Why do beavers build dams?
The two types of beaver are the North American beaver (castor canadensis), which lives in north America and Canada; and the Eurasian beaver (castor fiber), which live across Europe and Asia.
A beaver is a large, mostly nocturnal rodent that is well adapted for life in the water. Beavers are a keystone species. A keystone species is a species that has an impact on the environment around it to the extent where a new ecosystem arises. Beavers are responsible for entire wetland habitats that support a wide range of plants and other wildlife. Beaver dams can also purify water, slow erosion and raise the water table. When beavers desert their lodges, they become host to a wide variety of aquatic plants that spread and take over the entire pond. Shrubs and other plants soon begin to follow and the pond becomes a meadow. Once the shrubs and plants provide enough shade tree saplings arise and take over ultimately becoming woodland.
In America, the beaver is the largest rodent; they can grow up to over a meter long and weigh up to just under 30kg. The front legs of the beaver are short with large claws, the back legs are longer with the feet webbed to help with mobility when swimming. The tail of a beaver is practically hairless. When a beaver enters the water, its nose and ears close and a transparent membrane resembling a third eyelid covers the eyes so that it can see underwater. It has glands on its body that enable it to waterproof its fur with an oily substance called castoreum. Beavers have a thick layer of fat that keeps them warm whilst underwater. They can swim at speeds of up to five miles an hour and remain underwater for 15 minutes without having to come to the surface for air.
Beavers have a powerful bite and long sharp teeth that grow continuously throughout its life. They use them to cut down trees, by chewing around the base of the tree creating an hourglass shape, and then continue to chip away until the tree can no longer support itself and falls down. A beaver can cut down an average aspen tree that's 15cm in diameter in just 20 minutes.
Beavers mate with one partner for life as long as both beavers survive. They reach sexual maturity at an age of about 3 years and mate between the months of November and March, depending on the warmth of the region - in warmer areas they will breed in winter and in colder regions in spring. Female beavers are pregnant for around 3 months before they give bird to their kits. New born beaver kits can swim within 24 hours and will leave the lodge with its parents within a matter of days.
Beavers live in colonies, which comprise of one breeding male and female beaver and their children. The offspring remain with their parents for a few years before heading off to start a colony of their own. Beavers are incredibly territorial and will mark their territory with piles of mud that are covered with scent to deter any other beavers.
Where do beavers live?
Beavers are rodents whose natural habitat is closely related to water. They usually live near lakes as well as rivers and streams. Beavers live in the near vicinity of an water source such as a river, stream, pond, lake or marsh. They will desert their homes when food supplies dwindle, as travelling too far for food exposes them to potential predators.
What do beavers eat?
Beavers mainly eat tree bark and cambium - the soft material that grows directly under tree bark. Preferred bark types for beavers tend to be willow, maple, birch, aspen, cottonwood, beech, poplar and alder tree. Beaver is a unique species as it is one of very few animals that can digest tree bark. They can do this because they have colonies of microorganisms within their intestines which can digest up to 30% of the cellulose found within tree bark; they even gain nutrition from their faecal matter when it is re-ingested. Beavers will fell large trees, but not for construction - they use the bark of large trees for food or to access the smaller branches high up the tree.
Beavers, being herbivores, eat most of the plants that grow along the coast, those that are available in freshwater's littoral zone, and hydrophytes – plants that grow underwater. During winter the ponds freeze, supplying beavers with hefty amount of food they can live off of. The mud that is used to build beaver dams absorbs a lot of nutrients from flowing water, which gives the mud the perfect properties for plants to grow. Their favourite foods are water lily tubers, clover, apples and leaves. Most of these are only available during the summer, in winter they survive off of bark, saplings and shrubs.
Why do beavers build dams?
Beaver dams are a natural fortification for beavers and provide them with a line of defence against their predators. Some predators of the beaver include hawks, owls, otters, coyotes, wolves and bears.
Beaver dams provide food for the beavers during winter. Inside of the 'walls', the water level rises creating a small pond that provides a hideout for underwater entrances to a beavers' lodge, where they live. The higher water level also makes it easier for beavers to swim in, dive, and transport building materials safely, without worrying about predators attacking.
So is the beaver one of nature's geniuses? Master engineers? Amazing architects? An experiment recorded in the Nature Almanac, conducted by a grad student on why beavers build dams, suggests that this may not be the case. He realised that beavers do not always build dams - those that live in ponds, lakes and rivers seldom build dams. It was assumed that beavers build dams as an outlet for all their energy and that those that do not build dams just do alternative activities instead. The experimenter decided to take a number of pairs of beavers that have all built dams and release them in different environments and observe what they did.
