism living close together.

conflate
Verb

to merge or fuse into one entity.

Noun

management of a natural resource to prevent exploitation, destruction, or neglect.

coral reef
Noun

rocky ocean features made up of millions of coral skeletons.

crucial
Adjective

very important.

crustacean
Noun

type of animal (an arthropod) with a hard shell and segmented body that usually lives in the water.

culture
Noun

learned behavior of people, including their languages, belief systems, social structures, institutions, and material goods.

dam
Noun

structure built across a river or other waterway to control the flow of water.

designate
Verb

to name or single out.

Noun

foods eaten by a specific group of people or other organisms.

dispersal
Noun

spread of something to a new area.

distinction
Noun

difference.

Noun

the way something is spread out over an area.

economic
Adjective

having to do with money.

Noun

community and interactions of living and nonliving things in an area.

ecosystem engineer
Noun

organism that creates, modifies, maintains or destroys a habitat.

Noun

organism threatened with extinction.

environment
Noun

conditions that surround and influence an organism or community.

eradicate
Verb

to destroy or remove.

Noun

mouth of a river where the river's current meets the sea's tide.

exoskeleton
Noun

the hard external shell or covering of some animals.

feeding behavior
Noun

methods by which an organism obtains food and eats.

filter feeder
Noun

aquatic animal that strains nutrients from water.

fishery
Noun

industry or occupation of harvesting fish, either in the wild or through aquaculture.

flagship species
Noun

organism that serves as a symbol for an environmental habitat, movement, campaign, or issue.

Noun

all related food chains in an ecosystem. Also called a food cycle.

foraminifera
Plural Noun

(singular: foraminifer.) Type of microscopic organism (protist) that forms a shell and lives in marine or salty conditions.

forest
Noun

ecosystem filled with trees and underbrush.

foundation species
Noun

species that creates or maintains an ecosystem.

functional redundancy
Noun

characteristic of species whose contributions to an ecosystem are matched by other species.

gene flow
Noun

movement and exchange of genes between interbreeding populations.

Noun

heat energy generated within the Earth.

government
Noun

system or order of a nation, state, or other political unit.

grassland
Noun

ecosystem with large, flat areas of grasses.

grazing animal
Noun

animal that feeds on grasses, trees, and shrubs.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

Noun

organism that eats mainly plants and other producers.

herd
Noun

group of animals.

human geography
Noun

the study of the way human communities and systems interact with their environment.

indicator species
Noun

organism that is very sensitive to changes in its environment. Also called a sentinel species.

Adjective

characteristic to or of a specific place.

inhibit
Verb

to slow or prevent.

introduced species
Noun

a species that does not naturally occur in an area. Also called alien, exotic, or non-native species.

Noun

type of plant or animal that is not indigenous to a particular area and causes economic or environmental harm.

juvenile
Noun

animal that is no longer a baby but has not reached sexual maturity.

keystone mutualists
Noun

organisms that participate in mutually beneficial interactions, the loss of which would have a profound impact on the ecosystem.

Noun

organism that has a major influence on the way its ecosystem works.

livestock
noun, plural noun

animals raised for sale and profit.

mammal
Noun

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

marine
Adjective

having to do with the ocean.

mascot
Noun

animal, person, or thing representing a group of people or organization.

meadow
Noun

wide area of grassland.

microscopic
Adjective

very small.

migratory
Adjective

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

modify
Verb

to change or alter.

mollusk
Noun

large phylum of invertebrate animal, all possessing a mantle with a significant cavity used for breathing and excretion, a radula (except for bivalves), and the structure of the nervous system. 

native species
Noun

species that occur naturally in an area or habitat. Also called indigenous species.

nectar
Noun

sweet plant material that attracts pollinators.

nest
Noun

protected area built by birds to hatch their eggs and raise their young.

niche
Noun

role and space of a species within an ecosystem.

Noun

substance an organism needs for energy, growth, and life.

overgrazing
Noun

process of too many animals feeding on one area of pasture or grassland.

Patagonia
Noun

large plateau in southern South America, stretching from the Andes Mountains to the Atlantic Ocean.

physical geography
Noun

study of the natural features and processes of the Earth.

Noun

flat, smooth area at a low elevation.

plankton
Plural Noun

(singular: plankton) microscopic aquatic organisms.

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).

pollinator
Noun

animal, object, or force such as wind that transfers pollen from one plant to another, allowing seeds to develop.

pollutant
Noun

chemical or other substance that harms a natural resource.

polyp
Noun

a type of animal with a fixed base, a tubelike body, and tentacles for catching prey.

population
Noun

total number of people or organisms in a particular area.

predator
Noun

animal that hunts other animals for food.

prey
Noun

animal that is hunted and eaten by other animals.

primary consumer
Noun

organism that eats plants or other autotrophs.

producer
Noun

organism on the food chain that can produce its own energy and nutrients. Also called an autotroph.

professor
Noun

highest-ranking teacher at a college or university.

pummel
Verb

to strike violently and repeatedly.

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.

sapling
Noun

young tree.

savanna
Noun

type of tropical grassland with scattered trees.

Noun

organism that eats dead or rotting biomass, such as animal flesh or plant material.

Noun

solid material transported and deposited by water, ice, and wind.

shelter
Noun

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

shrub
Noun

type of plant, smaller than a tree but having woody branches.

significance
Noun

importance.

social
Adjective

having to do with a community or other group of organized people.

soil
Noun

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

Noun

native, geographic area in which an organism can be found. Range also refers to the geographic distribution of a particular species.

subarctic
Noun

region just south of the Arctic Circle.

subsequent
Adjective

coming after.

sustain
Verb

to support.

symbol
Noun

something used to represent something else.

temperate
Adjective

moderate.

territory
Noun

land an animal, human, or government protects from intruders.

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.

timber
Noun

wood in an unfinished form, either trees or logs.

top-down trophic cascade
Noun

ecological phenomenon in which a top predator is removed from the environment.

tourist
Noun

person who travels for pleasure.

trophic cascade
Noun

ecological phenomenon triggered by the addition or removal of predators from an environment.

umbrella species
Noun

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

uproot
Verb

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

vibrant
Adjective

lively.

water pollution
Noun

introduction of harmful materials into a body of water.

Noun

area of land covered by shallow water or saturated by water.

Noun

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

zoology
Noun

the study of animals.