What is a Keystone Species?

Mary McMahon

A keystone species is an organism that plays a significant role in its environment. Like the keystone in an arch, this species holds the ecosystem together. If removed, the structures it supports will collapse. Different environments can have entirely unique makeups and a keystone species in one environment may not be one in another. Identification of such species as important, as they can be a critical factor to consider when developing conservation initiatives and other projects.

A keystone species is an organism that plays a significant role in its environment.
A keystone species is an organism that plays a significant role in its environment.

Typically, the population of a keystone species is relatively small, with the animals, plants, or other organisms making up a small percentage of the overall biomass. The influence exerted by the organisms is disproportionate to their overall numbers in the natural environment. This can make them difficult to single out and identify as keystone species, as scientists may not realize the importance of a seemingly minor population of plants or animals until it is too late.

If animals, like deer, become too numerous they can upset the natural balance.
If animals, like deer, become too numerous they can upset the natural balance.

Many species considered keystones in their environments are predators. While predator populations tend to be small, they are critical members of the ecosystems they live in. Predators keep prey numbers within reasonable limits, reducing the risk that prey will overrun the environment. When prey such as deer become too numerous, the natural balance can be disturbed. The animals continue multiplying, strip the environment of all edible plants, and eventually starve. With predators, their numbers are kept in check, allowing fewer animals to survive but conserving resources so that they are all healthy.

Another example of a keystone species is a natural engineer like a beaver. Beavers literally shape the environment around them by constructing dams. These dams have a profound impact on the natural world, creating habitat and a source of fresh water. Other engineers can shape plant populations, as well as the natural environment. Other keystone species are mutualists, organisms that maintain mutually beneficial relationships with other organisms.

When a keystone species is disturbed, it can have a rippling impact. As the numbers decline, the function they were serving is no longer performed. Interconnected systems may start to fall apart as the environment struggles to adjust to the loss of the keystone species. A common problem emerges when animal and plant populations that are normally kept at low numbers by prey start to multiply, driving out weaker organisms and decreasing biodiversity. The decrease in diversity in an ecosystem can leave it very vulnerable to disease, climate change, and other events.

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Discussion Comments


@stl156 - I think the wolf is an interesting example since it could also possibly be considered a flagship species like was mentioned earlier. A lot of people sort of have a majestic image of the wolf. I don't think the two groups necessarily have to be mutually exclusive.

Yet again using the wolf example, I have heard of cultural keystone species, too. I think these are usually linked to Native Americans, I guess just because they had a closer relationship with nature, but it is just what it sounds like - a species that has a disproportionate effect on a culture.

To be honest, though, I don't know what all it includes. I would say it could include anything from spiritual to food usage. I would say another example besides the wolf could be the eagle or bear. A more interesting example could be cats in ancient Egypt, since they were worshiped by the people.


I think another good example of a keystone species would be the wolf. Before we started moving west and killing them, they helped control the populations of deer and other grazing animals. I live in Minnesota, and now that we don't have wolves, deer are a huge problem. It caused the rippling effect that the article talks about. I think it is a similar situation in most northern states.

I know they are starting to release wolves back into the wild in some areas. I don't know how successful it has been, though, and if they are helping to control the ecosystems like they used to.

I would be curious to know if anyone else has any examples of a good keystone species. Since the article mentions the possibility of keystone species decreasing in numbers, I am also wondering if there are any extinct species that were keystones species and what caused them to go extinct.


@jcraig - Actually, animals like the panda and whales would belong to a group called flagship species. Whereas keystone species are defined as having a disproportionate effect on their environment given their population, flagship species are usually used as a symbol of an ecosystem.

From an ecological perspective, keystone species are the most important for regulating different food web functions, but flagship species are things that people really want to see and are willing to give money to save. Because people want to save animals like pandas, many of the other animals that live in similar habitats are also protected. If you are curious, in biology, these animals are called charismatic megafauna.

As far as a threatened species, these are usually the flagship species, as well. People want to save them because they have a low species abundance. Keystone species could go either way. Like the article mentions, there don't have to be a lot of them, so they may become threatened relatively easy.


Wow, I guess I didn't really think about how important some species were to their environments. I am assuming things like pandas and whales would be keystone species in their respective ecosystems. Would that be right?

Also, are keystone species usually animals that are on the endangered species list? What about plants, can they be keystone species, too?

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