Beaver reintroduction and its effects on freshwater biodiversity in Britain

Written By Geneve Bartholomew-Brand

FBA Info Note 3. May 2024

There is a growing interest in the restoration of the Eurasian beaver (Castor fiber) to Britain. The return of this iconic mammal not only has ethical significance, but also holds the potential to restore vital ecosystem functions and contribute to mitigating the climate and biodiversity crises.

However, due to the impoverished state of freshwater habitats in Britain, there are some uncertainties about the effects beavers may have on some priority species and natural processes. There are still unknowns regarding their interactions with some species of high conservation value, as well as the overall benefits they may bring to the modified habitats they create.

The ongoing collation of ecological and environmental data will help us to further understand the beaver's influence on different habitats and track their effects over time as their populations grow and spread. To fully utilise beaver restoration as a tool for ecosystem management and rewilding, it will require coordinated research efforts and collaboration to better understand the potential trade‑offs and predict future change.

Authors: Bernd Hänfling (Freshwater Biological Association & University of the Highlands and Islands), Guy Woodward, Laura Kahane, Lucy Jennifer Somekh, Juliet Everson, and Lucas De Ruig (Imperial College London), Martin Gaywood (NatureScot), Claire Howe (Natural England) and Nigel Willby (University of Stirling).

Background

The once-abundant populations of beavers (Castor fiber) in Europe and Asia were greatly reduced due to over-hunting for their pelts and the use of castoreum in perfumery and foods [Box 11–8]. Their populations declined so dramatically that they are believed to have gone extinct in Britain by the 16th century, although some sources suggest that a few may have persisted in certain areas for longer. Over recent decades, beaver reintroduction programmes and natural recolonisation have led to the reoccupation of much of the beaver's historical range in Europe9. Beaver reintroductions are frequently touted as model rewilding initiatives, as beavers are considered ecosystem engineers10,11. This is because of the perception that they have the capability to restore ecosystem services and alleviate or reverse the damage to freshwater habitats12 [Fig. 1]. Beavers modify landscapes primarily through dam building and tree felling or coppicing behaviours1,13. These activities can alter the physical environment including hydrological connectivity and extend biodiversity-rich wetland areas14–19. The feasibility and desirability of reintroducing beavers to Scotland started to be examined in 1995 by Scottish Natural Heritage (SNH)20,21, now called NatureScot. Since then, beavers have returned to British ecosystems through both licensed releases and unauthorised releases and escapes from captive populations. Wild, free-living beaver populations now occur at several locations in Britain. By far the largest is in Tayside, Scotland. There are three other known wild populations in Scotland, and seven areas in England and two in Wales where beavers have been recorded in the wild [Fig. 222]. There may be other areas with unrecorded beavers. There have also been several releases into fenced enclosures containing natural and semi-natural habitat.

Beaver reintroduction and management in Britain

The recognition of the potential benefits of beavers for freshwater habitat restoration led to extensive feasibility and desirability studies in Scotland in the 1990s and beyond. This comprehensive programme of work, widely recognised as one of the most thorough assessments for a species reintroduction project, resulted in a landmark report ‘Beavers in Scotland’20. This report also served as a basis for subsequent feasibility studies for beaver reintroductions in England23. As a result, beavers were formally reintroduced into Knapdale, Scotland in 2009, marking the first ever reintroduction of an extirpated mammal species in Britain21. Since then, further licensed conservation translocations have taken place in Tayside, Knapdale (a reinforcement), Loch Lomond and the Cairngorms, with others planned in Scotland. In England, wild releases are not currently authorised, but beavers can be released into large, fenced enclosures in naturalistic habitat settings under licence. The first official trials releasing beavers into semi-natural fenced enclosures started in England in 2003, and the number of releases into enclosures, primarily in England, has steadily increased [Fig. 324] . In September 2022 Scotland’s Beaver Strategy 2022-204525 was published setting out a vision of wider beaver restoration balanced with appropriate management and mitigation, a ten‑year action plan and details of how the process will be governed.

Box 1: Quick Facts

  • Common name: beaver, Eurasian beaver

  • Scientific name: Castor fiber

  • Habitat: Freshwater streams, rivers and lakes

  • Diet: Tree bark, leaves, buds and roots, aquatic and semi-aquatic plants

  • Predators: Wolves and brown bear in mainland Europe. In Britain, kits may be predated by species such as otter, pine marten, badger and fox.

