All ecosystems on Earth are made up of a variety of interdependent plant and animal life. Keystone species are those plant or animal species that make their presence felt to the point that they are critical to the survival of other species in the ecosystem.
Not only are keystone species particularly important for their habitats, but they are usually relatively few in number, so have a disproportionate impact on their environment relative to their abundance.
Keystone species are often dominant or apex predators that keep a balance in the ecosystem, but there are plenty of keystone species listed below that are not predators at all.
The concept of keystone species was introduced by zoologist Bob Pain in 1969. It has since become a popular idea in conservational biology (alongside ideas such as flagship and umbrella species), though has been criticized for oversimplifying complex ecosystems and being a theory that is technically limited.
What is a keystone species?
A keystone species is defined as a species that has a disproportionately large effect on its natural environment relative to its abundance.
Its presence helps to determine the types and numbers of various other plant and animal species in its habitat, and removal would cause the ecosystem to be dramatically different.
Now we have the basics cleared up, read on below for our pick of 11 classic examples of keystone species, or skip to the end of the page to see our lists of keystone species by habitat:
Ochre Sea Star (Pisaster ochraceus)
The ochre sea star is recognised as a keystone species as it’s an organism that helps keep populations of mussels in check.
Mussels have no other natural predators aside from ochre sea stars. They grow on rocky surfaces underwater, taking up space and reducing the growth of other species – such as seaweed and the creatures that feed on seaweed. An experiment that involved removing the sea star from an ecosystem saw the mussel population exploding, and reducing the number of other species by 50% by reducing the amount of seaweed available.
Sea Otter (Enhydra lutris)
North Pacific sea otters are perhaps the most important keystone species in their kelp forest habitats, which is home to many species from sharks to sea stars. Sea otters feed on sea urchins (which themselves feed on the kelp), so help to maintain a healthy sea urchin population, which would destroy the environment – and food source for dozens of other species – if their numbers grew out of control.
When sea otter numbers were reduced to less than 1,000 through the 18th and 19th centuries by fur hunters, sea urchins were responsible for the disappearance of kelp forests and associated species in large areas off the North American Pacific coast. Reintroduction of sea otters in the 20th century allowed for the restoration of the kelp forests and the species that live off them.
Snowshoe Hare (Lepus americanus)
Unlike many keystone species, the snowshoe hare is prey for a large number predators in its Canadian Boreal forest habitat. Their helthy populations are actually the biggest food source for most carnivores in the forest, so removal of the species would mean a huge reduction in the numbers of animals that rely on them as a food source – lynx, fox, coyote, and more – or even a collapse of the entire ecosystem.
African Elephant (Loxodonta africana)
Because of its status as the biggest animal on land the African elephant is a clear keystone species. These creatures are a destructive force in their habitat, killing trees and feeding on young saplings in savanna grasslands. This behaviour reduces the number of trees and increases the areas for grass to flourish, ensuring enough grazing land for the herbivores of the savanna.
One interesting study found that elephants tend to graze and trample on trees smaller than 30 centimeters in diameter, allowing surviving trees to grow larger by eliminating their competition. So counterintuitively, although elephants destroy trees and reduce their density, they actually cause an increase in both average tree diameter and the total biomass.
Gray Wolf (Canis lupus)
Gray wolves are the primary keystone species in the Great Yellow Ecosystem made up of meadows, mountains, and forests. The ecosystem is home to a variety of herbivores including elk and rabbit, and without predation from gray wolves, herbivores begin to over-graze. This affects the area’s plant populations, leads to soil erosion, and has a knock-on effect of herbivores grazing in new areas and impacting beaver populations.
Hummingbirds around the world are sometimes referred to as keystone mutualists as they work in partnership with other species. These birds are key pollinating species, aiding the spread and growth of various plant species, and so maintaining healthy forest growth which provides a habitat for thousands of animal species.
As individual hummingbirds pollinate over a large area, and decline in their number would lead to a reduction in vegetation cover, ultimately meaning the end for certain plant species pollinated by the hummingbird alone.
Honey Bees (Apis mellifera)
Bees play a similar keystone species role to hummingbirds, being a pollinator that contributes to the reproduction of up to 90% of the world’s flowering plants.
Along with pollinating vegetables, fruits, and other crops that humans depend on, bees help ensure that new sources of nuts, berries, and seeds are continually growing to support numerous other species all around the world.
