A keystone species is a type of organism that has a much bigger impact on its environment than its numbers suggest. The idea was introduced in 1969 by zoologist Robert T. Paine. These species are very important for keeping an ecosystem balanced. They influence many other living things in the environment and help decide how many different types of species exist in an area. If a keystone species were removed, the ecosystem could change greatly or even stop existing. Some keystone species, like wolves and lions, are also top predators in their ecosystems.

The role of a keystone species in an ecosystem is similar to the role of a keystone in an arch. A keystone is the stone that holds the other stones in place, even though it is not under the most pressure. If the keystone is removed, the whole arch collapses. In the same way, an ecosystem can change a lot if a keystone species is removed, even if that species is not the largest or most numerous in the ecosystem. This idea became important in conservation biology, along with other concepts like flagship and umbrella species. While the idea helps scientists and policymakers describe strong connections between species, it has also been criticized for making complex ecosystems seem simpler than they are.

History

In 1969, zoologist Robert T. Paine introduced the idea of a keystone species. He created this concept to explain his findings from experiments on marine animals in the intertidal zone, which is the area between high and low tide lines. Paine studied starfish and mussels, and he removed starfish from an area to observe how the ecosystem changed. In his 1966 paper, Food Web Complexity and Species Diversity, Paine described a similar system in Makah Bay, Washington. In his 1969 paper, he used Pisaster ochraceus, a type of starfish commonly called the ochre starfish, and Mytilus californianus, a type of mussel, as an example of a keystone species. The ochre starfish eats many animals, including chitons, limpets, snails, barnacles, echinoids, and decapod crustaceans. Its favorite food is mussels, which are strong competitors for space on rocks. By controlling mussel numbers, the starfish allows other sea plants like seaweeds, sponges, and anemones, which the starfish does not eat, to live together. When Paine removed the starfish, mussels grew rapidly and pushed out other species. At first, rock pools had 15 species that clung to rocks. After three years, only 8 species remained. Ten years later, the pools were mostly filled with mussels. The keystone species idea became important in conservation efforts. It was used to support conservation, especially in places where human actions, like removing keystone predators, had harmed ecosystems.

Definitions

A keystone species is a species that has a much bigger effect on its environment than its numbers suggest. In 2003, Davic described it as a species that strongly interacts with others and has a large influence on the variety of species and competition in its area, even though it may not be the most common species in its group.

A well-known example is a predator that stops a certain herbivore (plant-eating animal) from overeating the most common plants. Even if the predator is not very numerous, it can still control the herbivore’s population. Without the predator, the herbivore might grow too large, destroy the dominant plants, and change the ecosystem in major ways. While each example may differ, the main idea is that a species with few numbers can greatly affect how an ecosystem works. For example, the weevil Euhrychiopsis lecontei helps keep aquatic plant diversity by eating invasive watermilfoil in North American lakes. Another example is the wasp Agelaia vicina, which is considered a keystone species because of its large nests, big colonies, and high reproduction rate. The wide range of prey it eats and the large amount of food needed to support its growth directly affects other species in its environment.

The keystone concept is based on how a species influences its ecosystem, which makes it important for conservation. This idea overlaps with other conservation terms, such as flagship species, indicator species, and umbrella species. For example, the jaguar is a large cat that fits all these categories.

— David Maehr et al, 2001

Predators

Sea otters help protect kelp forests by eating sea urchins. When sea otters on the North American west coast were hunted for their fur, their numbers dropped to fewer than 1,000 in the north Pacific Ocean. This decline made it impossible for them to control the sea urchin population. The sea urchins then ate the parts of kelp that hold them in place, causing kelp forests to disappear. Many species that relied on the kelp forests also disappeared. Reintroducing sea otters helped restore the kelp ecosystem. For example, in Southeast Alaska, 400 sea otters were released, and their population has grown to nearly 25,000.

Keystone predators can increase biodiversity by preventing one species from becoming too dominant. They play a major role in keeping ecosystems balanced. Adding or removing a keystone predator, or changing its population, can greatly affect other species in the ecosystem. For example, grazers in a grassland can stop one plant species from taking over.