  • Estimated wild-living population (individuals):
    Europe:* >1.5 million4 (up from <1200 in 19003)
    Britain:*
    England: 420-5005,6,8
    Wales: 127
    Scotland: 1500 individuals9

Ecological effects of beavers - a tool for improving degraded freshwaters?

Beavers act as ecosystem engineers as they can modify the environment considerably through dam building, digging canals and burrows, and felling and coppicing of riparian trees. In small rivers and headwater regions, beaver activity can increase standing water and wetland habitats along with the creation of water channel networks. This results in higher water tables and a diversification of habitats, ranging from actively maintained to fully abandoned ponds, fluctuating wetlands, and wet meadows. The actions of beavers contribute to the heterogeneity of habitats through the creation of features such as dams, canals, coppiced tree stands, standing and fallen deadwood, and vegetation debris13. In larger lakes and deep rivers, beavers may not engage in much dam building, but the riparian habitats still experience ecological disturbance from their burrowing and tree-cutting activities, and through herbivory of other plants.

Effects on physical processes

Beavers have the potential to cause dramatic and rapid changes to the hydromorphology of a catchment and can help mitigate the effects of extreme flows. A review of 120 studies found that these changes generally have a positive impact on local ecosystem services, but the impact on human land use needs to be taken into account26. Dams built by beavers can increase water storage capacity and reduce average flood flows by up to 6016. For example, between 2013 and 2016, 13 beaver ponds on the River Otter in the England added 18,000 square metres of water storage, which held an additional 1 million litres of water. During storm events, water took an average of 29% longer to pass through the system, and the total volume of water discharged was 30% lower than without the beaver dams27.

The impact of beaver activities on a catchment depends on its characteristics. In small rivers and upland areas, the impact may be different from rivers in lowlands, urban areas, or areas with heavily modified floodplains28. Dams built by beavers can reduce downstream flooding by slowing the flow of water, but they can also cause local flooding. For example, beaver activity has been linked to increased flooding of agricultural fields in Tayside, Scotland29,30. Such negative impacts can sometimes be mitigated by using artificial dam modification and flow control devices, but the success of these strategies varies depending on the local situation1,27.

Beaver dams and ponds can help improve water quality downstream by trapping sediment31, but beavers can also be responsible for increased levels of sediment in a watercourse resulting from their burrowing activities and dam breach events. Excess sediment can have a significantly detrimental effect on water quality through reducing visibility, inhibiting invertebrates, aquatic macrophyte and algal development through smothering or siltation and decreasing ability of fish to find prey or detect predators32.

Two photos of the same pasture

Figure 1: The impact of beavers on a drained pasture near Blairgowrie, Scotland a. 1 year post-release (2003)
b. 12 years post-release (2014) where plant species richness has increased by 46% and cumulative number of species has increased by 148%34.

Effects on freshwater species

The evidence from monitoring data indicates that beavers can play a significant role in restoring the characteristics of healthy ecosystems. Their presence has an overwhelmingly positive impact on the abundance and diversity of species in freshwater and riparian ecosystems, particularly on a large scale. These findings have been extensively reviewed in multiple studies13,24,26,33. The partial recovery of beaver populations in North America over the last century has been estimated to have generated tens of thousands of additional square kilometres of wetlands34. The creation of standing water bodies in beaver modified landscapes in Britain will be more modest but will complement efforts to replace some of the estimated 571,000 British ponds lost between 1880 and 1996 and will enhance habitat diversity across the aquatic and riparian ecosystem10,35. An individual beaver pond might not necessarily host more species than the free-flowing channel it replaced; however, it is the creation of a mosaic of diverse and dynamic freshwater and riparian habitats at a landscape scale which is most beneficial for biodiversity13,20,26,33 of many groups including vascular plants12,18,36,37, bryophytes20,26, invertebrates18,38,39, fish20,26,40, mammals13,33, amphibians41 and birds13.
There is also evidence that several declining or threatened species benefit from the habitat created by beavers13. Detailed species surveys on the River Otter revealed an increase in distribution and abundance in newly wetted areas of water vole, snipe, and teal24,27. These effects on individual species however are likely to be variable, site specific and the status of some species, potentially including some of conservation concern, might be negatively affected26,33. Aquatic and semi-aquatic macrophytes, on which beavers often feed extensively during the summer, are probably the freshwater group most directly affected. Beavers utilise a wide range of plants but tend to target those with large fleshy rhizomes such as water lilies, bulrush, bur-reed, bog-bean, saw sedge and iris and can significantly diminish populations of these species at a local level, with recovery likely to be slower in less productive environments21. However, herbivory of dominant plant species by beavers in more productive wetlands has also been found to lead to significant diversification of vegetation with benefits for smaller species36.