Jaguar (Panthera onca)
The jaguar is the largest cat in the Americas, and an apex predator. They play an important role in balancing the food chain in the Amazon rainforest basin and nearby Pantanal wetland, controlling the populations of other species such as alligators, turtles, and deer. (Not sure the differences between a jaguar and leopard?)
Grizzly Bear (Ursus arctos)
Grizzly bears are keystone species a number of reasons:
- They control salmon populations, which in turn maintains the river ecosystem by ensuring the seaweed ecosystem is not overrun by salmon.
- These bears are what’s known as ‘gardener’ species, taking prey deep into the forest to eat. Prey carcasses end up rotting and fertilize the forest, supporting strong tree growth.
- Also on the gardening angle, bears feed on plant roots, digging up roots and mixing and aerating the soil.
Lion (Panthera leo)
The lion is another keystone species that fills an important ecological role as an apex predator. These big cats cull populations of both herbivores and other carnivores through predation, balancing their savanna ecosystem to ensure no overpopulation of these other species.
Great White Shark (Carcharodon carcharias)
Perhaps the most feared predator in the world, the great white shark truly is the king of the oceans, and thanks to movies such as Jaws is generally feared – and even hated – by humans.
The shark is one of the largest fish in deep waters – and top of the food chain – meaning they help regulate life in their deepwater ecosystems. As well as feeding on live fish they prey on sick and weak fish, so not only manage fish numbers but also reduce the spread of disease between fish stocks.
Keystone species by habitat
Here are some of the most crucial keystone species, grouped and listed by habitat:
- Bears – facilitate the riverine seaweed habitat by controlling salmon populations, and promote healthy forest soil by digging and leaving prey carcasses which fertilize the land.
- Gray wolves – keep herbivore numbers in check, which prevents overgrazing, land erosion, and beaver habitats.
- Snowshoe hares – serve as the primary food source for multiple predator species, including the threatened Canada lynx.
- Willow and Aspen trees – provide critical habitat for smaller organisms like lichens, fungi, and insects, as well as birds.
- Wild red raspberries – provide a critical food source for myriad species, from bears to bees.
- Australian dingos – keep the outback intact by preying on a wide range of herbivores, as well as predators in the middle of the food chain.
- Mojave desert tortoise – dig burrows that other animals rely on for protection from both the heat and predators.
- Saguaro cactus – provide the only available nesting spots for woodpeckers and red-tailed hawks, as well as their flowers and fruit giving sustenance to birds, bees, insects, bats, and other mammals.
- Ivy tree coral – provide a habitat for over 300 invertebrate species.
- Krill – va ital food source for many whale, seal, and seabird species.
- Mangrove crabs – manage leaf litter and improve underwater soil health by burrowing.
- Ochre sea stars – regulate the mussel population to provide room for seaweed to grow, which in turn provides a habitat for many ocean species.
- Sea otters – regulate sea urchin populations that would otherwise destroy the kelp forest habitats which supports dozens of species.
- Sharks – control disease spread among fish and keep other prey populations in check.
- African Elephant – help shape the savanna habitat by destroying young saplings and maintaining space and light for the grassland to proliferate, providing food to myriad grazers.
- Lions – cull populations of both herbivores and other carnivores through predation, balancing their savanna ecosystem.
- Fig trees – grow fruit that supports over 1,200 species of birds, insects, bats, and primates year-round, when other food resources are scarce).
- Jaguars – play an important role in balancing the rainforest food chain by controlling the populations of other species such as alligators, turtles, and deer.
- Seed dispersers such as Western lowland gorilla, Southern cassowary or Javan rhino – promote continuous new flora growth as they move through the rainforest leaving seed-filled droppings.
- Dovekies – these birds are prey for arctic foxes and polar bears, and provide crucial compost for the tundra vegetation.
- Lemmings – are a food source to numerous predators including arctic foxes, weasels, and snowy owls, and impact the amount and health of the vegetation they graze.
Why are they called ‘keystone’ species?
In stone arches the ‘keystone’ is the central stone at the top of the arch which locks the whole arch together. Without the keystone, the arch would collapse.
The role that a keystone species performs in its ecosystem is comparable. Without the keystone species an ecosystem can experience a dramatic shift, or even collapse.
Below we’ve picked out 11 examples of keystone species, along with some examples of keystone species by habitat:
And that’s your lot for our round-up of keystone species. Did any surprise you? Or have you had the opportunity to see any of these important animals in the wild? Please share your experiences in the comments section below!