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The impacts of beavers Castor spp. on biodiversity and the ecological basis for their reintroduction to Scotland, UK

Authors:
  • Forestry England
  • NatureScot

Abstract

1. In Scotland, UK, beavers became extinct about 400 years ago. Currently, two wild populations are present in Scotland on a trial basis, and the case for their full reintroduction is currently being considered by Scottish ministers. Beavers are widely considered ‘ecosystem engineers’. Indeed, beavers have large impacts on the environment, fundamentally change ecosystems, and create unusual habitats, often considered unique. In this review, we investigate the mechanisms by which beavers act as ecosystem engineers, and then discuss the possible impacts of beavers on the biodiversity of Scotland. 2. A meta-analysis of published studies on beavers’ interactions with biodiversity was conducted, and the balance of positive and negative interactions with plants, invertebrates, amphibians, reptiles, birds, and mammals recorded. 3. The meta-analysis showed that, overall, beavers have an overwhelmingly positive influence on biodiversity. Beavers’ ability to modify the environment means that they fundamentally increase habitat heterogeneity. As beavers are central-place foragers that feed only in close proximity to watercourses, their herbivory is unevenly spread in the landscape. In addition, beaver ponds and their associated unique successional stages increase habitat heterogeneity both spatially and temporally. Beavers also influence the ecosystems through the creation of a variety of features such as dams and lodges, important habitat features such as standing dead wood (after inundation), an increase in woody debris, and a graded edge between terrestrial and aquatic habitats that is rich in structural complexity. 4. In Scotland, a widespread positive influence on biodiversity is expected, if beavers are widely reintroduced. For instance, beaver activity should provide important habitat for the otter Lutra lutra, great crested newt Triturus cristatus and water vole Arvicola amphibious, all species of conservation importance. 5. Beavers are most likely to have detrimental impacts on certain woodland habitats and species of conservation importance, such as the Atlantic hazelwood climax community and aspen Populus tremula woodland. A lack of woodland regeneration caused by high deer abundance could lead to habitat degradation or loss. These are also of particular importance due to the variety of associated dependent species of conservation interest, such as lichen communities in Atlantic hazelwoods.
1
INTRODUCTION
The reintroduction of a species is often used as a tool to
improve the conservation status of the focal species.
However, it may also be used as a tool to improve the
overall species richness of the release area, to increase habitat
quality, or to improve ecosystem functioning (NSRF 2014).
Beavers Castor fiber and Castor canadensis are herbivorous
rodents (Rodentia) of riparian areas. They are widely considered
ecosystem engineers: species that create, modify or maintain
REVIEW
The impacts of beavers Castor spp. on biodiversity and the
ecological basis for their reintroduction to Scotland, UK
Andrew P. STRINGER Scottish Natural Heritage, Great Glen House, Leachkin Road, Inverness,
IV3 8NW, UK. Email: stringea@tcd.ie
Martin J. GAYWOOD* Scottish Natural Heritage, Great Glen House, Leachkin Road, Inverness,
IV3 8NW, UK. Email: Martin.Gaywood@snh.gov.uk
Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
Keywords
Castor spp., conservation translocation,
ecosystem engineer, keystone species,
meta-analysis
*Correspondence author.
Submitted: 10 September 2015
Returned for revision: 4 November 2015
Revision accepted: 18 December 2015
Editor: KH
doi: 10.1111/mam.12068
ABSTRACT
1. In Scotland, UK, beavers became extinct about 400 years ago. Currently, two
wild populations are present in Scotland on a trial basis, and the case for their
full reintroduction is currently being considered by Scottish ministers. Beavers
are widely considered ‘ecosystem engineers. Indeed, beavers have large impacts
on the environment, fundamentally change ecosystems, and create unusual habi-
tats, often considered unique. In this review, we investigate the mechanisms by
which beavers act as ecosystem engineers, and then discuss the possible impacts
of beavers on the biodiversity of Scotland.
2. A meta-analysis of published studies on beavers’ interactions with biodiversity
was conducted, and the balance of positive and negative interactions with plants,
invertebrates, amphibians, reptiles, birds, and mammals recorded.
3. The meta-analysis showed that, overall, beavers have an overwhelmingly posi-
tive influence on biodiversity. Beavers’ ability to modify the environment means
that they fundamentally increase habitat heterogeneity. As beavers are central-place
foragers that feed only in close proximity to watercourses, their herbivory is
unevenly spread in the landscape. In addition, beaver ponds and their associated
unique successional stages increase habitat heterogeneity both spatially and tem-
porally. Beavers also influence the ecosystems through the creation of a variety
of features such as dams and lodges, important habitat features such as standing
dead wood (after inundation), an increase in woody debris, and a graded edge
between terrestrial and aquatic habitats that is rich in structural complexity.
4. In Scotland, a widespread positive influence on biodiversity is expected, if
beavers are widely reintroduced. For instance, beaver activity should provide
important habitat for the otter Lutra lutra, great crested newt Triturus cristatus
and water vole Arvicola amphibious, all species of conservation importance.
5. Beavers are most likely to have detrimental impacts on certain woodland habitats
and species of conservation importance, such as the Atlantic hazelwood climax
community and aspen Populus tremula woodland. A lack of woodland regeneration
caused by high deer abundance could lead to habitat degradation or loss. These
are also of particular importance due to the variety of associated dependent species
of conservation interest, such as lichen communities in Atlantic hazelwoods.
Mammal Review ISSN 0305-1838
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2Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
A. P. Stringer and M. J. Gaywood
The ecological basis for beaver reintroduction
habitats (Jones et al. 1994). This is due, in part, to their extraor-
dinary ability to fell large trees and their ability to dam water-
courses (Wright et al. 2002, Müller- Schwarze 2011).
In Scotland, UK, the case for reintroducing beavers is
currently being considered by Scottish ministers, following
20 years of study (Gaywood 2015). Currently, two wild
populations are present in Scotland on a trial basis, and
the case for their full reintroduction is currently being con-
sidered by Scottish ministers. The reintroduction of an
ecosystem engineer may have profound consequences for
the ecosystem and landscape in the release area. Importantly,
the effects of beavers in the modern landscape may be dif-
ferent from those in the past environment (Moore et al.
1999). This is because the distributions of some species and
habitats are now much more restricted than when beavers
were last widespread, frequently as a result of human pres-
sures. In addition, woodland remnants that have persisted
in riparian areas have often become a more important
habitat for woodland species, and the abundance of large
ungulates is now likely to be much higher than previously
recorded. These factors mean that the modern Scottish
landscape does not resemble the past environment, and that
the reintroduction of an ecosystem engineer may have nega-
tive effects on some habitats or species.
The aim of this review is to assess whether the reintro-
duction of beavers to Scotland would have a broadly positive
or negative overall impact on biodiversity. Habitats and
species of conservation concern are identified that may
particularly benefit from, or may be vulnerable to, beaver
reintroduction. We use a meta- analysis to review compre-
hensively the effects of beavers on biodiversity and species
abundance. This is accompanied by a review of the mecha-
nisms that lead beavers to influence biodiversity. We use
the meta- analysis and review to identify Scottish species
and habitats that may be positively or detrimentally
impacted by beavers, following reintroduction.
METHODS
Castor fiber and Castor canadensis
It is frequently reported that North American beavers Castor
canadensis have either a greater propensity or a greater
ability to build dams than Eurasian beavers Castor fiber
(Müller- Schwarze 2011, Kemp et al. 2012). The only evi-
dence for this was found in the Russian north- west, where
invasive North American beavers and Eurasian beavers could
be directly compared. Early data suggested that there were
differences in dam- building behaviour. However, beavers
have now expanded into more comparable, adjacent areas,
and no difference in dam- building behaviour has been
observed (Danilov et al. 2011). The authors suggest that
previous results were based on comparing beavers living
in different habitats. Therefore, literature from both species
is utilised in this paper. The Eurasian beaver Castor fiber
is being considered for reintroduction to Scotland.
Meta- analysis
The online data bases ‘Scopus’ and ‘Zoological Record’ were
searched for literature relating to the two beaver species in
July 2014. All English- language literature identified as a result
of using the search terms ‘Castor fiber’, ‘Castor canadensis’,
and ‘Castor spp’ was archived. The results were then searched
for references in which the impacts of beavers on particular
species groups (e.g. amphibians) were investigated.
Studies were categorised as explicitly showing a positive,
neutral, or negative effect of beavers on species diversity,
abundance or both. The effect was then evaluated, and only
papers that included a statistical test of the effect, a suitable
control, or both, were retained for further analysis. For
example, a study in which areas affected by beavers were
compared with those unaffected would be considered to
have a suitable control.
