Understanding the Ecosystem Engineer

What is a beaver?

The Eurasian beaver is a large, semiaquatic, herbivorous rodent which is native to Scotland.  It is found in freshwater environments such as rivers, streams and lochs, and lives in family groups typically of 2 to 5 animals which will contain a mated pair and any offspring from that year and the year before.

They were hunted to extinction in Scotland roughly 400 years ago for their fur, meat and scent glands but have been reintroduced as part of the Scottish Beaver Trial and through unlicenced releases in Tayside. The population in Scotland is now estimated to be >1,000 animals.

Beavers are known as a keystone species due to their ability to shape freshwater habitats which benefits many other animal and plant species, but their engineering can conflict with human infrastructure and farming systems.

Below we look at common beaver activities and how they can benefit biodiversity but also present people with challenges.

Foraging and Tree Felling

Beavers are herbivores and browse on a wide range of bark, shoots, leaves and roots, from riparian to aquatic vegetation. They fell trees for food but also building materials for constructing their dams and lodges. They have a preference for tree species with soft wood such as willow, birch, alder, aspen, ash, poplar and hazel, but what they will eat is relative to what is available. The felling of native trees results in coppicing and root growth, which can extend the life of some species and result in the strengthening of river banks.

Beavers mostly feed along or close to the water’s edge to avoid predation. As a result most foraging activity occurs within around 20m or so of the water’s edge with beavers rarely moving more than 60m from water to forage.

Foraging by beavers brings a range of biodiversity benefits. Selective feeding and tree felling opens up woodland canopy, allowing more light to reach the ground, enabling the regeneration of understorey vegetation, increasing the diversity of plant, lichen and bryophyte species, and improves feeding opportunities for insectivorous birds and bats.

Opening up of the canopy also results in increased light hitting water-bodies which has been shown to increase breeding in amphibians and can raise the water temperature leading to an increase in the number of aquatic invertebrates.  But increased temperatures may favour non-salmonid species, which have a greater tolerance for lower oxygen levels in the water.

The increase in standing and felled deadwood on land and in water from beaver foraging has also been shown to provide valuable habitat for fungi, invertebrates and beetles which act as a food source for many species of amphibians, birds and fish. Deadwood, both on land and in water, provides shelter for many species from their predators, along with breeding sites. Deadwood in water increases scour which can help with the naturalisation of rivers, but may present conflicts in some locations.

Tree felling can be undesirable and hazardous in some locations. Sensitive trees in the immediate riparian zone may require mitigation to prevent them from being felled. They can be protected individually using wrapping and painting methods or as a stand using fencing. Find out how you can protect trees by wrapping them in wire mesh.

Making space for nature and creating a buffer from intensive land use along the river is an alternative form of mitigation which will allow natural regeneration of native riparian vegetation and will reduce the need for beavers to forage on mature trees.

Beaver Damming

Beavers build dams to increase and maintain the water depth in their territory to protect them from predation as they navigate it, open up new areas for browsing, and to keep the entrances to their multiple burrows and lodges submerged.

In freshwater habitats, with wide water courses and/or where water depth is naturally sufficient, such as the main channel of a river or a loch, beavers will not be physically able to build a dam.

Water levels at a site is the main decisive factor in triggering dam building activity, not the sound of running water as is often reported.

Dam building tends to occur in late summer and autumn, but beavers repair and build dams throughout the year whenever necessary.

Beavers will utilise natural and artificial features in their territory when dam-building, such as narrowing of a channel, large rocks, culverts, or fallen trees already in place. Dam placement and building technique will vary according to the hydrology of the site.

Dam construction will typically begin with a beaver depositing a base layer of mud, stones and vegetation. Larger sticks are then placed on the structure pointing upstream and weighed down with more mud and stones. Finally vegetation and mud will be used to plug any remaining gaps. Beavers have been found to skillfully integrate all sorts of available materials into their leaky structures without tree felling.

 

Beaver dams provide a host of benefits for beavers and people. The creation of new complex wetland behind a dam provides new habitat for a diverse range of plant and invertebrate species. This in turn provides breeding, foraging and shelter opportunities for a range of birds, bats, mammals and amphibians.

By impounding water and slowly releasing it through leaky structures beaver damming activity makes river systems more resilient to drought by increasing base flows in dry periods and decreasing the risk of flooding downstream during high flows.

Research has even shown that by impounding water beaver created wetlands keep plants green and soil wet during droughts and even help reduce wildfires providing a refuge for wildlife which may even help limit fire spread.

Beaver damming can cause localised flooding problems for people. Any land-manager tackling a beaver damming conflict should seek professional advice from NatureScot, as work needs to be carried without causing pollution and consideration needs to be given to animal welfare and potentially fish passage depending on the site. 

Our beaver mitigation team can be contacted by emailing [email protected].

Burrowing and Lodge Building

Lodges and burrows act as the focal point of a beaver's territory. They both provide a safe space where beavers can rest and breed protected from water and predators. Lodges and burrows are a protected feature under current legislation and cannot be disturbed or damaged without the appropriate licence.

Both lodges and burrows normally contain an entrance tunnel starting underwater, a feeding chamber at water level and a sleeping chamber above water level. 

Whether a beaver digs a burrow or builds a lodge is based on water depth and bank characteristics of their territory. In wetlands or water bodies with very low profile banks, beavers tend to build upwards, creating very visible lodges. In water bodies with deep water and steeper banks, beavers may live entirely in a range of burrows. 

Most beavers have a mixture of burrows and bank lodges in their territory, with a bank lodge built around a chamber of burrows with a visible cap on top.

Burrowing by beavers can create weakness in flood banks, increasing erosion and resulting in more dynamic river systems but it can be a health and safety risk as burrows can collapse damaging or flooding nearby property or machinery.

Any land manager experiencing an issue from beaver burrowing should seek professional advice from NatureScot, as work needs to be carried out without causing pollution and consideration needs to be given to animal welfare.

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