What is a Keystone Species? (Why They Matter and What You Should Know About Them)

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Published on October 19, 2022
Last Updated on October 11, 2023

A keystone species is an organism that other organisms depend on to such a high degree that its local environment would significantly change if the keystone species were to disappear.

While all organisms are important and have important roles in their local environment, in most cases, the environment would remain relatively unchanged.

However, if a keystone species, such as the honey bee were to disappear, it could throw its entire habitat out of balance. Its disappearance would mean that the important role it plays, such as pollinating flowers, would no longer be carried out, and a long chain of other organisms would be seriously affected.

A slight adjustment will not be enough; instead, the whole ecosystem will need to change to avoid an ecological disaster. In the case of the honey bee’s disappearance, numerous crops that both humans and animals are dependent on for food would in effect get close to extinction, triggering a catastrophic population decline not only in herbivores but also in the predators that feed on them.

But what is it that makes these organisms so essential, and what should we know about them? Join us in Floofmania as we answer these questions about keystone species and more.

What Does It Mean to Be a Keystone Species?

A keystone species is a particular animal, plant, or microorganism that plays a significant role in its environment. Keystone species are important to the point that their very existence is crucial to their ecosystem’s survival.

In biology, an ecosystem is defined by its organisms as well as specific factors such as the climate and the physical environment that interact closely with one another. While ecosystems can vary in size, composition, and location, their main function is to keep everything in balance. There should be food, shelter, and space in order for every member to thrive.

An organism’s “keystone” role can come in many forms, such as being a significant component of the food chain like it’s the case with the snowshoe hare, reshaping or maintaining the physical ecosystem like elephants, or having a special relationship with its specific environment that is otherwise crucial to the way the ecosystem functions as a whole.

These are just broad categories, however, as keystone species can come in all shapes and sizes. There are no specific criteria for things like the size or appearance of organisms to be keystone species.

Researchers work on a case-by-case basis in each environment to identify a keystone species, which means that there can be a lot of debate on whether or not an organism really counts as a keystone species.

Keystone Species Aren’t Just The Most Common Animals

The size of an organism’s population is not what makes it a keystone species. It doesn’t matter if a specific organism’s numbers are in the thousands or the millions. What matters is that the keystone species plays an important role that no other creature can replace. 

Predators are often outnumbered by prey, but that doesn’t mean they can’t be a keystone species. Keystone predators help maintain the population dynamics of an environment, preventing their prey from growing too numerous or for too many predators to settle in that area.

It doesn’t matter if the predators are outnumbered ten to one because that ten percent has a more significant impact on the environment. As long as they can fulfill that role, they are still keystone species.

Why are Keystone Species Important for the Environment?

The best way to describe a keystone species is with an example. Picture an ecosystem as a Jenga tower, with each brick being a species. Removing most bricks will cause the tower to wobble and shake, but it wouldn’t collapse. After a bit of shaking, the building corrects itself, albeit weakened.

A few of the bricks, though, are harder to remove. They are the ones that seem to carry the weight of the tower, and if you try pulling them out, you will end up causing the building to collapse. These bricks are the keystone species.

Keystone species are what help define an ecosystem. Since they are so important, the ecosystem is shaped around that species and its behavior. An organism considered a keystone species has a low ‘functional redundancy.’ It means that the animal has a unique role that no other organism in the area can replace.

Where Can You Find Keystone Species?

I think a better question is where can’t you find a keystone species? In all the world’s major ecosystems, there is bound to be at least one keystone species present. It is only natural for this to happen because when various species interact with one another, some form relationships with other organisms that specifically contribute more to the environment.

The ocean is one of the largest ecosystems in the world with millions of inhabitants and a diverse array of species. To keep this menagerie of creatures in balance, several keystone species such as sharks, starfish, and coral reefs serve critical roles.

How Do You Know a Species is a Keystone Species?

Trying to determine a keystone species is a lot more complicated than people think. It is not as simple as pointing out an animal or plant and saying it’s important enough to be one. 

The ecosystem is a web where all the plants and animals are interconnected with each other and serve each their purpose, and scientists have to consider each ecosystem on a case-by-case basis.

Not only do they have to study each organism and its role in the environment, but they also have to observe outside factors like geography and climate to see how effective a species’ role is.

Research on that level can take years to accomplish facing all the different factors. Only after doing all of that, scientists can get an idea of which species is essential in a given ecosystem. 

From there, ecologists can begin to analyze the roles of each animal and determine how important they are to the environment. They can check the effects on the environment if they were to hypothetically remove a species.

However, even with the data, scientists may not be able to determine all keystone species because some of their roles might not be as clear as others.

Sometimes, biologists may not even realize a plant or animal is a keystone species until they are already starting to disappear and the effects on the environment are starting to happen.