We considered presenting the total number of species
positively or negatively affected by beaver activity. However,
with this approach, certain papers dealing with a high diver-
sity of species (such as those from the southern USA) would
dominate the analysis, and hence potentially bias any result.
Conversely, if papers were simply counted as reporting an
overall positive, neutral, or negative effect, then the result
would be biased towards species that have been the subject
of more research. The latter approach was used; however,
to reduce the bias, studies in which previously described
interactions were repeated were not included. This means
that some reported effects are much better supported by
the literature than others. In total, 49 studies were included
in the meta- analysis, with some studies included within
more than one taxonomic group. Full details of every paper
used in the meta- analysis and a description of each interac-
tion are included in Appendix S1.
Extensive reviews have already been performed for two
of the species groups (aquatic invertebrates and fish; Collen
& Gibson 2001, Hering et al. 2001, Kemp et al. 2012). A
repetition of this extensive work was judged to be unneces-
sary, hence summary of the results of those reviews are
presented here. The plant meta- analysis revealed a difference
of opinion on the impact of beavers on some tree species,
but a consensus when investigating the effects on biodiver-
sity. Both are reported here in the text, but only the effects
on biodiversity are reported in the table (Table 1).
Predicting beaver interactions in the Scottish
context
The meta- analysis findings, together with the expert judge-
ment of specialists (partly based on experience gained from
3
Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
The ecological basis for beaver reintroduction
A. P. Stringer and M. J. Gaywood
field- based projects run in Scotland, Gaywood 2015),
were used to try to predict what may happen if beavers are
reintroduced widely throughout Scotland. The potential
interactions between beavers and relevant terrestrial and
freshwater habitats and species of European conservation
importance were estimated. The species considered were those
listed in Annexes II and IV of the Habitats Directive and
occurring in Scotland, all non- marine birds listed on Schedule
1 of the Wildlife & Countryside Act 1981, and a small number
of other species of particular conservation importance. It
was not possible to assess the hundreds of other species and
habitats of conservation concern, for example, the UK
Biodiversity Action Plan and Scottish Biodiversity List species.
Some of these species may need further consideration in the
event of any future, or wider, beaver reintroduction.
REVIEW OF THE IMPACTS OF BEAVERS ON
BIODIVERSITY
Habitats and associated plants
mechanisms of beaver influence
The ability of beavers to fell very large trees is remarkable,
and perhaps only equalled by that of elephant species
(Elephantidae). This ability, alongside the propensity of
beavers for constructing structures such as dams and lodges,
means that they have a larger impact on local ecosystems
than many other herbivores (Rosell et al. 2005).
In the meta- analysis, we found 10 studies that reported
the effects of beaver activity on plant biodiversity, and
specifically effects on aquatic macrophytes, herbaceous
(vascular) terrestrial plants, and trees. Seven studies
reported a positive effect and three a neutral effect on bio-
diversity. A combination of beaver flooding and herbivory
may produce distinctive riparian habitats. The transition
from aquatic to terrestrial areas may be characterised by
flooded emergent vegetation (Grover & Baldassarre 1995,
Brown et al. 1996, Ray et al. 2001), a grass–forb–shrub
layer next to ponds (Edwards & Otis 1999, Martell et al.
2006), and then coppiced and open woodland, where forest
gaps have been created by beaver herbivory (Bulluck &
Rowe 2006). This gradual edge provides a rich structural
complexity and a variety of habitats, ultimately resulting
in high levels of plant diversity. Since dams tend to be
established irregularly along a watercourse, and because
beavers are central- place foragers (Fryxell & Doucet 1991),
the impacts of beavers are not consistent along a water-
course. Hence, landscapes that contain beavers have a
patchwork mosaic of different levels of beaver influence,
and are structurally diverse at many scales. There is also
the further influence of temporal heterogeneity caused by
the multiple successional pathways that may develop from
beaver ponds (Naiman et al. 1988). For instance, wetland
vegetation composition changes with the age of a pond
(Bonner et al. 2009). Due to either siltation or dam failure,
beaver ponds are often temporary. After a beaver pond has
returned to a terrestrial state, a beaver meadow may form.
Plant succession within beaver meadows is slower than after
other disturbances, such as fire, due to the extirpation of
soil mycorrhiza during flooding (Terwilliger & Pastor 1999).
There is also succession within the watercourse, as lentic
habitat reverts to lotic habitat. The timescale of these changes
is variable, but may be long term. For instance, beaver
meadows may persist for many decades, while ponds may
develop into emergent wetland, bogs, or forested wetland
that can remain stable for centuries (Naiman et al. 1988).
Beavers prefer to feed on tree species such as willow Salix
spp. and aspen Populus spp. Herbivory of preferred species
promotes the abundance of non- preferred species, altering
the species composition of the plant community (Donkor
& Fryxell 2000). However, there seem to be a number of
mechanisms that ensure preferred species are rarely extir-
pated. For example, aspen and willow can show rapid
regrowth after beaver browsing (Jones et al. 2009), and
aspen regrowth may be in a juvenile form avoided by beavers
(Basey et al. 1990). In contrast, willows invest in rapid
compensatory growth after herbivory, although this
regrowth may be more palatable to beavers (Veraart et al.
2006). This suggests that preferred species may have evolved
responses to beaver herbivory. In addition, the felling of
large trees opens the canopy, allowing higher light levels at
Table 1. Results from a meta- analysis of evidence investigating the im-
pacts of beavers on biodiversity. The total number of papers in which a
positive, neutral, or negative influence of beavers on species abundance or
biodiversity is shown. Papers replicating studies using the same species
were not included. Results include both beaver species. However, num-
bers within parentheses refer to Castor fiber only. Only papers in which
impacts on plant biodiversity are reported are included, impacts on spe-
cific plant species abundance are not included due to a lack of consensus
in the literature. A full explanation of interactions is provided in Appendix S1
Species group Total Positive Neutral Negative
Plants 10 (4) 7 (4) 3(0) 0 (0)
Aquatic
invertebrates
See Hering et al. (2001)
Terrestrial
invertebrates
5 (2) 5 (2) 0 (0) 0 (0)
Fish See Kemp et al. (2012)
Frogs and toads 10 (2) 8 (2) 1 (0) 1 (0)
Salamanders
and newts
8 (1) 4 (1) 2 (0) 2 (0)
Reptiles 2 (0) 1 (0) 1 (0) 0 (0)
Birds 17 (4) 15 (3) 0 (0) 2 (1)
Mammals 11 (3) 6 (2) 4 (1) 1 (0)
Total 63 (16) 46 (14) 11 (1) 6 (1)
Percentage 73% (88%) 17% (6%) 10% (6%)
4Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
A. P. Stringer and M. J. Gaywood
The ecological basis for beaver reintroduction
ground level, and aiding the recruitment of a range of spe-
cies. Furthermore, flooding and the raised water table caused
by beaver dams promote the growth of willow and alder
Alnus spp. due to their preference for wet, marshy soils
(Donkor & Fryxell 2000, Marshall et al. 2013).
In certain situations, beaver herbivory has been predicted
to have negative effects on overall biodiversity. Beavers have
been shown to prefer to feed on certain tree species.
However, this preference may change depending on the
abundance of different species in the environment. For
instance, in the Biesbosch in the Netherlands, beavers were
reintroduced into an environment dominated by willows.
Beavers were observed to select species other than willows.
It was suggested that this increased the diversity of their
diet, and allowed them to avoid dietary deficiencies (Nolet
et al. 1994). This selective herbivory of the less common
species was predicted to decrease tree biodiversity over the
long term.
On temporal and landscape scales, beaver herbivory is
variable. Beaver settlement may not be permanent and there
may be a variety of reasons for territory abandonment,
such as the depletion of resources in the area. After aban-
donment, there may be many years before recolonisation,
allowing plant species time to recover (Fryxell 2001). On
a landscape scale, beavers browse predominantly in close
proximity to water (<10 m), exhibit tree size selectivity
with distance to water, and are central- place foragers, which
results in gradients of herbivory pressure along watercourses
(Jenkins 1980, Fryxell & Doucet 1991, Hood & Bayley
2009). These mechanisms help to create a dynamic equi-
librium, preventing preferred species extirpation (Donkor
2007).
In some habitats, 60–80% of beavers’ diet has been shown
to be made up of aquatic vegetation (Milligan & Humphries
2010). However, due to the variation in abundance of
aquatic vegetation that occurs in different habitats, aquatic
vegetation may be a more important component of the
diets of pond- dwelling than stream- and river- dwelling
beavers. Beaver ponds are often rich in macrophyte diversity
(Ray et al. 2001). Indeed, by reducing dominant species
cover and increasing habitat heterogeneity, beavers have
been shown to triple macrophyte diversity within ponds
(Law et al. 2014). However, these positive effects may be
restricted to degraded habitats, and beavers may have a
neutral effect in high-quality habitats (Willby et al. 2014).
