A keystone species is a life form that affects an entire ecosystem.

If the keystone species disappear from the ecosystem, no other species can do its job. The ecosystem would have to radically change, allowing new and possibly invasive species to take over the habitat.

Any being, from an animal down to fungi, may be a keystone species. 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 a predator. Just a few predators can control the population of large numbers of species they prey on.

The entire idea of the keystone species started with studying predators. Zoology professor Robert T. Paine was researching Tatoosh Island in Washington state. He found that the purple sea star had a huge effect on the ecosystem along the shore. These sea stars are a major predator of mussels and barnacles on Tatoosh Island. With the sea stars gone, mussels took over the area. The mussels crowded out other species, including a special algae that sea snails and bivalves ate. Without a keystone species, the wetland habitat's biodiversity, or overall variety of creatures, was cut in half within a year.

In the Greater Yellowstone Ecosystem (GYE), gray wolves are a keystone species. The GYE stretches across the U.S. states of Montana, Wyoming, and Idaho. The GYE includes mountains, forests, meadows, bubbly geysers, and freshwater habitats.

The elk, bison, rabbit and bird species in the GYE are partly controlled by the presence of wolves. These are prey species, and how they eat and where they choose to make their nests largely depend on what wolves do.

The U.S. government set aside land for Yellowstone National Park in the late 19th century. At that time, hundreds of wolves roamed the GYE. They hunted the abundant herds of elk and bison. The government feared that wolves would too heavily hunt these animals, as well as local livestock, like cows. So the government removed wolves from the GYE. The last remaining wolf pups in Yellowstone were killed in 1924.

The removal of this key predator started a top-down change in the Greater Yellowstone Ecosystem.

Lacking a top predator, elk populations in Yellowstone exploded. Elk herds competed for food resources, and plants such as grasses and reeds did not have time or space to grow back. This hurt other species populations, such as fish, beaver, and songbirds. These animals rely on plants and their roots, flowers, wood, and seeds for survival.

It wasn't just animals who were affected. Stream banks eroded, or wore away, as plants were no longer there to hold the soil in place. Lake and river temperatures increased as trees and shrubs no longer provided shade.

Starting in the 1990s, the U.S. government began bringing wolves back to the Greater Yellowstone Ecosystem. The results have been astonishing. Elk populations have shrunk, willow tree heights have increased and beaver and songbird populations have recovered.

Herbivores

Herbivores—plant eaters—can also be keystone species.

In African savannas such as the Serengeti plains in Tanzania, elephants are a keystone species. Elephants eat shrubs and small trees that grow on the savanna. Even if the acacia tree grows to a yard high or more, elephants are able to knock it over and uproot it. This feeding behavior keeps the savanna a grassland, rather than a forest.

With elephants to control the tree population, grasses thrive. This feeds 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 finding prey.

Keystone Mutualists

Keystone mutualists are two or more species that work as a team. A change in one species would impact the other, and change the entire ecosystem. These are often pollinators, such as bees.

In the woody grasslands of Patagonia, a region in South America, a species of hummingbird and native plants work together. Local trees, shrubs, and flowering plants have evolved to only be pollinated by a hummingbird called the green-backed firecrown. Green-backed firecrowns pollinate 20 percent 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. No other creature can pollinate these plants.

Other Organisms Crucial To Ecosystems

Umbrella Species

Umbrella species are often confused with keystone species. Each of these life forms has other life forms that depend on it. The main difference between the two is that an umbrella species often travels far.

Most umbrella species are migratory—that is, they move from place to place. These species expect much out of their habitat, and this affects other animals.

If a part of nature is about to be protected, umbrella species play a key part. The government may measure the border of the protected area by looking at how far the umbrella species travels.

The Siberian tiger, an endangered species, is an umbrella species with a range of more than 998 kilometers (620) miles in eastern Russia. 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 foundation species of many islands in the South Pacific Ocean. These tiny animals grow as a colony of thousands and even millions of individual corals. The rocky outer layers of these corals create enormous structures around islands: coral reefs.

Coral reefs are awash with colorful creatures. Tortoises, crabs, sponges, sharks, and fish are all often part of healthy coral reef ecosystems.

Coral reefs, when their outer shells wear away, break down into a soft sand. This creates coral sand beaches, which are very popular with (human) tourists and travelers.

Ecosystem Engineers

Like foundation species, ecosystem engineers contribute to the physical world around where they live. Ecosystem engineers modify, create, and maintain habitats.

Some engineers change their environment by changing their own biology. Corals and trees are such engineers. As they grow, they are a living part of the environment, providing food and shelter to other organisms. The hard outer shells left behind as corals die continue to define the ecosystem.

Other engineers physically change their environment. Beavers are a classic example. They help maintain woodland areas by thinning out older trees and allowing young ones to grow. They turn these trees into dams, which greatly changes forests and streams—turning them into wetland habitats.

Invasive species are often ecosystem engineers. They don't have predators or other factors holding them back. So, these species move to new environments and change them greatly. This holds back the growth of native plants and animals, that are used to the old environment.

Kudzu is an invasive plant that altered the environment of the southeastern United States. Kudzu vines regularly fight native species for space and nutrients, crowding them out. As it crowds out native species, kudzu limits the pollinators, insects and bird species that live in an area.

Indicator Species

An indicator species is a life form that is very sensitive to environmental changes in its ecosystem. They can give us early-warning messages that a habitat is suffering.

Indicator species can give us clues to any water pollution, air pollution, or climate change that may be happening.

In the Chesapeake Bay, in the northeastern U.S., oysters are an indicator species. Oysters filter water as they strain it for food particles. Oysters filter nutrients, sediments, and pollutants that enter the bay. Oyster beds help protect fisheries and coastal habitats. The health of oyster populations in the Chesapeake, therefore, is used to help understand 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.

Flagship species are often large animals that are loved for how they look. They often appear in movies, TV, or books.

Flagship species can sometimes be symbols of general ideas about protecting nature. Polar bears are seen as a flagship species associated with climate change. The earth is getting warmer, and large ice sheets are melting. This greatly affects how polar bears live and hunt.

The giant panda is perhaps the most familiar flagship species. Pandas are the global symbol of endangered species.