Ecosystem engineers

Giving a dam about beavers

Bronte McHenry
November 5, 2023
November 5, 2023
min read
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Beavers are one of the most influential and critical ecosystem engineers on Earth.  

By building a dam to protect their lodges from predators, and channels along which to ferry tree branches, they slow and spread water in a landscape, which creates a wetland ecosystem. This might not sound like a big deal, but it is. Wetland ecosystems provide numerous services, including drought prevention, water filtration, habitat creation and wildfire mitigation. 

And given the world is facing down increased droughts and wildfires, as a consequence of both climate change and biodiversity loss, wetland ecosystems — and the beavers that build and maintain them — have never been more important.

But before we get into how beavers build dams, and the main ecosystem services provided by wetlands, let’s get some facts straight. 

Contrary to popular misconceptions, beavers are vegetarian, eating bark, leaves, roots and twigs from a variety of trees. Specifically, they feed primarily on cambium, a layer of soft living tissue under the bark that carries moisture and nutrients to the tree’s leaves and branches. Beavers are plump, with webbed hind feet and large, paddle-like tails. They are the largest rodents in North America, Europe and Asia. 

At the beginning of the 20th century, beavers were hunted to near extinction in Europe and the central and southern regions of North America. According to estimates, there were 10 million beavers in Europe before the hunting began, and only several thousand survived, living in small, isolated populations across the continent.

Thankfully, the narrative has changed. Instead of being seen as a source of fur, or as a pest, beavers are increasingly recognised for the marvel they are. In fact, there’s a growing number of projects returning beavers to ecosystems, for the collective benefit of all.

A beaver dam in Grand Teton National Park.

How beavers engineer

When beavers move into a new area, they build a lodge from boulders, branches and mud. These lodges can only be accessed through underwater channels, and are so strong they’re bear-proof. 

Beavers then build a dam to surround their lodge with water, offering additional protection from predators. To do this, they construct a wall from one side of a river (or a stream, or a trickle of water) to slow the water flow significantly, flooding the landscape. The whole family works together, interlocking the timber and dredging mud from the pond bottom to seal the dam. The main body of the dam wall is boulders. The downstream side is then lined with logs, some big and very heavy. On the dam side, the wall is packed with mud and vegetation.

These dam walls are a marvel of engineering. When David Attenborough visited a dam in America, he commented on how level the dam wall was. “It's been built so accurately, that it’s horizontal to within a few inches, spanning 137 metres,” he said.  

Once the wall is built, and the water has slowed and flooded the surrounding area, beavers then dig channels that lead into the very heart of the woodland so they can swim safely to their main source of food and ferry entire branches back to the dam wall. 

In summary, beavers build dams to protect their lodges, and channels to swim for their food. But how? How does a rodent build a wall and fell a tree? The answer lies in their mouths. 

A beaver’s incisors contain so much iron that the enamel is bright orange. These teeth never stop growing and they even self-sharpen. Beavers use their powerful incisors and lower jaw muscles to raze through branches and tree trunks. A popular tree-felling technique is to slice halfway through the trunk of a tree and let gravity do the rest. (Here’s a good video if you’re intrigued.)

Beavers use their iron-strengthened incisors and the power of gravity to fell huge trees.

Beaver-boosted benefits

In creating a dam and channels, and flooding a woodland, beavers create wetland ecosystems. 

Wetlands are among the most productive ecosystems on earth, providing a swathe of services for humans and flora and fauna alike. But there are four main ecosystem services worth mentioning.

1. Drought prevention

The dam and interconnected channels slow down the flow of water and spread it out in the landscape. “This gives the water more time to soak into the soil, which ultimately keeps plants green and lush even during periods of drought,” explains Dr Emily Fairfax.

“You can think of the channels that beavers dig as drip irrigation systems running throughout the entire riparian zone.”

A beaver-dammed wetland in Baugh Creek, Idaho served as a firebreak and refuge for other species during a wildfire. Credit: Joe Wheaton/UTAH State University.

2. Fire breaks

A fire will often take the path of least resistance and burn through the dry vegetation away from beaver ponds instead of smouldering through the wet vegetation near beaver ponds. In this way, beaver wetlands become natural fire breaks where other animals can take refuge.

One study of five large wildfires in the United States found that beaver-dammed riparian corridors were relatively unaffected by wildfire when compared to similar corridors without beaver damming. 'On average, the loss of vegetation in areas without beaver dams was 3.05 times greater than in areas with beavers.

3. Filtered water

In a fast-flowing stream or river, nutrient-rich sediment is swept away. But in a dam with slowed water flow, sediment instead sinks and collects on the bottom. This abundance of minerals filters and breaks down harmful materials such as pesticides, and leaves areas downstream of dams healthier and less polluted than upstream.

4. Habitat creation

Beaver dams, ponds and channels have a range of depths and temperatures, which means a range of habitats for species with particular needs and preferences. In fact, research published in Oecologia found that the beaver presence boosted the diversity of herbaceous plant species by 33%.

Yes, some trees can’t survive in wetland conditions and die, and the ecosystem is recognisable for its gnawed-off stumps, but these dead trees provide homes for wood ducks and other cavity-nesting birds. Even the inside of beaver lodges provide homes for other animals, such as muskrats, mink and river otters. Birds nest on top of the lodges too, while fish take cover under the submerged sections.

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