Plant biodiversity within beaver meadows is no greater
than in adjacent riparian communities. However, the com-
munity composition of these meadows is fundamentally
different from that of other riparian ecosystems. Hence,
the presence of beaver meadows increases habitat hetero-
geneity, which has been recorded increasing herbaceous
plant species richness by 33% on a landscape scale (Wright
et al. 2002).
implications for scottish biodiversity
The meta- analysis and literature review suggests that beavers
may have a range of positive benefits on plant biodiversity
in Scotland. However, their impact on preferred species may
be a concern. For instance, European aspen Populus tremula
has a restricted range in Scotland, and is a highly preferred
species of beavers. Despite the ability of aspen to regrow
rapidly, the local loss of Populus termuloides has been
reported in close proximity (30 m) to some beaver impound-
ments (Martell et al. 2006), and also on 4–5% of stream
reaches within beaver- occupied habitat (Beier & Barrett
1987). However, both the reduction in overstory density
and the transport into watercourses of felled branches (that
may act as propagules for Populus balsamifera) may increase
aspen recruitment on a wider scale (Rood et al. 2003, Runyon
et al. 2014).
An unknown factor is the influence of beavers on the
age class structure of affected woodlands. Old woodland
with large trees is important for woodland- associated com-
munities, such as lichens, and large dead wood is important
for saproxylic insects. In Scotland, aspen woodlands and
Atlantic hazelwoods harbour particularly important com-
munities. If beaver herbivory shifts the age- structure of these
woodlands towards younger growth, this may have detri-
mental effects on overall biodiversity (Gaywood 2015).
Numerous tree species can be coppiced and produce
suckers. Indeed, it has been argued that the reintroduction
of beavers into Scotland would increase the diversity of
aspen age classes throughout the landscape, with subsequent
positive impacts on biodiversity (Jones et al. 2009). However,
deer (Cervus elaphus, Capreolus capreolus, and some further
non- native Cervidae) may prevent regrowth, depending on
the amount of browsing and the tree species that is browsed
(Kuijper et al. 2010, Runyon et al. 2014). For instance, wil-
low can regrow vigorously when deer density is at medium
to low levels, particularly as the raised water tables created
by beaver impoundments can greatly improve willow
recruitment (Jones et al. 2009, Marshall et al. 2013). When
ungulate browsing is high, willow regrowth may be restricted
to hedge height (Baker et al. 2005).
By the end of the 5- year Scottish Beaver Trial at Knapdale
in mid- Argyll, 26% of beaver- browsed tree stumps were
showing regrowth. Regrowth was not equal between species.
For instance, very poor re- sprouting was observed on alder,
although overall impacts on this species were low. Ash
Fraxinus excelsior and willow showed vigorous re- sprouting,
suggesting that species differ in their ability to respond to
beaver browsing. By the end of the study, >68% of re-
sprouting stumps or tree stems from four preferred species
had been browsed by deer (Iason et al. 2014). This highlights
how high deer density could reduce the regrowth of beaver-
browsed woodland.
5
Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
The ecological basis for beaver reintroduction
A. P. Stringer and M. J. Gaywood
Invertebrates
mechanisms of beaver influence
Beaver impoundments convert lotic habitats into lentic habi-
tats. Within the ponds, the aquatic invertebrate community
changes to reflect the newly created lentic habitat. Under
such circumstances, shredders and scrapers become less
abundant, while collectors and predators become more
abundant (McDowell & Naiman 1986). Beavers may also
create unique aquatic habitats, such as channels and canals,
which support taxa that are not found in other wetland
habitats (Hood & Larson 2015). Beaver dams can support
a high diversity of invertebrates (Rolauffs et al. 2001). In
particular, the turbulent water flowing over a beaver dam,
and the increased stream velocity directly downstream of a
dam due to the head of water behind dams, may both create
rare habitat for lotic species on low- gradient stream reaches
(Clifford et al. 1993, Smith & Mather 2013).
Hering et al. (2001) thoroughly reviewed the literature
on the aquatic invertebrate community in beaver- impounded
streams and un- impounded streams. They reported that,
on a landscape scale, beaver impoundments have positive
impacts on aquatic invertebrate abundance and diversity.
The few exceptions include gravel- preferring species and
macro- invertebrate grazers that may be affected by sedi-
mentation within the beaver pond. Caddisflies (Trichoptera)
and stoneflies (Plecoptera) may also be negatively affected
due to their preference for fast- flowing reaches.
Beavers are therefore expected to increase the diversity
of aquatic invertebrates at the landscape scale. However,
beaver dams may also influence downstream areas and dis-
rupt the river continuum. Therefore, it is possible that
patches of lotic habitat between beaver impoundments will
not support the same communities as lotic habitat on beaver-
free catchments. Beaver impoundments may affect the water
chemistry, nutrient composition, sediment load, and tem-
perature of downstream reaches, and effects may be highly
variable (Rosell et al. 2005). Indeed, different types of
impoundment will have different downstream effects. For
instance, beaver impoundments with a high head dam and
low surface area force water into the ground, causing a
greater amount of cool groundwater upwelling, which ulti-
mately cools downstream temperatures. Conversely, low head
dams containing ponds with large surface areas will absorb
high levels of solar radiation that warm downstream waters.
These contrasting effects have different implications for
downstream aquatic invertebrates. Water temperature, for
example, affects the size of adult mayflies (Ephemeroptera),
which has direct implications for their reproductive success
(Fuller & Peckarsky 2011).
Numerous papers show no change in aquatic invertebrate
biodiversity downstream of beaver impoundments in com-
parison to upstream. However, species abundance and
community assemblage may change (McDowell & Naiman
1986, Arndt & Domdei 2011, Redin & Sjoberg 2013). The
influence of a beaver impoundment on downstream eco-
systems is expected to dissipate gradually with distance. For
instance, the effects of a beaver impoundment on down-
stream invertebrate assemblages has been shown to be much
reduced 100 m downstream of the beaver dam (Margolis
et al. 2001). In addition, stonefly abundance has been shown
to return to above- impoundment levels 250 m below an
impoundment (Smith et al. 1991). However, crayfish species
assemblages have been affected up to 2 km downstream
from beaver dams (Adams 2013).
Beavers may increase terrestrial invertebrate biodiversity
by increasing the abundance of dead wood, by providing
habitats such beaver meadows, and by providing beaver-
specific structures such as dams and lodges. Five studies
have investigated the impact of beavers on terrestrial inver-
tebrate diversity or species abundance, and all found a posi-
tive effect (Appendix S1). In particular, saproxylic beetles
may utilise dead, decaying, and rotting wood resulting from
beaver flooding and herbivory (Saarenmaa 1978, Zahner
et al. 2006, Horak et al. 2010).
implications for scottish biodiversity
Beaver impoundment will increase the diversity and abun-
dance of the aquatic invertebrate community at the landscape
scale. However, at high dam densities, lotic habitat may be
considerably reduced, with subsequent impacts on the inver-
tebrate community. This is important because short stream
reaches between impoundments may not resemble un-
impounded streams. This may affect some important lotic
obligates in Scotland, such as the freshwater pearl mussel
Margaritifera margaritifera. Juvenile Margaritifera margaritif-
era cannot survive in beaver ponds due to sedimentation
(Rudzite 2005). However, habitat may be improved down-
stream of dams due to a reduced water sediment load and the
regulation of stream flow (Campbell 2006). The abundance
of host fish is thought to be a key determinant of juvenile
recruitment (Johnson & Brown 1998). In Scotland, the pre-
ferred hosts for the parasitic juvenile stage of Margaritifera
margaritifera are brown trout Salmo trutta and Atlantic salmon
Salmo salar (Hastie & Young 2001). The former is expected
to benefit from beaver reintroduction, although the effects
on the latter are unknown, and so the implications for
Margaritifera margaritifera are unclear (Kemp et al. 2012).
Fish
mechanisms of beaver influence
Reviews of the impacts of beavers on a variety of fish spe-
cies are provided by Kemp et al. (2012) and Collen and
Gibson (2001). A variety of possible influencing mechanisms
6Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
A. P. Stringer and M. J. Gaywood
The ecological basis for beaver reintroduction
have been identified, and it is likely that beaver activity will
have differing effects on different fish species.