Removing gray wolves from the Greater Yellowstone Ecosystem had major effects on the food chain. Without wolves, herbivores overgrazed woody plants, harming plant populations. Wolves also kept animals from eating plants near rivers, which protected beavers from losing their food sources. Without wolves, beavers lost their habitat to grazing animals. Overgrazing of willows and conifers along Blacktail Creek caused erosion because beavers usually helped slow water flow and keep soil in place. Wolves also helped maintain healthy rivers and streams. When wolves were reintroduced, beaver populations and the riparian ecosystem recovered quickly.

As described by Paine in 1966, some sea stars, like Pisaster ochraceus, eat sea urchins, mussels, and other shellfish that have no other natural predators. If sea stars are removed, mussel populations can grow too large, pushing out other species.

Keystone predators do not always need to be the top predators. Sea stars are eaten by sharks, rays, and sea anemones. Sea otters are eaten by orcas.

The jaguar, which is near threatened in Central and South America, acts as a keystone predator by eating 87 different species. This helps balance the jungle ecosystem. The lion is another keystone species.

Mutualists

Keystone mutualists are organisms that help each other in ways that are important for the ecosystem. If one of these organisms disappears, it could cause big problems for the environment. In the Avon Wheatbelt region of Western Australia, there is a time of year when Banksia prionotes (acorn banksia) is the only source of nectar for honeyeaters. Honeyeaters help pollinate many plant species. If Banksia prionotes were to disappear, the honeyeater population might decline, which could harm the entire ecosystem. Another example is frugivores, like the cassowary, which eat fruit and spread the seeds of many trees. Some seeds can only grow if they have passed through a cassowary's digestive system.

Ecosystem engineers

The term "ecosystem engineer" is often used with "keystone." In North America, the prairie dog is an ecosystem engineer. Prairie dog burrows provide nesting areas for mountain plovers and burrowing owls. Their tunnel systems help direct rainwater into underground water storage, reducing runoff and erosion. These tunnels also improve soil by increasing air flow and fixing soil compaction caused by cattle grazing. Prairie dogs also trim vegetation near their colonies, possibly to remove hiding places for predators. Grazing animals like plains bison, pronghorn, and mule deer often eat the same land used by prairie dogs.

Beavers are well-known ecosystem engineers and keystone species. They change their habitat from a stream to a pond or swamp. Beavers cut down older trees near water to build dams, allowing younger trees to grow. Their dams alter riparian areas, turning stream edges into wetlands, meadows, or forests. These dams support many species, including amphibians, salmon, and songbirds.

In the African savanna, large herbivores like elephants shape their environment. Elephants destroy trees, creating space for grasses and habitats for small animals. Without these animals, much of the savanna would become woodland. In the Amazon river basin, peccaries create and maintain wallows used by many species. Australian studies show that parrotfish on the Great Barrier Reef are the only reef fish that consistently clean coral. Without these animals, the Great Barrier Reef would face serious challenges.

In the Serengeti, enough gnus in grasslands reduce the chance of wildfires, which helps trees grow. The documentary The Serengeti Rules shows this in detail.

Limitations

Community ecologist Bruce Menge explains that the idea of a keystone species has been used in many ways that go beyond what the scientist Robert Paine originally described. This expansion can be measured: researcher Ishana Shukla found that 230 different species were labeled as keystones in 157 studies over the past 50 years. Menge's own research shows that the purple Pisaster sea star, which Paine studied, acted as a strong keystone species in areas with strong wave action. However, in sheltered areas, this sea star had much less influence. Paine himself noted that in Alaska, without the specific mussel species it feeds on, the Pisaster sea star was "just another sea star." This means whether a species is considered a keystone depends on its environment and the relationships it has with other species.

The keystone species idea helps ecologists and conservation planners describe strong connections between species and communicate more clearly. However, scientists like L. S. Mills have criticized this concept for making complex ecosystems seem too simple. The term "keystone species" has been used in many different ecosystems and for various types of species, including predators, prey, and plants. For example, removing a predator might allow other animals to grow in number, leading to the loss of other species. Removing a prey species could cause predator populations to decline or force predators to drive other prey species to extinction. Removing a plant species might harm animals that rely on it, such as pollinators or seed dispersers. Even beavers are sometimes called keystones not because they eat other species, but because they change their environment in ways that affect other species. This shows that the term "keystone species" has been used with different meanings in different situations. Mills argues that while Paine's work showed some species can have strong effects in certain ecosystems, this does not mean all ecosystems have the same structure.