Beavers, ecological stress and river restoration

In February 2014 a family of wild beavers were photographed on the River Otter in Devon, South West England by a retired ecologist. The animals are believed to be the first evidence of populations living and breeding outside captivity in England for over 400 years.

Their (re)discovery prompts a number of questions for the form and function of British freshwaters. What impact will the beavers return have on freshwater ecosystems and human livelihoods? What reference conditions do we use to monitor and assess restoration and reintroductions?

How can the new ecological stresses and processes caused by beavers be managed in such environmental restoration, if at all? These questions are central to the MARS project’s wider research on stress and environmental restoration.

The Eurasian beaver in the UK

The Eurasian beaver (Castor fiber) was once common across Britain, a fundamental link in many freshwater ecosystems. Populations were hunted to extinction in the 16^th century for pelt, meat and the medicinal properties of a secretion ‘castoreum’, used to variously treat headaches, fever and ‘hysteria’.

Under the European Union’s Habitat Directive, governments are required to consider the reintroduction of extinct native species such as the beaver (although the concept of what we term ‘native’ often proves tricky to tie down). As a result, there is increased debate in Britain over whether beavers should be reintroduced, perhaps most prominently shown by the experimental Scottish Beaver Trial at Knapdale, South West Scotland.

Keystone species and ecosystem engineers

Beavers are termed ‘keystone species’ by ecologists because their presence is likely to significantly affect the form and function of the ecosystems in which they live. By damming rivers, beavers are ‘ecosystem engineers’, holding back silt (which can increase water quality), creating large, slow pools in rivers and wetland areas on the banks, which in turn provide new habitat for other plants and animals.

Beavers and flooding?

Beaver dam in Lithuania (Image: Wikipedia)

This regulation in river flow may also help reduce flooding and bankside erosion downstream. Following major floods in January 2014 (see our piece on the subject), Marina Pacheco, the chief executive of the UK Mammal Society, recommended that the UK Government promote beaver reintroductions as a means of reducing flood risk in the future, stating:

“Restoring the beaver to Britain’s rivers would bring huge benefits in terms of flood alleviation. These unpaid river engineers would quickly re-establish more natural systems that retain water behind multiple small dams across tributaries and side-streams. As a consequence the severity of flooding further downstream would be greatly reduced, at no cost to the taxpayer.”

By felling and coppicing bankside trees, beavers may create a more open, diverse riverside ecosystems, which support an array of new growth and ground vegetation such as wild flowers.

Experimental trials in Devon

Early reports from experimental trial populations of beavers undertaken by the Wildlife Trust, Devon – where a pair of animals to a small woodland enclosure – show that over a two year period the beavers acted as ‘ecosystem engineers’, opening and diversifying the woodland, creating a set of ponds which store water (potentially reducing flooding downstream) and provide new habitat for other plants and animals.

‘Wild’ beavers on the River Tay

Mother and kit on Tayside (Image: Ray Scott)

However, this process may not necessarily be positive. In 2012, it was revealed that dozens of beavers had escaped from private collections across southern Scotland and had colonised the River Tay in Perthshire. A Royal Zoological Society report from 2012 suggests that there may be as many as 140 animals living along the Tay valley.

A decision by Scottish Natural Heritage not to trap the ‘feral’ animals (as quoted in the Daily Telegraph) sparked outcry from angling groups concerned about the beaver’s presence on populations of migratory fish such as salmon, and from farmers concerned about loss of land and woodland due to tree felling and dam building.

Reintroductions and restoration: what reference conditions?

The reintroduction of plants and (particularly) animals to environments from which they once went extinct is a process fraught with numerous practical and conceptual considerations. Practically, beavers have not existed in Britain for around 500 years, and freshwater ecosystems have developed in their absence over this period. This means that whilst beavers might be considered ‘native’ to Britain, their reintroduction potentially poses new stresses to the ecosystems to which they return.

Ecological reference conditions and the Water Framework Directive

Fish pass on the River Otter in Devon – what is a natural ecosystem here? (Image: Wikipedia)

A journal article “The European reference condition concept: A scientific and technical approach to identify minimally-impacted river ecosystems” published by Isabel Pardo and colleagues in Science of the Total Environment in 2012, and co-authored by MARS partner Sebastian Birk, discusses how ecological reference conditions for the restoration of ‘good status’ in rivers are set.

One of the major challenges for the Water Framework Directive – Europe’s most important policy document on freshwater health and conservation – has been to find common approaches across different countries in defining what reference conditions should be set as targets for freshwater restoration.

Given extensive human alteration of the natural world, coupled with species introductions, migrations and extinctions, selecting a set point in time or ecosystem state as a target for restoration is likely to be problematic.

The Water Framework Directive addresses this variability by stating that ‘good status’ can be defined by geographical characteristics and location. So for example, reference conditions would be set differently for a Scandinavian mountain stream compared to a lowland river in Germany.

Setting criteria for reference sites, stress and ‘ecological thresholds’

Reflecting on restoration (Image: Per Harald Olsen)

The journal article seeks to establish levels of pressure (or stress) on freshwater ecosystems at reference sites with ‘acceptable’ or ‘good’ ecological status. The paper outlines a set of criteria that the authors see as crucial for identifying reference sites, including: pollution, water abstraction, surrounding land-use and alien species.

A key concept here is ‘ecological thresholds’, which describe points where an ecosystem may significantly change form, processes and function in response to external factors such as stress.

I spoke to MARS member and article co-author Sebastian Birk about the challenges reintroduced species such as beavers pose to setting appropriate reference conditions:

“The return of the beaver to our rivers was not really anticipated when we defined the near-natural conditions. Stream impoundments are usually man-made and are thus penalized when assessing the ecological status. The activities of the beaver, however, indicate high status conditions. This may require some rethinking among water managers as well as farmers and anglers. Especially when the riparian zones along our rivers are enhanced, creating a suitable habitat for the beaver … “

Questions for restoration

The case of beaver reintroduction provides a set of challenges to this process of setting ecological reference conditions. The animals are considered ‘native’ to Britain, yet their reintroduction has been shown to significantly alter the ecosystems in which they live and ‘engineer’.

In the case of the Devon beaver trial, the ecosystem passed a threshold from woodland with a small stream, to a more open, diverse habitat with a number of ponds and marshes within two years of the reintroduction of a pair of beavers.

In a sense, beavers bring new stresses to ecosystems that have developed in their absence over recent centuries. Despite the Pardo et al 2012 paper outlining a set of quantitative criteria for ‘natural’ reference conditions for freshwater ecosystems, it could argued that, due to the endlessly altered (and altering) environment, any definitions of ‘naturalness’ are essentially subjective.

In the case of the beaver in Britain, what choices do we make over the form and function of the freshwater environments that we want to live with and enjoy?  If beaver populations continue to slowly grow across Britain, how will what we define as ‘natural’ in our rivers and lakes change, if at all?