Overall, beaver impoundments replace terrestrial habitat
with aquatic habitat, thereby increasing aquatic and wetland
habitat abundance. The abundance of lentic habitat is
increased, which increases habitat heterogeneity in areas
where lotic habitat dominates. The head of water created
by a dam increases stream velocity downstream. This results
in important habitat for lotic- dependent fish species in low-
gradient watercourses. Therefore, beaver dams both increase
and decrease stream velocity at different points along the
stream reach. This fundamental increase in habitat hetero-
geneity has been shown to have positive impacts on overall
fish biodiversity (Hanson et al. 1963, Snodgrass & Meffe
1998, Smith & Mather 2013). Temporal heterogeneity is
also created due to the creation and abandonment of beaver
impoundments, and the differing effects of beaver ponds
of different ages; this has further positive impacts on fish
biodiversity (Schlosser & Kallemeyn 2000). Restoring
degraded watercourses through impoundment and increas-
ing the abundance of dead wood also increases total fish
biomass present within a stream reach (Acuna et al. 2013).
Importantly, although this describes the general impacts of
beaver activity on habitat heterogeneity and subsequent
impacts on biodiversity, there will be variation in how these
impacts influence, positively and negatively, the abundance
of any single species (Kemp et al. 2012).
implications for scottish biodiversity
Through increases in habitat heterogeneity, beavers are likely
to influence fish biodiversity positively in Scotland. However,
previous reviews identify a number of species that may be
either positively or negatively affected by beaver activity, and
the cumulative effects of different mechanisms are unknown.
For instance, lamprey Lampetra spp. and Petromyzon
marinus and Atlantic salmon Salmo salar may be unable to
pass certain dams at certain times; however, beaver activity
may also improve water quality and food abundance.
Ultimately, multiple mechanisms will interact, with unknown
repercussions on population performance (Collen & Gibson
2001, Kemp et al. 2012, BSWG 2015).
Amphibians
mechanisms of beaver influence
In the meta- analysis, we considered the frogs and toads
(Anura), and newts and salamanders (Caudata) separately,
due to common differences in habitat requirements. A posi-
tive impact of beaver activity on the abundance or diver-
sity of frogs and toads was found in eight studies. One
study found no impact, and one study found a negative
impact.
The meta- analysis highlights numerous positive effects of
beavers on frog and toad populations. A number of mecha-
nisms were proposed including increasing the size, number,
and diversity of lentic zones, which provides essential breed-
ing habitat for many amphibian species (Cunningham et al.
2007, Stevens et al. 2007). Indeed, beavers may introduce
ponds where few occur, for example in upland areas where
streams dominate (Dalbeck et al. 2007). Beaver activity may
also increase the connectivity between ponds, due to the
increased density of lentic habitat, but also due to the crea-
tion of canals by beavers (Cunningham et al. 2007). Beaver
lodges and dams may provide valuable habitat for amphib-
ians that can be used for predator avoidance, for larval food
provision and development, or as hibernation sites (Karraker
& Gibbs 2009, Browne & Paszkowski 2010, Alvarez et al.
2013). Only lotic obligates were negatively affected by beaver
activity (see Appendix S1).
It has been proposed that a higher abundance of preda-
tory fish within beaver ponds may reduce amphibian abun-
dance. However, Dalbeck et al. (2007) reported that the
increase in habitat heterogeneity caused by beaver activity
means that Salmo trutta, a key predator, does not extirpate
amphibians from impounded upland streams. In particular,
it was suggested that the creation of ponds with shallow
pond margins containing areas of submerged vegetation
and woody debris provides amphibians with protection from
predators.
Beaver activity was found to have a positive impact on
abundance or biodiversity in four studies of salamanders
and newts. Two studies found no impact, and two studies
found a negative impact. The impact of beavers on newt
and salamander species is variable. Many species of sala-
mander prefer flowing water and cannot utilise beaver ponds
(Metts et al. 2001, Dalbeck et al. 2007). On a landscape scale,
beavers may reduce the abundance of lotic habitat and replace
it with lentic habitat, hence reducing the abundance of habi-
tat for these stream- dependent species. However, there is
limited research on whether beaver impoundments degrade
lotic species habitat downstream or are barriers to migration,
and therefore the effects on lotic species at the whole stream
level. Initial data show that, on beaver- modified streams,
stream- dependent species may be abundant in un-
impounded reaches (Cunningham et al. 2007).
implications for scottish biodiversity
There are six native species of amphibian in Scotland. All
species prefer lentic habitat over lotic habitat, and hence
should be positively impacted by beaver activity. In par-
ticular, impoundment by beaver may create suitable habitat
for Triturus cristatus, as two other species from the genus
Triturus were shown to utilise older beaver ponds heavily
in central Europe (Dalbeck et al. 2007).
7
Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
The ecological basis for beaver reintroduction
A. P. Stringer and M. J. Gaywood
Reptiles
mechanisms of beaver influence
A number of researchers have observed reptiles utilising
beaver- created habitat. Cottonmouth snakes Agkistrodon
piscivorus have been observed basking on beaver lodges
(Graham 2013), while a variety of terrapins have been
observed utilising beaver ponds (Reddoch & Reddoch 2005).
The older a beaver pond was, the greater the diversity and
abundance of reptiles (Russell et al. 1999).
In two studies, the usefulness of beaver ponds as habitat
for reptiles was investigated. One showed that beaver ponds
had higher reptile abundance and biodiversity than un-
impounded streams (Metts et al. 2001). In particular, the
creation of lentic habitat, and of open habitats around ponds
due to beaver browsing, was viewed as important for ter-
rapins and lizards, respectively. However, the effects on
snakes were mixed. Yagi and Litzgus (2012) found that
terrapins exploited new aquatic habitats created by beavers;
however, flooding also reduced nesting opportunities.
implications for scottish biodiversity
The reptiles native to Scotland are the adder Vipera berus,
common lizard Zootoca vivipara, and slow worm Anguis
fragilis. Recent reports suggest that a grass snake Natrix
natrix population may also be present, and this may expand
in response to climate change. The grass snake is the only
one of these species that specialises in freshwater and wet-
land habitats and, although no research has tested the
effects of beaver impoundment on it, an increased abun-
dance of food, such as amphibians, is likely to benefit the
grass snake.
Birds
mechanisms of beaver influence
Thirty of 47 papers showed that bird species use beaver
ponds or beaver- created habitat, but this use was not com-
pared with the use of areas not affected by beavers. In the
remaining 17 studies, the differences between beaver-
impacted and non- impacted areas were investigated. Beaver
activity was found to have a positive effect on the abun-
dance of a species or on overall bird biodiversity in 88%
(n = 15) of studies, and a negative effect in 12% (n = 2)
of studies.
Numerous mechanisms were cited as reasons for increased
bird abundance or diversity. The increase in wetland area
caused by beaver impoundments is a key determinant of
avian biodiversity (Peterson & Low 1977, Grover &
Baldassarre 1995). In particular, the aquatic characteristics
of beaver ponds, such as large shallow- water areas, may be
particularly important for a variety of waterfowl (Anatidae;
Brown et al. 1996, Longcore et al. 2006).
The gradual edge characteristic of beaver habitat (see
‘Habitats and associated plants’) may be a key driver of
high bird biodiversity. It provides a structurally complex
area that may improve nest concealment, reduce predation,
increase food production, and ultimately provide a diverse
range of ecological niches to be exploited (Edwards & Otis
1999, Bulluck & Rowe 2006). The interspersion of different
vegetation types seems to be a key component of this habitat,
which can provide cover for waterfowl in particular (Beard
1953, Edwards & Otis 1999).
The ponds created by beaver dams often flood and kill
trees in the riparian zone. This attracts woodpeckers
(Picinae), since standing dead wood is an important nesting
and feeding habitat (Grover & Baldassarre 1995, Sikora &
Rys 2004, Tumiel 2008). Woodpeckers are often classified
as ecosystem engineers themselves, due to the use of wood-
pecker holes by a range of secondary cavity- nesting species
(Robles & Martin 2014). Dead trees and snags are also
important for raptors (Ewins 1997).
The habitats created by beavers provide a more abundant
food supply for birds. Beaver impoundments contain an
abundant aquatic assemblage including a diverse range of
macroinvertebrates that are an excellent food source for
ducks (Longcore et al. 2006, Cooke & Zack 2008, Nummi
& Holopainen 2014). Furthermore, an increased abundance
and diversity of fish and amphibians within beaver
impoundments provides food for species such as herons
(Ardeidae) and kingfishers (Alcedines; Beard 1953, Elmeros
et al. 2003).