As the old saying goes, you don’t realize what you have until you lose it. This was actually the method used by some early zoologists to study the importance of different species.

Testing For Keystone Species By Species Removal

The very first experiment done in studying keystone species was conducted by the Zoologist Robert Paine, the man who would coin the term “keystone species”.

He conducted this experiment along the shores of Washington State in the 1960s where he removed a species of starfish (sea star) from an area and observed its effects on the other animals. His research showed that removing this one starfish had completely altered the ecosystem in the area.

Several species that he normally found along the shore completely disappeared once all the starfish were gone. Robert realized that this was proof that one species can have a direct impact on the other animals inhabiting the ecosystem.

While the experiment was a success and is one of the most straightforward ways to determine the effects of a species, it also has multiple ethical problems. The experiment was criticized for knowingly tampering with the environment and directly causing environmental damage by forcibly removing an entire species.

What Are Some Types of Keystone Species?

An organism can be considered a keystone species if it serves an important role that cannot be replaced, but what that role is can vary. Keystone roles are too numerous to list, but there are some broad categories that keystone species can fit into:

  • Keystone predators 
  • Keystone prey animals
  • Ecosystem Engineers
  • Mutualists 
  • Keystone plants

Keystone Predators 

Most predators play a critical role in maintaining the environment they inhabit. These predators can help keep their prey population in check through hunting. 

A healthy environment requires a balance of predators and prey. Having too many of one can cause problems with the population. But what is it that sets a keystone predator apart from regular ones?

This role is where having a low functional redundancy is essential, and where the keystone predator plays a role that no other animal can easily replace. All predators eat meat, but they have different ways of doing it.

Some predators have a specialized diet and hunt animals that none of their fellow meat eaters eat, such as sea otters. Conversely, other keystone predators might be generalists and hunt dozens of species, which means no one meat eater can fill their role.

Otters are a good example because, in their habitat, they are among the only predators which hunt sea urchins. Without them, the sea urchin population would grow out of control as they would have no other natural predators, which would mean that sea urchins would quickly eat up local kelp forests, removing important hiding and breeding grounds for numerous fish and crustaceans.

Keystone Prey

At the end of the day, all predators need food, and keystone prey animals are considered some of the most essential parts of an ecosystem’s food chain. Normally, if a predator can’t find a certain prey, it will just hunt a different species. That might not be the case when it comes to keystone prey. 

A keystone prey serves as the primary food source for countless predators in a habitat, making it a vital element in keeping the ecosystem in balance.

While almost any animal can be seen as prey (even some predators become prey for larger predators), what sets keystone prey apart is their ability to serve as the main food source for many different predators while still maintaining a healthy population.

In this case, having a large population does matter for keystone prey. Without these keystone prey, there might not be enough animals for predators to hunt.

Think of it like this: while raccoons or squirrels are often prey for larger animals, they lack the numbers to really sustain large populations of predators. If the ecosystem’s predators were to hunt them exclusively, the raccoon and squirrel populations would be decimated. 

On the other hand, this is why snowshoe hares and wildebeest are keystone prey. There can be as many as 1,500 snowshoe hares per square mile in a given environment, which is an impressive ratio that gives dozens of predators easy access to prey.

Snowshoe hares are hunted by countless predators but still breed quickly enough that their population can easily bounce back and recover any losses.

Keystone Ecosystem Engineers

Of course, the ecosystem isn’t just about who eats who. If you study ecology, you know that living things help maintain, create, and modify the current ecosystem. Creatures responsible for that are called ecosystem engineers and are some of the most important keystone species.

Instead of impacting the food supply, these animals go beyond and alter the physical environment in some way, shape, or form. Some animals do this in the most unlikely ways that provide shelter or create ideal conditions for plants to grow. 

If we’re talking engineers, the most famous of these critters are probably the beavers. They are among the only animals to actively modify their environments to suit their needs by creating dams and lodges.

Their dams do not just assist the beavers though, as all animals feel its effects. Instead, the dams transform the habitat, turning it into a wetland that allows for other species to thrive.

Mutualists Keystone Species

A beneficial relationship between two organisms in an ecosystem is called mutualism. How the relationship works is that the two species involved both benefit from their interactions. The interaction can come in different forms, such as the creatures providing nutrients, getting food, or getting protection from either one.

A keystone mutualist is a species whose relationships impact the broader ecosystem. In cases like this, it isn’t just the two species who benefit from the relationship but the whole environment. 

An example of this relationship is between the green-backed firecrown, a hummingbird found in South America, and the local shrubs and trees. These indigenous plants provide pollen and nutrients for the hummingbird, and in turn, the hummingbird helps pollinate the plants.

The rest of the environment benefits because these plants are food sources for other animals. Without the firecrown, the plants will struggle to reproduce, and without the plants, the firecrown loses its main food source.

