Devon beavers are officially working their magic

The latest data from Okehampton reveal incredible results. Sara King and Cain Blythe from ecological consultancy Ecosulis, and Mark Elliott from the Devon Wildlife Trust, describe the boom in biodiversity seen at the Devon Beaver Trial

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We walk down a Devon country track in late summer and emerge in a tussock-covered field where marbled white butterflies flit around. One of our group disappears with her net in pursuit, while others turn their attention, with a palpable air of anticipation, to an area of scrub, enclosed by electric fencing.

This enclosure is different, not a sight you would expect to see in Britain. Trees lie on their sides, stripped bare at the base, straddling wide, terraced pools. Thousands of delicate flies dance at the water’s edge and the pools are lined with aquatic vegetation. Mud and sticks form an array of crescent-shaped dams, some only 40cm high, others well over a metre. Each holds back large areas of ponded water and, below, a braided series of rivulets spill out, creating habitats for aquatic plants and insects. 

Only a few years ago, an ill-defined channel passed through this area of willow scrub. Now, large sunlit glades complete the picture, with flushes of culm grassland re-establishing, abuzz with a colourful insects and wetland plants

The electric fence is in place to contain a family of beavers, the makers of this exciting and novel landscape, part of a privately owned County Wildlife Site and part of the Devon Wildlife Trust’s enclosed Devon Beaver Project, near Okehampton.

Devon Beaver Enclosure
Devon Beaver Project enclosure: thriving biodiversity

An ecologist’s perspective

As an ecologist, it’s clear to see how the beavers have had a huge impact within the enclosure. Habitat variety and structure are the first things that have changed – wet areas, ponds, deadwood, open grassland, scrub and trees and areas of sphagnum. Visually, there also seems to have been overall improvement in biodiversity.

But not everyone is an ecologist and sometimes we take it for granted that everyone sees what we do. Others may just see an electric fence, or a flooded area, or not really see it at all. So how can we influence political and economic decisions if we can’t relay this message to those who don’t appreciate or see nature and wildlife in the same way? How can we say for sure that biodiversity has been improved? Based on our experience, we would expect this to be the case, but in what way has change occurred? And how does this relate to other disciplines such as hydrology?

Beavers and biodiversity

Ecosulis, driven by its shared vision of rewilding Britain, uses a Biodiversity Quality Calculator, developed by Dr Alan Feest, which measures change in biodiversity quality. 

The bespoke calculator has been used in many ways to measure change as a result of management prescriptions and to gauge the effectiveness of biodiversity off-setting schemes. More recently, it was used to measure the change in biodiversity quality, using a range of indices, as a result of the beaver reintroduction at the experimental site in Devon. 

Particular focus was given to finding out if the beavers could help maintain the open grasslands in the face of encroaching scrub species. This could allow us to see how biodiversity changes over time and could also be linked to other environmental changes, such as nitrogen or hydrology. This could then also be used to influence decisions on whether reintroductions should be undertaken on a wider scale or if management plans and prescriptions should be modified.

To do this we picked what we agreed to be the most relevant indicator groups related to change associated with the beavers: bryophytes, bats and aquatic invertebrates. One of the key benefits of the calculator is that historical as well as current data can be analysed, allowing for trends to be determined. We measured the changes in biodiversity quality between 2012 (one year after beaver introduction) and 2015 data collected by Ecosulis for bats and bryophytes (invertebrate data yet to be assessed). The data revealed some very interesting trends:


+ Increase in species richness

+ Increase in species evenness, indicating less dominance of common species

- Decrease in species dominance

+ Increase in species rarity scores on the site, including rare grey long-eared and barbastelle bats

+ Increase in biomass, indicating an increase in invertebrate prey species on the site (and number of bats)


+ Increase in species richness

+ Increase in species evenness, indicating less dominance of common species

- Decrease in species dominance

+ Increase in biomass

+ Increase in nitrogen intolerant species (indicating lower nitrogen levels)

+ Increase in species associated with well-lit areas, and species associated with acidic soils


+ Increase in species richness

+ Increase in species evenness

- Decrease in species dominance of any one species

+ Increase in population density

- Slight decrease in species rarity

Rewilding – right here, right now

The scale and direction of the changes have been compelling. By taking a relatively simple, cost-effective and standardised approach to collecting biological records, a clear picture of biodiversity change has been recorded at the Okehampton site. The increase in indices such as biomass and species rarity reveals that habitat structure and the carrying capacity of the site have increased. A rise in biomass for bats indicates higher levels of invertebrate prey, which in turn benefits other species including birds. 

The beaver have turned what was an area of dense scrub and simple channel into a mosaic of scrub, pools, dead wood, banks, culm grassland and habitat piles. After an absence of 400 years, beavers are back in England and, within a few short years, are having an amazing effect. Associated species are now diversifying and thriving, instead of declining – this is rewilding in action!

The full results are due to be published in the next Devon wildlife Trust Beaver Project update.

Beavers build nitrogen sinks

One unexpected consequence of the beaver was a potential reduction of nitrogen levels at the site, as indicated by the bryophyte assemblage recorded. By linking bryophytes with their nitrogen sensitivity we discovered that our data supports recent research that indicates beavers produce nitrogen sinks (Geographical. October 2015). This could be a handy additional tool in the argument favouring the reintroducing of beaver to Britain.

Clearly one of the fundamental principles of rewilding projects is that there is no ultimate destination. Rewilding is a journey and one that is to be shared both by people and wildlife. Like any journey, it makes sense to have a reference point to determine whether you’re heading in the right direction and are not back in the same place you started.

Our assessment measures the changes in biodiversity quality without the added value judgement of one species being more important than another. Instead, it tells you whether you have a dominance of any particular species, if you’ve recorded all the expected species present, what the spread and biomass of the species are and how this can be interpolated against the expected outcomes.

Once this quantitative assessment has been made, it can be incorporated into biodiversity and rewilding decision-making related to issues such as the location of rewilding projects, appropriate management regimes and the effects of externalities.

The next steps for the method are to help inform the debate regarding the decision to reintroduce beavers more widely back to Britain. We can also consider if this method might be applicable to other potential reintroductions such as those for pine martens or even lynx.

Method supports miracle

The Devon studies indicate that beavers are improving habitat structure and diversity through building dams, thinning out dense vegetation and creating pools and deadwood. The results, along with those from studies by Exeter University, reveal the benefits that beavers can bring, from improvements in biodiversity and water quality to flood prevention.

The studies also highlight the importance of standardised and targeted survey and assessment methods when it comes to measuring change in biodiversity. This information can now be used to inform decisions on the future for this species.