Beavers may facilitate increases in bird abundance in less
obvious ways. In places where ponds are covered with ice
for much of the winter, it has been observed that beaver
activity causes the ice to melt earlier in the spring. This
brings benefits to Canada geese Branta canadensis, as it
allows them access to an important habitat for an extended
period (Bromley & Hood 2013). It may also benefit a range
of other species.
Beaver meadows can support diverse vegetation which
promotes bird biodiversity (Chandler et al. 2009), and may
be an essential source of habitat for grassland birds on a
landscape scale (Askins et al. 2007). Aznar and Desrochers
(2008) discovered that beaver meadows had the highest
levels of songbird biodiversity when compared to all other
adjacent riparian habitats.
In two studies, a negative association between birds and
beavers was found. Kuczynski et al. (2012) found that
Slavonian grebes Podiceps auritus avoid ‘borrow pits’ (man-
made ponds created during road construction) that con-
tained beavers. This may be because Podiceps auritus prefer
ponds with low surrounding forest cover (<33% within
500 m), and hence they prefer habitat less suitable for
8Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
A. P. Stringer and M. J. Gaywood
The ecological basis for beaver reintroduction
beavers. However, where sedge beds are not present, Podiceps
auritus use willow for nesting, and beavers may reduce the
abundance of willow in certain situations. Whitethroat
Sylvia communis abundance was also observed to decline
at local levels after beaver reintroduction into Denmark
(Elmeros et al. 2003).
In summary, beavers create a diverse habitat rich in struc-
tural complexity, which supports an avian diversity greater
than may be expected from a riparian area unaffected by
beavers, including bird species that may not normally be
associated with wetlands (Reese & Hair 1976). The structur-
ally and temporally heterogeneous habitat created by beavers
supports a highly diverse bird fauna on a landscape scale.
implications for scottish biodiversity
The meta- analysis shows that, given that beavers are known
to create diverse habitats rich in structural complexity, their
presence is likely to result in a greater avian diversity than
may be expected from the existing remnant riparian habitats
in Scotland. A potentially detrimental mechanism is the
change in age structure of riparian woodland; hence bird
species strictly dependent on old woodland may be detri-
mentally affected (Livezey 2009). This may be further exac-
erbated if tree regeneration is limited by deer grazing. If
deer grazing can be controlled, the increased structural
diversity resulting from the cyclical coppicing and regrowth
of riparian trees by beavers is likely to open niches for
species not found in mature closed- canopy woodland, for
example tree pipits Anthus trivialis. The increased shrub
layer will also create habitat for a range of insectivorous
songbirds, particularly warblers. Inundation of woodland,
leading to the death of standing trees, would also create
feeding and nesting opportunities for a range of bird spe-
cies, including raptors, dead wood feeders such as the nut-
hatch Sitta europea, and woodpeckers.
Mammals
mechanisms of beaver influence
Studies investigating the impact of beavers on mammalian
diversity and abundance were investigated. In 25 of 36
papers, mammalian species were described as using beavers
as prey, or utilising beaver ponds, or other beaver- created
habitat, but this was not compared to areas without beavers.
In the remaining 11 studies, differences were investigated
between beaver- impacted and non- impacted areas. Beaver
activity was found to have a positive effect on the abundance
of a species, or on overall mammalian species diversity, in
55% (n = 6) of these 11 studies. No difference was found
in 36% (n = 4) of the studies. In a single study, a negative
impact of beaver meadows on bat species diversity was
found.
Four studies within the meta- analysis were focused on
bats; in two, a positive impact of beaver activity was found.
Nummi et al. (2011) showed that beaver- created ponds
supported a higher abundance of bats than non- beaver
ponds. Bats are thought to benefit from beaver activity due
to an increase in prey abundance and availability and due
to improved foraging habitat due a reduction in woodland
density (Ciechanowski et al. 2011). Bats may also utilise
beaver habitat in other ways, for example, by roosting under
the exfoliating bark of trees killed by beaver flooding
(Menzel et al. 2001). When beaver ponds succeed into beaver
meadows, any benefits for bats seem to be lost, as meadows
are poorer bat habitat than adjacent riparian habitats
(Brooks 2009).
Otter species (Lutrinae) are likely to benefit from beaver
activity. Through impoundment, beavers increase the
amount of suitable aquatic otter habitat. The ponds formed
are often rich in prey species such as fish, amphibians, and
invertebrates. Abandoned beaver lodges and bank dens may
also provide important shelter for otters such as the North
American river otter Lontra canadensis (Newman & Griffin
1994, Swimley et al. 1999). Gallant et al. (2009) showed
that beaver- created habitat is an important predictor of
North American river otter distribution.
Small terrestrial mammals do not seem to be impacted
by beaver activity (Hanley & Barnard 1999, Suzuki &
McComb 2004). However, a diverse range of small mam-
mals are known to use beaver lodges (Ulevicius & Janulaitis
2007).
Beavers may influence large mammals, as creators of
habitats, sources of prey, and because trees felled by beavers
may provide food for numerous browsing ungulates (Baker
et al. 2005, Rosell et al. 2005). However, Nelner and Hood
(2011) reported that beaver activity had no influence on
large mammal diversity or abundance in either protected
areas or agricultural landscapes, although they did con-
clude that beavers were important for maintaining water
levels in agricultural wetlands, and therefore ecological
heterogeneity.
implications for scottish biodiversity
The beneficial effects of beavers on mammalian diversity
and abundance are likely to be seen in Scotland. Effects on
otters and bats are the examples best supported by the lit-
erature. In addition, beaver presence is likely to result in
new and improved habitat for the European water vole
Arvicola amphibious. Water voles have a strong preference
for slow- moving water with abundant aquatic, emergent,
and herbaceous bankside vegetation, all features that are
characteristic of beaver ponds. A key management technique
already used to improve water vole habitat is the thinning
of woody riparian vegetation (Field 2009), which beavers
9
Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
The ecological basis for beaver reintroduction
A. P. Stringer and M. J. Gaywood
will also do. However, predation of water voles by the non-
native American mink Neovison vison has been a major
factor in the extinction of water voles in many Scottish
main river stems and tributaries to date. Therefore, the
apparent avoidance of beaver- modified habitat by mink
reported from Patagonia (Schüttler et al. 2010) and Russia
(Kiseleva 2008) is interesting and, if this pattern is translated
to Scotland, could have important implications for the
future strategic management of mink.
Invasive non- native species
Beavers have been known to have both positive and negative
effects on invasive non- native species abundance (Perkins
& Wilson 2005, Parker et al. 2007). In Scotland, beaver her-
bivory may reduce invasive non- native species abundance.
For instance, Rhododendron maximum, a parent of the inva-
sive complex hybrid Rhododendron ponticum, the invasive
parrot’s feather Myriophyllum aquaticum, and Elodea spp.
(including Elodea nutallii and Elodea canadensis which are
invasive in the UK) are food species for beavers (Allen 1982,
Dams et al. 1995, Parker et al. 2007), although it seems
unlikely that beavers would exert a controlling influence on
these plants. However, herbivory may also increase the dis-
persal of some invasive species. For instance, beaver her-
bivory of Elodea canadensis may create numerous smaller
fragments of the pondweed. Each of these fragments may
act as a propagule for the species (Willby et al. 2014).
The wetland conditions created by beavers may also pro-
vide habitat for invasive non- native species, such as the
Mandarin duck Aix galericulata, as beaver ponds are a pre-
ferred habitat of the closely related wood duck Aix sponsa
in North America (Folk & Hepp 2003).
OVERVIEW
Results of meta- analysis
The results of the meta- analysis (Table 1) demonstrate that,
overall, beavers have an overwhelmingly positive influence
on biodiversity. Beavers influence biodiversity by increasing
habitat heterogeneity. The process of pond creation and
subsequent rescindment creates an abundance of temporal
habitat diversity, providing a variety of successional stages.
Hence, a mosaic of beaver impoundments at different stages
throughout a landscape, combined with beaver herbivory
that is unevenly spread in both time and space, is expected
to provide a high level of abundance of habitat heterogene-
ity, and hence biodiversity. Other ways in which beavers
may have a positive impact on the abundance or diversity
of a large variety of species include:
• Creation of pond habitat and associated changes in water
chemistry and bed substrate.
• Changes in water chemistry immediately downstream of
beaver ponds.
Direct creation of important habitat features such as dams
and lodges.
Indirect creation of important habitat features such as
standing dead wood after inundation.
Influx of woody debris into both lentic and lotic
environments.
Habitat created by the response of vegetation to herbivory,
such as coppiced stands and juvenile forms of woody plant
species containing high levels of anti-herbivory defence
chemicals.