The presence of beavers may also result in the localised loss of lotic habitat, potential barriers to fish migration and changes in the structure of old‑growth riparian woodland13. Beaver engineering can locally displace species that prefer faster flowing, well-oxygenated water, many of which are of high conservation or economic value, like Atlantic salmon or freshwater pearl mussel13,42. Equally, there could be downstream benefits for such species if silt loads are reduced as a result of interception by beaver dams26,43. Other species which are already under multiple pressures and are therefore potentially vulnerable to any kind of additional disturbance may be of particular concern. For example, the impacts on Britain’s few remaining populations of native, white-clawed crayfish in England could be complex, with both positive and negative impacts at differing times and situations26,44. In Scotland the predicted overlap of an expanding beaver population with woodland habitat types of conservation concern such as aspen and Atlantic hazelwood could potentially have negative consequences for conservation of these habitats and associated specialist biota26,45. However, these are predominantly hypothetical concerns and empirical evidence for such direct effects or more complex multi-species interactions and long-term effects is mostly lacking. Hence, there is an urgent need to monitor long-term impacts on a wider range of aquatic/riparian species and ecosystem responses across multiple sites to further understand if, how and where beavers might interfere with other specific conservation and biodiversity targets14,46. In turn this will allow informed management decision on a local scale.

Figure 2: Map of wild-living beaver populations and semi-natural enclosures in England41.

Impacts on migratory fish

The potential effects beavers may have on freshwater fish communities are complex; current evidence includes examples of both perceived impacts and benefits to wild fish populations, reviewed in several studies13,20,26,40. It is likely that beaver activity promotes an increased fish community diversity at a broad spatial and temporal scale26. However, there is some concern that beaver activity could negatively affect migratory fish populations through the localised loss of lotic spawning habitat and also those dams, that could potentially impair the connectivity of rivers42,47,48. This is especially pertinent as globally freshwater migratory fish populations have declined by an average of 76% between 1970 and 2016 with a more pronounced decline of 93% in Europe49. Atlantic salmon and eel have seen dramatic population collapses in recent years50,51. Britain is home to several migratory fish species, for example Atlantic salmon, eel, sea trout, shad and the lamprey species, some of which are of conservation concern. Salmon and trout populations are additionally of significant economic value through recreational fishing. A 2021 stock assessment revealed that 74% (highest rate since 1988) of the principal salmonid rivers in England and 91% in Wales were ‘at risk’. Meanwhile salmon stocks have declined to the lowest level on record and are at critically low levels50.
Although the primary drivers of population decline are believed to be poor marine survival, possibly linked to climate change, over-exploitation, habitat degradation, pollution and aquaculture, some fishery managers have suggested that the generation of a more retentive and potentially fragmented habitat by beavers could potentially incur further unquantified costs to such migratory species51. The current evidence to support such a claim is equivocal, suggesting that the effect of beavers on migratory fish might be highly context dependent. For example, a study in Lithuania concluded that beaver dams reduced the accessibility of upstream spawning habitat for salmon52, while a similar study in Norway demonstrated that beaver dams do not provide an effective barrier to salmon and trout movement42. However, evidence from British rivers is currently lacking. Influences on fish might also extend beyond longitudinal connectivity to include alterations in stream temperature regimes caused by impoundment and changes in riparian shading. There is evidence from a study in the Scottish uplands that beaver structures can positively influence brown trout size and abundance through habitat manipulation53, suggesting that potential impacts on some species could be offset by increases in habitat complexity and prey availability. The effect of beaver presence on migratory fish populations in Britain will need to be monitored, and appropriate management techniques developed and applied when necessary.

