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Keystone species

Updated: Aug 3, 2020

Everything in nature is connected to everything else to different degrees. However there are some species that have a large effect on the distribution of other species.

A keystone species is a plant or animal that holds the rest of the ecosystem together. It gets it's name from a keystone in an archway- the middle brick holds the balance of the archway. Remove the stone and the whole thing crumbles. In the same way, one plant or animal species may have a much larger impact than the others in the ecosystem, and help all the other species in its ecosystem survive.

There are three types of keystone species: predators, ecosystem engineers, and mutualists.

Predators are carnivores that help control the population of prey species, and are an important part of the ecosystem. The fear of predators keeps prey species moving around so that they don't overgraze in one place. Predators keep the population of prey species healthy. Predators normally target vulnerable animals such as the old and sick, which leaves the healthy and fittest animals to survive and reproduce. This also help prevent the spread of disease.

When the top predator is removed from the food chain, it impacts the populations of other predators and prey throughout the food chain, all the way down to the plants. The numbers of the prey species will initially increase, which then eats more of their food sources, and so on. This is called "trophic cascade" (see videos below).

Some examples of predator keystone species:

  • A tide pool was observed to have several species including starfish, barnacles, sea urchins, algae and mussels. The starfish were on the top of the food chain, and were removed to study the effect of predators. Starfish eat mussels, among other things. Over several years the number of species went from 15 to 1 without the starfish. The starfish regulate the whole tide pool ecosystem by controlling the number of mussels. Without starfish, the mussels reproduce and take over all the space in the tide pool, removing all other species.

  • Kelp forests are home to a variety of species including fish, sea lions, whales, sea otters, shrimp, and brittle stars. When sea otters are removed through hunting, they no longer eat sea urchins. The sea urchins eat the holdfasts that attach kelp to the ground. Without otters to control their population, the urchins destroy whole kelp forests, removing the habitat for all the species that live there.

  • The grey wolf was removed from Yellowstone National Park through hunting, but when they were reintroduced it became apparent the huge impact they made on the whole ecosystem. Without wolves, the population of prey species increased in number. These herbivores also stayed in one place for a long time since they had no fear of being hunted, eating everything in that area. They ate trees along the river that beavers relied on, which reduced the beaver population (beavers themselves are ecosystem engineers). Without beavers slowing rivers with their dams, the rivers became faster and deeper, and eroded the land. The shallow marshes made by beavers also disappeared, which had created a habitat for fish, amphibians, and birds.

How whales change the climate:

How wolves changed the landscape in Yellowstone:


Ecosystem engineers create, change, or destroy a habitat.


  • Prairie dogs dig underground tunnels to their various chambers, and these tunnels help move rainwater down into the water table and prevent erosion. By tunneling, they also aerate the soil and redistribute nutrients. Their underground burrows are used as nests for burrowing owls and mountain plovers, in addition to snakes, ferrets, and other animals. Prairie dogs maintain grassland habitats by grazing grass down low by their burrows and prevent growth of bushes. Prairie dogs enhance the quality of forage around their burrows which attracts many herbivores, like bison.

  • Elephants change the shape of the landscape. They pull up grass and knock down trees. By knocking down trees, they create a savanna from woodlands, which opens up grazing for other animals. New species of plants are able to grow up in the openings which increases species diversity. Elephant's feces fertilizes the soil, and disperses the seeds the elephants ate which rejuvenate the landscape. In areas where the elephants damage the woody vegetation, there are more species. Bits of debris left from broken trees make a good habitat for amphibians and lizards to hide. Elephants dig wells in areas where they know they can find underground water, which provides water for the other animals in the area.


Mutualists are two or more species that interact to the benefit of each other.


  • Honeyeaters are birds in Australia that pollinate many species of plants. During certain times of the year honeyeaters rely on the acorn banksia shrub as their sole food source. If the acorn banksia were to disappear the honeyeaters wouldn't be around to pollinate the other plants.


Activity #1

After watching the video that discusses the wolves in Yellowstone, draw two pictures that illustrate how the landscape looks with and without wolves.

  • Take out two pieces of paper. On the first paper, draw a landscape without wolves. Include the plants and animals that would be present. On the second paper, draw how the scene has changed with wolves- how the plants look different, how different species are present, and how there are different numbers of species.

Activity #2

Watch the video "how whales change climate."

  • Phytoplankton are the tiny algae that are the foundation of the food chain in the ocean. Phytoplankton (tiny algae) feeds zooplankton (tiny animals), which then feed small fish, and bigger fish. Phytoplankton are completely dependent on receiving nutrients in order to grow. Phytoplankton are estimated to produce 20% of Earth's oxygen. It also pulls carbon dioxide out of the atmosphere helping to reduce climate change.

  • Answer the following questions based on the video:

  1. If all whales were killed off, it would seem that there would be more fish because the whales are no longer eating them. In reality there would actually be less fish. Explain why.

  2. How do whales help phytoplankton grow?

  3. Considering carbon dioxide is a greenhouse gas that causes global warming, and that phytoplankton pull carbon dioxide out of the atmosphere, explain how whales change the climate.

  4. If whales disappeared, how would the ocean change? How would our atmosphere change?

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