• The creation of a unique vegetation structure due to the
combination of flooding with tree felling.
The unique successional stages that result from beaver
impoundment, such as beaver meadows.
Many of these are unique to beavers and hence result in
rare or unique habitats. Impacts may reverberate through
trophic levels. For instance, positive impacts on the abun-
dance or diversity of invertebrates may have a variety of
impacts on species that prey on them, such as amphibians,
fish, mammals, and birds. In Scotland, there are likely to
be positive impacts for a number of species of conservation
interest such as otters, water voles, and great crested newts.
However, a number of potential negative impacts were also
identified during this review, with potential implications
for Scotland. These include the following:
Beavers cause disturbance, and while disturbance is a
fundamental influence on ecological landscapes, it may
reduce the extent of old-growth riparian woodland com-
munities, or shift the age structure of a woodland
towards younger growth. This can be a negative impact
if old-growth woodland is rare and if a large propor-
tion is impacted, or if ecological continuity is affected.
Two habitat types of conservation importance that fulfil
these criteria in Scotland are aspen woodland and
Atlantic hazelwood. Deer in high abundance may also
prevent the regeneration of woodland species, which
may lead to localised effects on the quality of some
habitat types.
• The creation of lentic habitat often involves the replace-
ment of lotic habitat. At high dam densities, this may be
detrimental to lotic obligates, as the habitat of stream
reaches between impoundments may not be as suitable
as those in streams with no beaver impoundments or
with a low density of impoundments.
Overall impacts on certain fish species are unknown, in
particular on Atlantic salmon and species of lamprey.
While many positive and negative mechanisms have been
proposed, further research is needed to elucidate the
overall impacts on populations of affected species.
10 Mammal Review (2016) © 2016 The Mammal Society and John Wiley & Sons Ltd
A. P. Stringer and M. J. Gaywood
The ecological basis for beaver reintroduction
CONCLUSIONS
This review demonstrates that beavers, if widely reintro-
duced, can be expected to have many positive effects on
the biodiversity of Scotland. Beavers promote biodiversity
through a variety of mechanisms, primarily by increasing
habitat heterogeneity and creating unique habitats. Beavers
may also help restore riparian habitat and provide a
natural means of restoring incised streams (Pollock et al.
2014).
All native species in Scotland evolved alongside beavers.
However, the reintroduction of beavers may have detri-
mental impacts on certain species and habitats. Threatened
species may now rely on habitats in riparian corridors that
have become increasingly important refuges for them since
beaver extirpation. High deer density may affect tree re-
growth in some areas, resulting in beaver- influenced habitat
not resembling any past environment (Baker et al. 2005).
Climate change may also have important implications for
the distribution of species in Scotland. For example,
reduced rainfall may restrict some lichen communities to
riparian areas, so that a greater proportion of these com-
munities may be impacted by beavers than in the past
environment. However, beavers may also help to mitigate
against the effects of climate change by stabilising flow
within watercourses.
Atlantic hazelwood, European aspen, and some other
woodland habitats would require close monitoring where
they overlap with potential beaver habitat to assess any
potential impacts (Gaywood 2015). These vulnerable spe-
cies and habitats also harbour a number of important
dependent species, such as lichens associated with Atlantic
hazelwoods. In certain cases, these will require additional
management. In particular, woodland regeneration follow-
ing beaver activity is possible at low to medium deer den-
sities, but at the high deer densities currently experienced
over many parts of Scotland, regeneration could be sig-
nificantly affected. A co- ordinated approach to deer and
beaver management in such areas would therefore be
needed. If the decision is made to reintroduce beavers
more widely in Scotland, an appropriate management
strategy would be required to set out how negative impacts
can be minimised, and how positive impacts can be
promoted.
ACKNOWLEDGEMENTS
Many thanks to Iain MacGowan, Iain Macdonald, Dr Dave
Genney, Jeanette Hall, Dr Athayde Tonhasca, Dr Iain Sime,
Angus Tree, Prof. Colin Bean, John McKinnell, Simon
Cohen, Rob Raynor, Dr Roo Campbell, and Prof. Des
Thompson for their helpful comments on drafts of this
manuscript.
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SUPPORTING INFORMATION
Additional supporting information may be found in the
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Appendix S1. Details of the meta- analysis with mechanisms
and references as summarised in Table 1.
... Beaver created wetland habitats are heterogeneous and structurally complex in nature Sommer et al., 2019;Stringer & Gaywood, 2016;Wright et al., 2002) which allows them to support rich biodiversity (Smith & Mather, 2013;Willby et al., 2018). Beaver wetland habitats are capable of supporting greater biodiversity than anything humans can replicate (Willby et al., 2018), significantly increasing abundance of wildlife, including of plants (Law et al., 2014(Law et al., , 2017Willby et al., 2018), invertebrates (Bush & Wissinger, 2016;Stringer & Gaywood, 2016;Willby et al., 2018) fish (Kemp et al., 2012, amphibians (Dalbeck et al., 2007(Dalbeck et al., , 2020, in addition to birds and mammals (Nummi et al., 2019;Nummi & Holopainen, 2014;Stringer & Gaywood, 2016). ...
... Beaver created wetland habitats are heterogeneous and structurally complex in nature Sommer et al., 2019;Stringer & Gaywood, 2016;Wright et al., 2002) which allows them to support rich biodiversity (Smith & Mather, 2013;Willby et al., 2018). Beaver wetland habitats are capable of supporting greater biodiversity than anything humans can replicate (Willby et al., 2018), significantly increasing abundance of wildlife, including of plants (Law et al., 2014(Law et al., , 2017Willby et al., 2018), invertebrates (Bush & Wissinger, 2016;Stringer & Gaywood, 2016;Willby et al., 2018) fish (Kemp et al., 2012, amphibians (Dalbeck et al., 2007(Dalbeck et al., , 2020, in addition to birds and mammals (Nummi et al., 2019;Nummi & Holopainen, 2014;Stringer & Gaywood, 2016). Importantly, this increase in biodiversity occurs at the landscape level (Rosell et al., 2005;Sommer et al., 2019;Willby et al., 2018;Wright et al., 2002). ...
... Beaver created wetland habitats are heterogeneous and structurally complex in nature Sommer et al., 2019;Stringer & Gaywood, 2016;Wright et al., 2002) which allows them to support rich biodiversity (Smith & Mather, 2013;Willby et al., 2018). Beaver wetland habitats are capable of supporting greater biodiversity than anything humans can replicate (Willby et al., 2018), significantly increasing abundance of wildlife, including of plants (Law et al., 2014(Law et al., , 2017Willby et al., 2018), invertebrates (Bush & Wissinger, 2016;Stringer & Gaywood, 2016;Willby et al., 2018) fish (Kemp et al., 2012, amphibians (Dalbeck et al., 2007(Dalbeck et al., , 2020, in addition to birds and mammals (Nummi et al., 2019;Nummi & Holopainen, 2014;Stringer & Gaywood, 2016). Importantly, this increase in biodiversity occurs at the landscape level (Rosell et al., 2005;Sommer et al., 2019;Willby et al., 2018;Wright et al., 2002). ...
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Biodiversity is declining in the UK, which is considered one of the most nature-depleted parts of the world. The reestablishment of the Eurasian beaver (Castor fiber) has been posited as an effective means of facilitating a restoration of biodiversity in Britain, and following successful trials, nationwide reintroduction is being considered. This literature review considers the potential psychological benefits of such an initiative. Beavers could act as a 'super restorer', facilitating psychological as well as ecological restoration through a beneficial synergy of effects. Through their eco-engineering activities, beavers increase biodiversity at the landscape scale and facilitate habitat restoration and creation (creating a mosaic of green and blue space, and a sense of wilderness) all of which can increase the psychological well-being of visitors. Their creation of biodiverse natural settings offers the possibility of increased nature connectedness and nature-based psychological restoration amongst some of the human population of the UK. Beaver reintroduction may represent a partial antidote to 'shifting baseline syndrome' and beavers could act as a flagship species and become a totem of hope as eco-anxiety increases. Beavers can potentially have negative psychological impacts, and this will require appropriate planning, management and communication among stakeholders coupled with community-led initiatives to mitigate. Overall psychological benefits of beaver reintroduction likely exceed that of any other single species' reintroduction or conservation initiative of equivalent cost, and far outweigh the costs of their reintroduction and management.
... Beaver effects on species abundance and diversity were intensively studied and evaluated in the last decades. In their meta-analysis, Stringer and Gaywood (2016) evaluated the effects of beavers on species abundance and diversity. They found that 73% of the published studies reported a positive, 17% a neutral, and 10% a negative effect on biodiversity. ...
