Eager for beavers: the return of nature's elite eco-engineers

27th January 2022

Beavers are on the comeback, with Scotland making them a protected species and the Westminster government looking at their wider reintroduction. Huw Morris reports on these elite eco-engineers

For Harry Barton, the moment of truth came on Valentine’s Day in 2020. A trailblazing trial on a river in east Devon came to an end after five years, with revelatory conclusions.

Following the reintroduction of two beaver families to the River Otter, the trial discovered that this mammal could not just survive there, but thrive. The population grew to at least eight and as many as 13 family groups, with only three casualties known to observers.

Their effect on the landscape was noteworthy, particularly in terms of flood prevention (see ‘The River Otter beaver trial’, overleaf). They built dams using tree branches, vegetation, mud and rocks, while the ponds and wetlands they created hampered the river flows that risked causing flooding further downstream.

“It’s extraordinary how beavers have captured our imagination,” says Barton, chief executive of the Devon Wildlife Trust, which spearheaded the trial. “Apart from their obvious visual appeal, there is something special about bringing back a creature that has been lost for centuries – one that seems more linked to our primeval past than to our modern high-tech present. But then, beavers are no ordinary beasts.”

Indeed, these tree-felling, dam-building, wetland-creating, semi-aquatic rodents, which are capable of changing river courses, are rapidly becoming heroes in the UK’s conservation and biodiversity fightback – and they are making a comeback.


From zero to hero

Hunted to extinction in Britain 400 years ago for their fur and castoreum (a natural secretion used in perfumes and medicine), beavers were reintroduced in 2009, when the Scottish government authorised the release of beavers from Norway in Argyll’s Knapdale Forest – the UK’s first official reintroduction of a mammal species to the wild. After assessing the Knapdale trial in 2016, the Scottish government announced that the beavers could stay, and it made them a protected species in 2019. The success of the River Otter trial has encouraged the UK government to go further.

Plans to release beavers into the wild in England took a “cautious” step forward in 2020, with a government consultation on further reintroductions and the establishing of native beaver populations. Under the proposals, applications for licences to release beavers into the wild must demonstrate positive stakeholder engagement and local buy-in, as well as proof that

there has been a comprehensive assessment of their impacts on surrounding land, the water environment, infrastructure, habitats and protected species. Projects must also ensure that support is in place for landowners and river users.

The plan aims to contribute to the government’s 25-Year Environment Plan pledge on providing opportunities for reintroducing formerly native species where there are clear environmental, social and economic benefits. Ministers are considering options.

Joshua Larsen, senior lecturer in water science at the University of Birmingham, says beavers are “probably the best option available” for recovering river corridor biodiversity that thrives in or requires habitat where there is some frequency of disturbance. They are also great in wetland, meadow or freshwater habitats that have a mix of flowing and still water. “Conservation, or environmental management, and disturbance often don’t go together very well, yet disturbance can play such a critical role in creating a new mix of habitats that really benefits biodiversity – and nothing does this better in river environments than beavers.”

Busy beavers

  1. Beavers weigh between 16-30kg, and can grow to 1.2 metres long.
  2. Beavers can hold their breath underwater for 15 minutes, and their average speed in water can be up to five miles an hour.
  3. Beavers are nocturnal and spend up to 12 hours a day building and maintaining their territory, which can range up to 7km.
  4. According to the Beaver Trust, Britain’s current beaver population is estimated at 550 individuals; 450 are thought to live in the Tay catchment in Scotland, with 30 at the site of the official Scottish Beaver Trial on the nation’s west coast.

The benefits of beavers

Beaver dams slow river-flows, in turn slowing the spread of nitrates and phosphates from fertilisers, which damage fish and water quality. The dams

also create watery habitats that allow macro-invertebrates such as worms and snails to thrive – and, with them, creatures higher up the food chain. “In drier areas, they may create the only area of significant water availability and potentially act as fire buffers, while the ponds as a habitat refuge during drought,” Larsen adds.

Beavers are also good at trapping carbon by storing plant detritus in ponds, although these ponds can also be sources of carbon and methane. Larsen points out that long-term outcomes are less clear, as the amount of carbon that beavers trap depends on how long they stay in a river valley – and how willing humans are to let them remain. We need a clearer understanding of where beavers fit within river systems’ carbon cycle, he says.

“Disturbance can play a critical role in creating a new mix of habitats that benefits biodiversity”

“Beavers need the space to do their work, so obviously this can be a big constraint in areas with more human activity. However, where space is available, the tide is definitely turning. They are a bit unique in the rewilding space because they really stick to river corridors in terms of habitat range, and they are definitely the most active ecosystem engineer, compared to all other rewilding candidates.

“Rewilding definitely doesn’t mean ‘no management’, though – there needs to be very active oversight and potentially intervention to keep things on track, and I think we’re learning this to some extent from beavers through the active management and relocation programmes that exist.”

Forest and farmland factors

So what are the downsides? Beavers are natural disrupters and chomp their way through forest cover. They can also be responsible for flooding fields or some urban areas and their infrastructure.

“Beavers are definitely the most active ecosystem engineer, compared to other rewilding candidates”

Mike Jeffries, associate professor of ecology at the University of Northumbria, says the greatest potential drawback is flooding of productive farmland. He also acknowledges that the River Otter trial did not shy away from the thorny issue of returning a large mammal to a landscape that has been altered by humans – problems included blocked culverts, paths blocked by trees that had been felled by beavers, and orchard trees gnawed.

Much has been written about the wonders and horrors of reintroducing the beaver, notes Barton; he says people could forgiven for thinking beavers are either the answer to every river’s problems or will destroy the landscape. “As humans we like to think that we make decisions on hard evidence, but this is seldom the case. We are far more comfortable with our beliefs, and will accept ‘facts’ that support them and reject those that don’t.

“What we have lacked is real evidence of how these animals behave in a lowland English river, how extensive and intense their real impacts are, and how we can work with them to accentuate the good and minimise the problematic.”

The River Otter Beaver Trial

The Devon Wildlife Trust worked closely with landowners, social scientists, flood experts, biologists, geneticists, government bodies and fishing interests on a trial licensed by Natural England between 2015 and 2020.

The trial area covers the entire area of the River Otter’s 250km2 catchment, containing 594km of watercourse. The Otter rises in the Blackdown Hills before flowing through highly productive agricultural land in its middle and lower reaches, then entering the sea at Budleigh Salterton.

In March 2015, two families of wild-living beavers of unknown origin were captured from the River Otter. Detailed health examinations by experts from the Royal Zoological Society of Scotland confirmed that they were healthy Eurasian beavers, fit for re-release.

Since their introduction into the enclosed site, the beavers have built 13 dams, holding up to a million litres of additional water within ponds on the site. The results of the trial were:

  • The area influenced is 1.8ha, equating to 56 litres of surface water storage per m2 of land. During storm events, on average, peak flows were 30% lower leaving the site than entering.
  • The average lag time between peak flow into the site and peak flow out was one hour over 183 metres.
  • Even in saturated conditions and during the largest monitored flood events since the winter of 2013, the flood peaks were still reduced due to the hydraulic roughness of the dams and felled trees.
  • The water storage and gentle release effect resulted in significant and constant base-flow from the site, even in periods of drought.
Image credit | Sam Falconer | Shutterstock


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