Science Discussion: Beavers as a Keystone Species

The official definition of a keystone species is “a species on which other species in an ecosystem largely depend, such that if it were removed the ecosystem would change drastically.” Beavers fit this definition, and the fact that they do is important for wetland conservation efforts.

It’s sad when any plant or animal is removed from an ecosystem (via hunting, competition from invasive species, natural population dynamics, etc), but removing a keystone species in different. Think about it like dominoes. You have a bunch of dominoes in a line, and that line is the ecosystem. When a normal species is removed from the ecosystem, it’s like carefully taking a domino out of the line. It leaves a gap, but usually another species will move in and fill the niche the eliminated species left behind. The ecosystem as a whole can recover pretty easily. When a keystone species is removed from an ecosystem, instead of carefully removing the domino, you knock it into the next one and start a chain reaction that ultimately leads to the collapse of the entire ecosystem.

Removing a keystone species from an ecosystem is a lot like knocking down a chain of dominoes. Each species is a domino, and the line together is the ecosystem. You can carefully removed individual dominoes, without destroying everything, but removing a keystone species is like knocking over the lead domino -the whole ecosystem comes crashing down.
Removing a keystone species from an ecosystem is a lot like knocking down a chain of dominoes. Each species is a domino, and the line together is the ecosystem. You can carefully removed individual dominoes, without destroying everything, but removing a keystone species is like knocking over the lead domino -the whole ecosystem comes crashing down.

Conservation efforts tend to focus on protecting keystone species, which can be either plants or animals. Beavers have been identified as a keystone species in numerous ecological studies. In today’s science discussion, I will briefly discuss a handful of examples of how beavers are clearly a keystone species.

Only have a few minutes? No problem! Here’s the short answer.
Beavers are a keystone species for many reasons. It always boils down to removal of beaver leading to large gaps in the base of the natural food chain, which causes all higher trophic levels to subsequently collapse. A couple examples are as follows:

  1. Many trout and salmon populations rely on beaver dams to sort sediment and regulate water flow. Without beavers to build the dams, populations of these fish would decline rapidly. Fish are food for a lot of carnivores and omnivores.
  2. Beavers create wetlands, and there are many species of plants, frogs and birds that live exclusively in wetlands. Without beavers making wetlands, these species would have no home. Bird eggs are food for reptiles and rodents, and amphibians feed a whole host of creatures ranging from owls to bass. Aquatic plants feed just about every herbivore native to wetlands.

Have a little more time? Perfect! Here’s a more detailed explanation.
So why are beavers a keystone species? Aren’t I a little biased in my opinion of beavers seeing as how I’m dedicating my whole PhD to studying the effects of their dams? Yes. I am absolutely biased in my opinion of beavers. It should be pretty clear from reading this blog that I think beavers are awesome. That being said, I’m also a scientist who believes in the scientific process. My opinion of beavers is irrelevant in whether or not they are a keystone species. I like cats a lot too but you won’t ever catch me arguing that a domestic house cat is a keystone species. I argue as a scientist that beavers are a keystone species because the damage that happens in ecosystems the beavers are removed from has been demonstrated time and time again in the scientific literature through peer reviewed, reproducible, well thought out experiments and studies.

I BELIEVE IN THE SCIENTIFIC PROCESS! I could just tell you that beaver dams are big, or I could go out in the field with a ruler, and determine the actual height of a beaver dam and PROVE that they are big. This particular dam is about 8 feet tall at the high point. HUGE, and towards the upper limit of typical beaver dam height.
I BELIEVE IN THE SCIENTIFIC PROCESS! For example, I could just tell you that beaver dams are big because I like them and want them to be big, or I could go out in the field with a ruler and determine the actual height of a beaver dam and PROVE that they are big. This particular dam is about 8 feet tall at the high point. HUGE, and towards the upper limit of typical beaver dam height.

With that out of the way, let’s get to the facts. I’m going to briefly talk about two examples of how removing beavers and their dams would cause serious instability in the lower levels of the food web. That instability propagates upwards through the food chain and eventually the whole ecosystem comes crashing down. The fact that the ecosystem depends so heavily on the presence of beaver is what makes them a keystone species.