Keystone Plants

Who says only animals are the ones who help the environment? Plants are just as important a part of the ecosystem as any animal and, in some cases, much more so. Like animals, some plants have unique properties or fill a unique role that no one else can replace, making them keystone species.

Trees such as oaks and cherry trees are often considered keystone plants because of their roles in providing shelter for countless animals such as birds, insects, small mammals, and even some plants. Aside from that, they serve as a source of food for all the forest creatures.

Examples of Some Keystone Species

The best way to learn about keystone species is by looking at a few of those we share our world with. There are thousands of these plants, animals, and microorganisms (like fungi and bacteria), some of which we might not even know about or consider essential

Great White Sharks

One of the major keystone predators in the deep blue sea is the mighty Great White Shark. As apex predators of the sea, they aren’t an animal you can easily replace, and they serve a vital role in keeping the ocean healthy.

Being as large as they are, the great whites have few competitors and an extensive diet. They are generalist predators, which means they have a broad diet that consists of any fish smaller than them.

This matters because they can keep the populations of certain species, like manta rays, in check. It, in turn, prevents overpopulation among sea life.

With a diet as varied as that, it can also be big shoes for other predators to fill if sharks were to disappear, which is the very fact that makes them keystone species.

Sea Otters

Like sharks, sea otters also have a voracious appetite, though their eyes focus on specific types of prey, such as sea urchins. Sea urchins feed on kelp, and if they remain unchecked, they can quickly cause the entire underwater kelp forests to disappear.

These kelp forests are crucial places for fish and other sea life to live in. This means that sea otters help support, not only kelp but a wide array of species, only by feeding on sea urchins.

And if that wasn’t enough, sea otters only feed on bigger sea urchins, which in turn means that the small urchins are left alone, keeping the species and the cycle alive!

Snowshoe Hares

Although some people view these furballs as pests for the damage they do to our gardens, snowshoe hares are an essential part of the boreal forest ecosystem. Snowshoe hares are one of the primary food sources for predators like bears, bobcats, and wolves.

What sets the snowshoe hares apart from other forest creatures is their availability. While there are plenty of small mammals that predators can snack on, most of them aren’t nearly as numerous as the snowshoe hare. These little guys breed like bunnies, figuratively and literally.

Each litter of snowshoe hares can have as many as eight babies, which grow quickly. Young snowshoe hares can be running within a day of being born and venturing out on their own within the first month. That means snowshoe hares can reach adulthood and mate much faster than most forest animals.

The result is a consistently large population that can sustain many predators.

Studies have shown that snowshoe hares sometimes go through a cycle of population decline and increase. As they decline, the data shows that after a year or two, so will the populations of lynxes and bobcats.

The time gap is due to many lynxes being able to get by with the reduced population in the short term. However, in the long term, hunting at that rate is not sustainable.

With fewer hares around, female lynxes and bobcats are less likely to breed due to not having enough food for them or their cubs. That means there will be fewer lynxes and bobcats to hunt. This benefits the snowshoe hares as there will be fewer predators which make rebuilding the population easier.

Coral Reefs

Coral reefs are one of the most important parts of the ocean ecosystem. These animals not only shape the ecosystem they inhabit, but they also become part of the environment themselves.

Shelter can be hard to come by in the ocean as predators patrol the sea floor and along the waters. Coral reefs provide protection for small fishes to balance the food chain.

That is why places with large coral reef populations are some of the most biodiverse places in the world. It just shows how much of an impact these colorful corals have.

Plains Bison

The bison of the Great Plains have long captured the imagination of those who studied them, and for a good reason. Aside from being some of the largest land mammals in North America, their migration plays a major role in the plain’s habitat

When bison march along the Great Plains, their hooves compress the soil into a series of holes called wallows. When it rains, these wallows turn into makeshift pools and ponds for animals and plants to get water. 

Aside from their feet, bison fur is great for the ecosystem. Hair is an excellent building material for nests, so the migration provides free building materials for nesting birds. 

As they walk, seeds cling onto the thick bison hair, carrying them for miles where they are planted throughout the plains in ways that are impossible by other means.


There are few mutualist relationships as well known as those of bees and flowers. We learn about this relationship in books and in school. There are even movies that explore this relationship.

To make honey, bees need pollen and harvest it from wildflowers, but instead of just taking it all on their own, these busy workers make sure to sprinkle some on the other flowers, as pollen allows flowers to grow and reproduce.

Without the bees to handle this work, pollination becomes much more difficult as many flowers don’t have ways to spread the pollen on their own, and if flowers can’t pollinate, the effects can devastate the area.


One of the most unique plants in the world is the cactus. They can survive in harsh desert climates and are a crucial source of shelter and nutrients for animals.