Current knowledge gaps

The effects of beaver activity tend to be specific to the site, particularly in their early stages, and can vary greatly based on location. This can result in seemingly contradictory outcomes if a broader perspective is not taken into account26,31,54. The local context is likely to be important and the impacts in, for example, intensively managed lowlands or urban habitats could be very different from those in rural uplands in Britain. Although beavers are a well-researched species, the available evidence in Britain is limited. Most studies on beaver influences have been conducted on North American beavers (Castor canadensis)13, and much of the research on Eurasian beavers is from outside of Britain2,3. The 2015 “Beavers in Scotland” report and the 2020 Natural England evidence report provided a comprehensive assessment of potential beaver interactions with species, habitats, ecosystems, land and water management, and socio-economic factors, but there will be a need to further address fundamental ecological questions, particularly with respect to the reshaping of the wider food web and freshwater biodiversity-ecosystem functioning relationships. Examples of potential research include investigating the effects of beavers on:

  • species and habitats of conservation concern, which are likely to co-occur and overlap with beavers to understand how beavers may negatively or positively influence specific conservation goals.
  • greenhouse gas production, carbon sequestration and productivity28,55 . It is possible, for example, that carbon sequestration in beaver ponds might be favoured in cold, nutrient-poor waters such as in Scotland, while ponds formed in nutrient-enriched lowlands could prove to be net sources of methane.
  • aquatic-terrestrial linkages along environmental gradients. Will potential conflicts with other conservation efforts arise due to landscape re‑engineering by beavers?
  • standing water ecosystems, a topic in which beaver effects are currently less well understood. There are several fundamental differences in the hydrological processes which affect standing compared to running waters, which could lead to differing effects56 .
  • the water regime of seasonally flowing waters like winterbourne chalk streams: Britain’s chalk streams are a globally rare habitat which support a wide array of important species due to their stable, cool, nutrient-rich waters57 . If beavers dramatically alter the hydrology of these systems, chalk stream specialist species may be affected.

There is also a need to monitor and assess the efficacy of beaver management methods to help inform their improvement and development. This will require:

  • Efficient coordination of future research efforts: This will necessitate additional collaborative studies of beaver impacts in various relevant contexts in Britain, and a more ecosystem-based approach to establish the precise costs and benefits of translocating beavers in a given setting.
  • Developing effective methods to monitor the changes in biological communities in response to beaver reintroduction in various contexts: Molecular techniques (such as environmental DNA), automated analysis of audio and visual recordings, simple standardised bioassays, and remote sensing technologies can be upscaled to provide a comprehensive biodiversity baseline data. Further research and development are necessary to create metrics based on these data that can reflect the impact of these changes on ecosystem stability and functioning.

These are some of the crucial challenges that need to be addressed to effectively predict and manage the restoration of habitats by beavers across Britain. A robust evidence base is essential to make informed decisions regarding future reintroductions and other conservation translocations, and much of this can be accomplished by coordinating ongoing and planned projects to establish a solid baseline to compare against future changes, and to make these resources freely available.

bar chart showing number of fenced enclosures against year 2001-2021.

Figure 3: Cumulative growth in releases of beavers into licensed and unlicensed fenced enclosures in England between January 2000 and August 2021. Since December 2017 all releases have required a licence24.

Future perspectives
So, how do we move forward with beaver restoration to optimise positive outcomes for biodiversity, ecosystems and people, and on an individual species or community level? Scotland’s Beaver Strategy 2022-204525 attempts to address this challenge and was produced through the collaboration of over 40 stakeholder organisations from a wide range of sectors. Crucially important is to continue the debate which engages key stakeholders to identify and prioritise the most appropriate management principles for beavers29,58,59, that will provide the best balance for all parties. Critically, a comprehensive picture of the impacts of beavers must be considered from a suite of expert sources including scientists, species specialists, policy leads, land managers and vets to guide decisions about future wild releases. Our scientific understanding of beaver impacts is growing rapidly but remains incomplete in many critical areas. Co-creation of research and monitoring with stakeholders can help minimise potential conflicts and ensure that we make the best use of research insights across sites through better coordination of data collection methods and sharing. This will be facilitated by the establishment of the National Beaver Forum in England, which had its first meeting in April 2023, and which will provide a platform for sharing best practice in monitoring and managing wild beaver populations. The context-dependent nature of beaver effects means that a systematic collation of studies across a wider variety of environmental conditions will help inform management decisions in a more targeted manner. This will also help inform how to prioritise the restoration of beaver habitat and populations in the face of the wider global freshwater biodiversity crisis. Delivering maximum benefits is a key objective, and as beaver restoration continues in Britain there will be opportunities to increase the evidence base to underpin science-based management strategies over the coming decades.

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The authors

Bernd Hänfling (Freshwater Biological Association & University of the Highlands and Islands)

Guy Woodward

Laura Kahane

Lucy Jennifer Somekh

Juliet Everson

Lucas De Ruig (all Imperial College London)

Martin Gaywood (NatureScot)

Claire Howe (Natural England)

Nigel Willby (University of Stirling

Geneve Bartholomew-Brand
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