... However, the positive and negative effects were not evenly distributed across the different taxonomic groups. For example, the positive effects of beavers on the diversity and productivity of plant species were attributed to flooding and foraging, which create new habitats not commonly associated with forests and riverine areas (Stringer and Gaywood, 2016). Furthermore, the abundance of aquatic invertebrates and the productivity of the respective waters were shown to benefit from beavers because of the larger lentic zones they create (Hering et al., 2001). ...
... Most bats species benefit from the increased abundance and availability of food, as the expanded area of open water area provides them with better hunting opportunities (Nummi et al., 2011); however, for some bat species local detrimental effects were observed (Ciechanowski et al., 2011). With the exception of one study (Kuczynski et al., 2012), positive effects on bird species were also found, induced by an increase in food production and habitat heterogeneity (Stringer and Gaywood, 2016). For most terrestrial mammals, the positive effects are relatively small and occur mainly in areas where beavers have been present for decades (e.g., Nummi et al., 2019). ...
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Freshwater ecosystems are among the most threatened ecosystems on the planet. Beavers are important engineers in freshwater ecosystems and reintroduction programs have enabled the recovery of beaver populations in several European countries, but the impact on biodiversity conservation is still unclear. We studied the effects of beavers on the terrestrial biodiversity of eight taxonomic groups by comparing beaver ponds with river and forest habitats in a mountain forest ecosystem in Central Europe. Among the 1,166 collected species, 196 occurred exclusively at beaver ponds, 192 in plots at the river, and 156 in the forest plots. More species of conservation concern were found at the beaver ponds (76) than on the river (67) and forest (63) plots. Abundances of bats and birds were higher at the beaver ponds than at the river or forest sites. The number of bird species at the beaver ponds was higher than at the river. The community composition of birds, beetles, and true bugs differed significantly between the beaver ponds and river plots, and for seven taxonomic groups it differed significantly between the beaver ponds and forest plots. An indicator species analysis revealed eight indicator species for the beaver pond but none for the river and forest plots. Our results demonstrate that beavers, as ecological engineers, increase habitat heterogeneity in mountain forests and thereby promote biodiversity. The expansion of beaver populations into these ecosystems should thus be supported, as it may serve as a biotic restoration tool.
... Recognition is also growing that if beaver construct dams in targeted stream reaches, beaver conservation could be leveraged in various biomes for enhancing water quality (Burchsted et al., 2010), habitat heterogeneity (Wright et al., 2002;Law et al., 2016), and sedimentation in floodplains (Curran and Cannatelli, 2014;Pollock et al., 2014). Of particular importance is the modifying role beaver can have on biodiversity through increasing habitat heterogeneity and food availability at local and landscape scales (Aznar and Desrochers, 2008;Stringer and Gaywood, 2016;Willby et al., 2018). ...
... Several studies have recently summarized beaver impacts on aquatic and riparian environments (e.g. Kemp et al., 2012;Stringer and Gaywood, 2016;Ecke et al., 2017;Johnson-Bice et al., 2018), highlighting disparate results on the environmental impacts produced by beaver dam construction. For example, in a meta-analysis containing 89 studies, Ecke et al. (2017) reported both positive and negative beaver dam-related impacts on water quality, salmonid populations, and ecosystem function. ...
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Beaver are recolonizing previously occupied regions, expanding into new territories, and increasingly being introduced and protected for stream conservation and restoration across numerous biomes. However, beaver dam effects on the physical, chemical, and biological characteristics of streams may vary within and among biomes. A comprehensive review of these impacts is lacking. The goals of this review were to: 1) summarize the distribution of studies by biome on beaver dam effects related to channel morphology, hydrology, water quality, and aquatic biota, as well as on beaver habitat selection, 2) summarize the extent to which beaver dam impacts have been consistent within and among biomes, and 3) share testable hypotheses regarding beaver impacts within understudied biomes. We quantify the directionality of beaver dam impacts from 267 peer-reviewed studies. Results show that the majority of studies have been completed within temperate forest environments and that many biomes are understudied. Across biomes, beaver preferred sites for dam development characterized by relatively low gradients and unconfined reaches with small drainage areas. Overall, parameters related to stream morphology and hydrology showed relatively consistent responses to beaver dams within and among biomes, yet water quality and biotic responses were variable among biomes. Responses also varied by parameter within water quality and biotic impact categories. The findings of this study can be useful for stream conservation and restoration efforts that introduce or protect beaver. Additional studies are needed within arid and cold biomes historically occupied by beaver and in novel biomes where beaver populations are currently expanding.
... The Eurasian beaver (herein referred to as beavers) is a large mammal which lives in terrestrial aquatic environments and was historically present in Britain . They are often referred to as ecosystem engineers as they modify landscapes through dam-building and tree-felling behaviours (Campbell-Palmer et al. 2016;Stringer and Gaywood 2016). Beaver behaviours create habitats which support wider biodiversity (Law et al. 2019;Nummi et al. 2011Nummi et al. , 2019Holopainen 2014, 2020;Stringer and Gaywood 2016;Ward and Prior 2020), and dams slow water flows through landscapes, reducing downstream flood risk and improving water quality (Brazier et al. 2020a, b;Brown et al. 2018;Graham et al. 2020;Puttock et al. 2017Puttock et al. , 2018. ...
... They are often referred to as ecosystem engineers as they modify landscapes through dam-building and tree-felling behaviours (Campbell-Palmer et al. 2016;Stringer and Gaywood 2016). Beaver behaviours create habitats which support wider biodiversity (Law et al. 2019;Nummi et al. 2011Nummi et al. , 2019Holopainen 2014, 2020;Stringer and Gaywood 2016;Ward and Prior 2020), and dams slow water flows through landscapes, reducing downstream flood risk and improving water quality (Brazier et al. 2020a, b;Brown et al. 2018;Graham et al. 2020;Puttock et al. 2017Puttock et al. , 2018. Additionally, beaver tourism may benefit local businesses (Auster et al. 2020c;Campbell et al. 2007). ...
Article
Full-text available
Species reintroductions are growing in popularity, and example motivations include supporting species populations or the restoration of ecosystem function. Interactions between humans and the reintroduced species are likely to occur post-reintroduction. Coexistence between humans and wildlife is adaptive and dynamic, in part requiring management of conflicts between humans and wildlife, or of conflicts between humans over wildlife management. We seek to learn from the experiences of steering group members in a Eurasian beaver (Castor fiber) project in England and identify how governance of coexistence with reintroduced species may differ from the governance of coexistence with species that are already present in the landscape. Using a qualitative thematic analysis of an online survey, we identify a series of lessons in six key areas: (1) project governance, (2) stakeholder engagement, (3) research and monitoring programme, (4) strategy to manage arising conflicts, (5) public engagement, and (6) broad perspectives on reintroduction trials. We advocate for reflective evaluation as an essential component of reintroduction projects to enable knowledge-sharing from experiences, leading to improved practices in the future. Reflecting on our analysis, we identify and define 'Renewed Coexistence'-a new term that draws on pre-existing coexistence knowledge but identifies the unique elements that relate to governing coexistence with reintroduced species. Supplementary information: The online version contains supplementary material available at 10.1007/s10344-021-01555-6.
... In other cases, reintroduction can be motivated by desire to repair fragmented guilds by reinstating extirpated species, especially where charismatic species are involved or where people feel a moral obligation to replace species lost through direct human activity (Seddon et al. 2005;Hawkins et al. 2020;Lee et al. 2021). There might also be economic arguments, either relating to ecosystem services or ecotourism potential (Stringer and Gaywood 2016;Auster et al. 2020). Reintroduction can also be undertaken as part of species conservation: 43% of the reintroductions analysed by Seddon et al. (2005) involved species listed as threatened by the IUCN, with 21% being endangered or critically endangered. ...
Article
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Palaeoecology involves analysis of fossil and sub-fossil evidence preserved within sediments to understand past species distributions, habitats and ecosystems. However, while palaeoecological research is sometimes made relevant to contemporary ecology, especially to advance understanding of biogeographical theory or inform habitat-based conservation at specific sites, most ecologists do not routinely incorporate palaeoecological evidence into their work. Thus most cross-discipline links are palaeoecology → ecology rather than ecology → palaeoecology. This is likely due to lack of awareness and/or the misnomer that palaeoecology invariably relates to the “distant past” (thousands of years) rather than being applicable to the “recent past” (last ~ 100–200 years). Here, we highlight opportunities for greater integration of palaeoecology within contemporary ecological research, policy, and practice. We identify situations where palaeoecology has been, or could be, used to (1) quantify recent temporal change (e.g. population dynamics; predator–prey cycles); (2) “rewind” to a particular point in ecological time (e.g. setting restoration/rewilding targets; classifying cryptogenic species); (3) understand current ecological processes that are hard to study real-time (e.g. identifying keystone species; detecting ecological tipping points); (4) complement primary data and historical records to bridge knowledge gaps (e.g. informing reintroductions and bioindicator frameworks); (5) disentangle natural and anthropogenic processes (e.g. climate change); and (6) draw palaeoecological analogues (e.g. impacts of pests). We conclude that the possibilities for better uniting ecology and palaeoecology to form an emerging cross-boundary paradigm are as extensive as they are exciting: we urge ecologists to learn from the past and seek opportunities to extend, improve, and strengthen their work using palaeoecological data.