Beavers Keep Fish Populations Healthy, and Many Animals Eat Fish
First, beavers play a large role in the health of salmon populations. I have previously discussed in great deal the ways in which beaver dams are closely tied to salmon population health in a science discussion post called “Why You Should Care About Wetlands – Salmon is Tasty.” To summarize the main points, the positive relationship between beavers and fish is accomplished primarily through sediment sorting and regulation of stream velocity.

If you go high enough up the food chain, you find that huge animals like grizzly bears rely pretty heavily on big yearly salmon runs. If the salmon populations decline, grizzly bear populations will decline too. It can be weird to think that something as small as a beaver building a dam on a stream somewhere can wind up impacting grizzly bears hundreds of miles away, but it happens. It's a good example of the domino effect when a keystone species is removed. Species start toppling that are many degrees of separation from the actual beavers.
If you go high enough up the food chain, you find that huge animals like grizzly bears rely pretty heavily on big yearly salmon runs. If the salmon populations decline, grizzly bear populations will decline too. It can be weird to think that something as small as a beaver building a dam on a stream somewhere can wind up impacting grizzly bears hundreds of miles away, but it happens. It’s a good example of the domino effect when a keystone species is removed. Species start toppling that are many degrees of separation from the actual beavers.

Sediment sorting refers to the fact that beaver dams trap fine sediment in their ponds, and pass very “clean” water downstream. Salmon (and other related species like trout) eggs are very sensitive to fine sediments, and too much fine sediment deprives the eggs of oxygen when it settles on them, and suffocates the egg. Too much fine sediment in the water means the baby salmons and trouts can’t get born in the first place – the population crashes even before the tiny fish start swimming around. Beaver dams also slow the stream velocity downstream, which makes it easier for the very young salmon to conserve their energy and focus on feeding and getting larger instead of constantly swimming against the current.

A number of species of fish eat salmon and trout eggs, and an even greater number feed on young trout. If the beaver dams are removed then the streams are no longer as favorable of spawning grounds for these fish, and any animals that relied on the eggs or young fish for food is going to die or move on to other streams. Removing beavers causes a lot of fish populations to be removed or decreased, and the absence of fish brings down a whole host of carnivores and omnivores with it. It is clear from this example that beavers fit the definition of a keystone species.

Beavers Create Resilience in Incredibly Sensitive Wetland Ecosystems
Another example is the basic fact that beavers and their dams are responsible for creating a large number of wetlands. Literally hundreds of ecological studies have already shown that wetlands have enormous amounts of biodiversity (which just means species richness – number and variety of species) and that there are many species that can only thrive in wetlands (such as types of plants, frogs and birds). Those hundreds of ecological studies also routinely bring up the fact that wetlands are very sensitive systems and small changes in rainfall patterns, nearby land use/human impact, etc can destroy them.

A basic drawing of a wetland food web. Beavers help keep populations healthy and resilient from the bottom levels like algae and plants, all the way up to predators like birds. Thanks, beavers.
A basic drawing of a wetland food web. Beavers help keep populations healthy and resilient from the bottom levels like algae and plants, all the way up to predators like birds. Thanks, beavers.

Luckily, beaver dams naturally regulate water flow, create ponding, raise the water table, and create an environment that is nutrient rich. Having an active beaver colony in a wetland has been shown to increase that wetlands resilience to change. Even in serious droughts, the clever engineering of beavers can keep a wetland wet when all the surrounding land is parched. This implies that the beavers are also increasing the resilience of the wetland specific species of aquatic plants, frogs and birds. Frogs are food for a huge variety of animals, ranging from owls to largemouth bass. Bird eggs are a favorite snack for many lizards, rodents, and other bird species. And it should go without saying that aquatic plants are important – they are the primary food source for just about every herbivore native to wetland environments. Removing beavers causes a loss of resilience in populations of aquatic plant, amphibian, and bird species. Between the plants, frogs, and bird eggs, almost the whole base of the food web in wetland ecosystems is dependent on beavers for long term stability. It is clear from this example that beavers fit the definition of a keystone species.

A Teaser for the Next Science Discussion Post…
Another example of beavers and their dams and ponds making them a keystone species is the sheer variety and volume of insects that use their ponds and even their lodges for breeding. This example, however, is particularly complex and important, and thus I believe that it deserves its own science discussion post. Stay tuned for the next science discussion post to learn more about the relationship between beavers and insects.

One thought on “Science Discussion: Beavers as a Keystone Species

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s