Beavers

I was wandering around the woods the other day and came across a beaver dam. It wasn’t the first I’ve seen. It won’t be the last. It wasn’t even an impressively large dam either. I stopped though. I stared and I contemplated. I realized something. Beavers are cool. They construct. They shape and morph the world around them to meet their needs. Many animals simply react to the world. A deer runs when it feels threatened. It will wander around looking for food, never choosing one location as “home.” Of course other animals build dens and nests, and they may often return to them time after time. But they choose sites that fit what they need.

A beaver changes the environment around it to make a better home for themselves. They build dams to flood the area around their lodge in order to protect themselves. They build their structures using a mixture of logs, sticks, and mud. They gnaw a ring around a tree until it weakens enough to collapse. (Their teeth continually grow, making it a necessity for them to gnaw on something.) Once a tree is felled they set upon it and bite it into smaller, usable pieces. Dams are built from the bottom up. First, sticks are stuck into the bottom to provide a foundation. Next, they add sticks slowly raising the height of the dam. Mud is added to fill any gaps that might let too much water through. They continue this process of sticks and mud until the dam has reached its desired height and is strong enough to hold back the flow of water.

A beaver dam near human housing.

Beavers build dams for their own sake. But the ecological effects of these dams spread beyond them. Their dams provide a whole list of positive changes in an ecosystem. Let’s look further.

Beaver dams have the ability to flood their surrounding areas. Spreading water out over the landscape has the potential to create or increase the area of wetlands (basically an area that retains standing water throughout the year). Why are wetlands good? Wetlands are one of the most important types of ecosystem on the planet. They filter the water that comes through, removing sediment, excess nutrients, and other contaminants. Wetlands provide habitat for many species of plants, animals, insects, and bacteria. Many of which wouldn’t survive without access to wetlands.

Wetlands trap and store water. These are important effects. This helps control flooding by trapping water during periods of high flow, especially during spring snow melt. Later in the summer when the water flow drops this stored water is slowly released back into the stream system. In this way wetlands help lower the flow of water during high water times, and also raise the flow of water during low water times. They balance the flow of water throughout the year.

A crosscut of a beaver lodge. Beavers

build dams to create a pool of water deep enough 

that it doesn't freeze solid in the winter so they can 

enter their lodges all year round.

Flooded areas help recharge groundwater by increasing the surface area of water in contact with the ground. This increase in groundwater helps plants grow along the stream which in turn increases the stability of stream banks and decreases erosion.

Beavers create fish habitat. The dams slow the water down, allowing the fish to have an easier time swimming, and the pools are deep enough to not freeze. This provides winter habitat for certain fish species. (This slower water speed has added benefit of reducing streambank erosion.) The random assortment of sticks and logs in and around the dam provide protection for young fish from predators. Dams also catch plant material, which in turn can help increase the presence of aquatic insects, food for the fish. The increase of plant growth along the streambank helps shade the water. This helps keep the water cool for the fish in the hot summer.

Beavers gnaw around a tree fairly evenly
until it weakens enough to fall.

Trapped plant material plus any excess nutrients in the water can lead to increase in the amount of bacteria in the water. Bacteria usually seem like a bad thing, but in this case they’re doing a favor by removing excess nutrients in the water. Excess nutrients lead to an increase in algae present, which clouds the water. Fish prefer clear, cool water that’s free from algae and other growths. These bacteria can also metabolize some of the pollutants that may find their way into the water.

I often hear about the negative things beavers do. They remove trees near their water sources. While that is true, the damage can be mitigated by wrapping the trees with wire mesh. Also, many of their favored species of trees to use for dam building and for eating will resprout from their roots. So even if the tree is cut down by a beaver, a new one will often sprout from the roots. Beaver dams don’t block the passage of fish as much as it would seem. Many fish are able to jump over the beaver dams. Regardless of the direction they’re traveling.

The negative effects we associate with beavers are more a matter of human perception than anything else. We look at what they do in a negative light because it interferes with what we want to do in an area. Decreases in the number of fish are more from human impacts than they are from a beaver dam blocking the stream. Fields flooded by a beaver dam may indicate that the field was too close to the stream to begin with. Plant enough trees that beavers prefer and they’ll probably leave your prized tree alone. 

Beavers are cool. There are so many positive benefits from their presence that it seems silly to not embrace them. Current beaver counts in North America range anywhere from 10 to 15 million, which is decent considering they were nearly wiped out for their pelts in the 19^th century, but that number is still a fraction of the nearly 100 million that are estimated to have been present at one time. Beavers are second only to humans in their ability to impact and shape their environment. Next time you see a beaver dam or lodge think about the positive impact of them on the ecosystem.