The beavers that were released at ponds and large rivers burrowed into the bank and did not build anything else. Beavers released at streams deepened them by building dams at the narrowest and shallowest parts of the stream. This gave the experimenter an idea, what if the beavers felt compelled to build dams because of the sound of water rushing over stones, as the location of the dams were at the loudest parts of the stream. The sound at these locations were recorded and the recording played through speakers in locations known to be frequented by beavers.
When the experimenter went to check on his speakers the next morning they were buried deep under a pile of sticks, gravel and mud. The piles on top of the speakers were so that the sound emitting from the speakers could no longer be heard. So why do beavers build dams? Because they are programmed to cover the sound of gushing water. A beautiful example of how evolution can combat complex problems with simple solutions.
Beaver dam construction
The key building material for a beaver dam is wood. Beavers use logs and big branches as the base of a dam. Then they strengthen the construction with mud, turf, and bits of plants.
Beavers work mainly at night. They use their paws to carry mud and stone while using their mouths to carry lumber. They have specially adapted skin flaps behind their teeth so that they can carry wood without water getting into their mouths. They can carry their own body weight in building material. They use the environment around them to aid them in moving materials, and drag heavier logs across wet mud into the water.
Beavers tend to construct most during autumn, in preparation for winter. Beavers build their dams at the narrowest point of a stream. The shallowest water can be to support a beaver lodge is 0.6-0.9 meters, this is so that it cannot be blocked by ice in winter and so it can prevent predators getting into the beaver lodge. If the water is not deep, beavers will build dams to raise the water level of at the entrance of the beaver lodge.
Beavers first divert the water flow of the stream to make it easier for them to build the dam. A foundation is built on the bed of the stream using large branches and logs. Once the foundation is completed the beavers build the main structure of the dam out of small pieces of wood, rocks, mud and plants. Beaver dams tend to be around 2m tall, 1m thick and 4.5m long, they very much vary depending on where they are built. Where water is moving slowly, the dams tend to be straight, but where water flow is rapid they tend to be curved.
The dams are built with internal passages that allow overflows of water to drain through so that the structure of the dam is undamaged when there is excess water. Beaver dams are wider at the base and thinner at the top and the top tilts upstream to resist the force of the current in the stream.
After the dams are finished, and once it has flooded the desired area, the beavers then move onto construction of their lodge.
Beaver lodges are large dome structures, with one or two underwater entrances leading to a chamber above the water, which is covered in woodchips to help absorb water. They have a vent built into them to allow for fresh air circulation within the chamber. Not all beavers build lodges, they are only built if there are no suitable places to burrow into a lake or river bank. Beaver lodges are often located in the centre of the pond.
World's largest beaver dam
World's biggest beaver dam is located within the Wood Buffalo National Park in Alberta, Canada. It has a staggering length of almost 850m and it's still growing! It was discovered on Google Earth and has existed for over 30 years.
Advantages and Disadvantages of Beaver Dams
There are both positives and negatives about the impact beaver dams have on the environment. Building them can have certain benefits on water quality, neutralising acidic run-off, acting as sediment traps and sinks for pollutants. Dams can also moderate irregular flow of a stream. However, sometimes increased water levels can cause a build-up of silt and flooding in the surrounding area, especially after heavy downpours; this can kill off species that were native to the area too. Using wood as a building material can lead to deforestation of the inhabited area. Beavers can also damage human homes, infrastructure and crops - meaning many people see beavers as pests!
Advantages of Beaver Dams
- Cancels out 'ditching effect' on water tables.
- Slows down channel scouring and bank erosion.
- Filters and purifies water.
- Creates new wetland habitats.
- Creates new meadows when dams are abandoned.
- Increases biodiversity of areas, more varieties of fish, plants, birds and amphibians.
- Stabilises water supplies that can be used by other wildlife and plants.
- Reduces the impact of droughts.
- Forms new lakes and ponds.
Disadvantages of Beaver Dams
- Beavers annually cause millions of dollars of damage in the US alone.
- Damage trees and woodlands.
- They can ruin crops and other agriculture in surrounding areas.
- Can damage roads, buildings and other infrastructure.
- Can cause flooding.
- Trees felled by beavers can damage anything they land on.