Many birds use the cactus as a nesting area, providing shelter and elevation for them. The fruits of the cactus are some of the only sources of wet food in the desert, making it an essential part of many animals’ diets.


These titans of the savannah have a crucial role in shaping their habitat into what it is. Elephants require lots of vegetation to survive, and they get most of that from shrubs on the forest floor.

This matters because these shrubs can prevent grasses from growing by absorbing all the sunlight and water. By keeping the shrubs and small trees in check, they allow grass to grow, which other animals can eat in turn.


Microorganisms like fungi are the unsung and often unseen (because we would need a microscope to visualize them) heroes of many ecosystems.

More specifically, the Arbuscular Mycorrhiza Fungi (AMF) is crucial for plant growth. They naturally thrive in our cropland and help recycle the soil’s nutrients while detecting and adjusting nutrient allotment for many plant species.

Unfortunately, we unknowingly strip our lands of these helpful microorganisms by using toxic chemicals and industrial tools in our crops. Perhaps it would be wise to sometimes put the chemical fertilizer away and trust nature to run its course.

The fungi work to keep a plant healthy despite the stress that it can experience from the weather, parasites, and bacteria.

What Happens When a Keystone Species Disappears?

While concluding if an animal is a keystone species is complex, the results of losing one are pretty simple drastic ecological changes. We say changes because a keystone species’ disappearing doesn’t necessarily mean the ecosystem collapses. 

Most of the time, the ecosystem can survive, but it will undergo massive changes. The results depend on what role the keystone species filled and how well the other organisms can fill that role.

Keystone Species Examples of the Effects of Their Disappearance
Keystone PredatorThe loss of a major predator causes its prey population to surge. Some prey can multiply rapidly without an animal to keep their numbers in check which puts more strain on the ecosystem. Sudden surges might lead to resource shortages.
Conversely, losing a major predator might allow new, invasive species to emerge in the habitat that was kept away by the keystone predators’ presence.
Keystone PreyLosing one of their primary food sources can affect the population of predators. They may suffer a population decline or a change in diet as they search for another animal to hunt. Some may leave their habitat altogether to search for greener pastures. The result is a loss of biodiversity in the habitat.
Keystone PlantsAs they are often the start of the food chain, losing keystone plants can cause effects across the food chain, affecting herbivores and the carnivores that eat them by providing less plant matter. Aside from food, these changes can also affect shelter and plant material for animals to use.

These are just a few possible effects of losing a keystone species. We didn’t even mention the keystone ecological engineers and keystone mutualists because their effects are too broad and varied to list.

It is important to note that a keystone species does not even need to go extinct to affect the environment. But if the species becomes ecologically extinct or so severely depleted that it can no longer fulfill its niche function, that might be more than enough to trigger drastic changes.

Have Keystone Species Disappeared in the Past?

Researchers have observed a few instances of a keystone species disappearing and how it affected the environment. The effects can vary, but they are often far-ranging and detrimental to the environment’s health. 

One example was the Great White Sharks disappearing from the Atlantic Ocean.

Due to hunting and climate change, the number of these massive predators has begun to drop around the ocean. The effects of this change have led to a spike in ​​cow nose rays, one of the shark’s prey animals. Unfortunately, these stingrays grow unchecked and decimate the populations of scallops, clams, and oysters.

Another was the purple sea star which used to live in intertidal areas around the American west coast.

A sea star is, by the way, what we commonly call a starfish. However, biologists believe we should refer to this five-armed echinoderm as a sea star since it technically isn’t a fish that swims. Instead, the sea star has tiny feet-like projections in its arms to help them move and bounce on the ocean floor!


During an experiment, the zoologist, Robert Paine removed all the sea stars in a given area to observe the effect. The mussels which the sea stars usually fed on had a dramatic increase in numbers, but the other species on the coast began to decline. 

Some of these species might have been predators of the sea star, while others were affected differently. But this experiment eventually led to Robert Paine developing the idea of keystone species. It also paved the way to determine the needs of the other species living within the ecosystem. 

Author: Quade Ong

Hello there, my name is Quade. I have been a writer for three years but an animal lover for over two decades. I grew up in one of the most biodiverse areas in the world, which has given me the blessing of seeing all sorts of beautiful animals. Now I strive to learn not just about the animals I am from, but those all over the world!


  • Quade Ong

    Hello there, my name is Quade. I have been a writer for three years but an animal lover for over two decades. I grew up in one of the most biodiverse areas in the world which has given me the blessing of seeing all sorts of beautiful animals. Now I strive to learn not just about the animals I am from, but those all over the world.

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4 thoughts on “What is a Keystone Species? (Why They Matter and What You Should Know About Them)”

  1. Hi Quade great information and site! Thank you I will be using a link to direct people to my page in a seminar I am giving on Citizen Science. I hope you get lots of traffic!


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