Some 500 years ago they were coveted for their fur and meat and disappeared from Britain. Now beavers are making a comeback – at least in small pockets in Scotland and Devon, south-west England. And that may be good news for flood protection and regeneration of wetlands, as reported on last week. 

It seems that beavers’ dams might help prevent flooding, cleanse water as well as help boost fish populations and wetland ecosystems.

The dams regulate the water flow both during heavy rains and droughts. “When it rains, more water gets stored in ponds behind the dams, and when it’s drier, water is gently released to keep rivers flowing,” says Richard Brazier of the University of Exeter, UK, head of the study of half-a-dozen beavers confined to an isolated woodland in Devon.

Rich habitat

Four years ago, when the two founder beavers were released, the site was a tiny stream flowing through deserted woodland. Within a couple of years, they’d transformed it into a rich, wetland habitat. “They’ve built 13 dams and had a profound effect on how the water flows through the site,” says Brazier, who presented his preliminary findings in Exeter at a national beaver conference hosted by the Mammal Society, Devon Mammal Group and Devon Wildlife Trust.

Behind the staircase of dams the beavers built ponds that stored 650,000 litres during heavy rains in November last year, around a quarter the volume of an Olympic swimming pool.

This is the kind of measure that some experts believe would have eased the impact of the 2014 floods in England.

If reintroduced, they could be of most use in narrow tributaries and headwaters near the sources of major river systems where holding back water could potentially have most impact on preventing floods.

Brazier’s study, due to finish next March, also found that the staircase of dams filtered pollutants washed off farms. “We found that on average each litre coming in contains 150 milligrams of sediment, but only 40 milligrams on the way out,” he says. Likewise, nitrates arrived at average concentrations of 6 milligrams per litre, but left at less than a milligram per litre, and phosphorus levels dipped from 0.16 to 0.02 milligrams per litre.

Recycled nutrients

The sediments and nutrients that would have tainted water were instead being recycled. The silt built up in the ponds, attracting mosses and plants, followed by insects and frogs, and then large birds. “It’s the first time these multiple environmental benefits have been proven,” says Brazier.

But beavers don’t always do what humans would like them to do.

In Scotland, problems have emerged around the River Tay where there are now around 50 to 200 animals.

Martin Gayford of Scottish Natural Heritage reported in June that although the benefits echo those found by Brazier, there have also been problems, especially in areas of intensive agriculture, where beaver dams can block drainage ditches and create flood risk. They also started to gnaw through trees beside a busy road, and breached anti-flood embankments.

The National Farmers Union is opposed to reintroducing beavers because of such concerns. They say there should be a robust legal framework in place to manage beavers.
Right strategy

“If you wanted to manage beavers strategically, it’s perfectly feasible if given the time, effort and resources,” Brazier says.

Other countries where beavers have been reintroduced, such as Denmark and Norway, manage what the beavers do, relocating them if they cause disruption and removing dams if they are a threat to farmland.
So what do these varied impacts mean for reintroducing beavers?

The fate of beavers in Scotland is now under consideration by Aileen McLeod, Scottish environment minister.Gayford’s report sets out four possible options ranging from eradicating all beavers to unconstrained reintroduction.

South of the border, the Environment Agency is also keeping an eye on new research. “We are particularly interested to see whether the overall benefits of reintroducing beavers outweigh any potential risks,” says Alastair Driver, the agency’s head of biodiversity. “We’re confident these trials will provide the necessary evidence to assess the true costs, risks and desirability of future releases.”


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