... Hydrologically complex riverscapes provide a diversity of intermingled habitats that support a vast array of plant and animal species. Naturally occurring beaver dam complexes are uniquely rich and varied components of riverscapes that contain highly heterogeneous water velocities, temperatures, depths, vegetation communities, and geomorphic structures within relatively small areas of the riverscape (Larsen et al., 2021;Rosell et al., 2005;Stringer & Gaywood, 2016). This heterogeneity results in particularly diverse and resilient habitats and is a large part of why beavers are keystone T A B L E 1 Briefly summarizes how connected-and disconnected-floodplain riverscapes generally respond to several key aspects of climate change (with abbreviated selected references) Fairfax & Whittle, 2020, Wohl et al., 2022, Weirich, 2021, Whipple, 2019 species (Hammerson, 1994;Naiman et al., 1986;Naiman et al., 1988;Pollock et al., 1995). ...
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Rivers and streams, when fully connected to their floodplains, are naturally resilient systems that are increasingly part of the conversation on nature‐based climate solutions. Reconnecting waterways to their floodplains improves water quality and quantity, supports biodiversity and sensitive species conservation, increases flood, drought and fire resiliency, and bolsters carbon sequestration. But, while the importance of river restoration is clear, beaver‐based restoration—for example, strategic coexistence, relocation, and mimicry—remains an underutilized strategy despite ample data demonstrating its efficacy. Climate‐driven disturbances are actively pushing streams into increasingly degraded states, and the window of opportunity for restoration will not stay open forever. Therefore, now is the perfect time to apply the science of beaver‐based low‐tech process‐based stream restoration to support building climate resilience across the landscape. Not every stream will be a good candidate for beaver‐based restoration, but we have the tools to know which ones are. Let us use them. This article is categorized under: Science of Water > Hydrological Processes Water and Life > Nature of Freshwater Ecosystems Water and Life > Conservation, Management, and Awareness Beaver connected floodplains are climate change adaptation and mitigation features of riverscapes.
... Restoration of riparian habitats using BDAs created unique habitat patches that substantially contributed to aquatic invertebrate gamma diversity. Previous studies of natural beaver-mediated habitats suggest that enhanced gamma diversity may be a function of increased habitat heterogeneity associated with beaver foraging activities and changes in hydrology (Hering et al. 2001;Wright et al. 2002;Smith & Mather 2013;Law et al. 2016;Stringer & Gaywood 2016). Similar to natural beaver dams, BDAs created patches of lentic habitat within predominantly lotic ecosystems, increasing habitat heterogeneity across the landscape (Hering et al. 2001;Bush & Wissinger 2016). ...
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Restoration has the potential to increase habitat heterogeneity through the creation of unique habitat patches that, in turn, increase regional species richness or gamma diversity. While biological diversity and habitat heterogeneity are important factors to consider under a shifting climate, restoration actions and outcomes rarely examine these components. In this study, we examined the effects of riparian beaver dam analog (BDA) restoration on aquatic invertebrate diversity and habitat heterogeneity. While the effects of BDAs on hydrology, geomorphology, and salmonid habitat have been explored, we are unaware of any studies assessing their effects on aquatic invertebrate diversity and the food web that supports them. We sampled aquatic invertebrates, basal carbon resources, dissolved nutrients, turbidity, and water temperature in pre‐ and post‐BDA pond, side channel, and mainstem habitat over a three‐year period. The BDAs functioned similarly to natural beaver dams and created slow‐water environments that accumulated fine particulate organic material (FPOM) and increased pelagic phytoplankton production. Nonmetric multidimensional scaling (nMDS), permutation multivariate analysis of variance (PERMANOVA), and Mantel's tests demonstrated that these changes led to the formation of a unique invertebrate community populated by lentic macroinvertebrates and zooplankton, which increased beta‐diversity and gamma diversity. Further, BDAs in our study maintained high densities of invertebrates and buffered water temperatures in comparison to adjacent lotic habitats. These results support our hypothesis that BDAs can enhance invertebrate beta and gamma diversity through the creation and colonization of unique pond habitat and improve habitat and resource heterogeneity for native fishes under variable climate conditions. This article is protected by copyright. All rights reserved.
... Science contributing towards this decision includes evidence of flow attenuation impact of beavers, which is of particular interest due to projected increases in UK flood risk Dadson et al., 2017;Graham et al., 2020). Other factors include benefits for biodiversity (Law, Levanoni, Foster, Ecke, & Willby, 2019;Nummi & Holopainen, 2020;Stringer & Gaywood, 2016), water quality (Puttock et al., 2017;Puttock, Graham, Carless, & Brazier, 2018) and wildlife tourism (Auster, Barr, & Brazier, 2020c;Campbell, Dutton, & Hughes, 2007). ...
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Natural flood management (NFM) methods work with natural processes to reduce flood risk, while often providing additional benefits such as water quality improvement or habitat provision. Increasingly, the activity of an animal—beavers—is recognised to potentially provide flow attenuation, along with multiple benefits for the environment and society, but there can also be associated challenges. We use Q‐Methodology to elicit and understand human perspectives of beavers and their potential role in flood management among communities living downstream of beavers at three sites in England (Cornwall, Yorkshire and the Forest of Dean). This is the first time a study has focused on downstream communities as the primary stakeholders. We identify diverse perspectives that exhibit a range of value judgements. We suggest a catchment‐based approach to beaver management and public engagement may facilitate deeper recognition of contextual perspectives in decision‐making and enable knowledge dissemination with communities. Further, we examine the relationship between beavers and other NFM methods through these perspectives. In doing so we identify features that relate to the unique element of relying on the natural behaviour of beavers for flood management, rather than human flood managers being the primary decision‐makers.
... It is critical to study resource and habitat selection of Eurasian beavers in their environment to understand and regulate the processes that interact with them and the ecosystem [5]. In this study, we use land cover types, distribution of river networks and observational records of Eurasian beavers in different locations to find their most preferred environmental resources and potential habitats. ...
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As ‘nature’s ecological engineers’ beavers can intentionally modify their habitat by building structures. This ability can have wider environmental benefits, including benefits for other habitats and species. However, this ability to modify the environment can sometimes be destructive, bringing beavers into conflict with land managers and others. To understand the complex connections between Eurasian beavers and ecosystems, this study was based on R language analysis tool that used land cover types, river network distribution and observational record studies of Eurasian beavers to find their most preferred environmental resources and potential habitats. The results found that reintroduced Eurasian beavers have a high potential for settlement and dispersal in restored areas.
... North American beavers (Castor canadensis) are native to the USA and Canada where they are official state symbols (Jernelöv, 2017). Eurasian beavers (Castor fiber) were almost hunted to extinction in the 19th century (Sjöberg et al., 2011), but were successfully reintroduced in Bavaria (Germany) in 1966 (Schwab and Schmidbauer, 2003), Hungary in 1996 (Bajomi, 2011) and the UK in 2009(Stringer and Gaywood, 2016. In Finland, 17 Eurasian beavers from Norway and seven North American beavers from the USA were reintroduced at around 1930 (Härkönen, 1999a). ...
Chapter
a.Aim Provides an overview of selected taxonomic groups of invasive freshwater species: plants, bivalves, crayfish, fish and mammals. b.Main concepts and main methods covered For invasive species in each taxonomic group, major introduction pathways are outlined as well as main impacts and management actions. c.Conclusions Invasive freshwater species can be found in a variety of taxonomic groups, only a few of which can be covered here. Many freshwater invaders are still expanding their ranges, and have large effects on biodiversity and socio-economics. Their ecology and impacts need to be more thoroughly investigated in order to improve their management.
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Even though beavers (Castor canadensis carolinensis) occur over most of the southeastern United States, the impacts of increasing beaver populations on riparian forests within the southern Appalachian mountains are. not been well documented. Long-tenor browsing and inundation by beaver may alter the composition and structure of riparian forests. A survey of 62 streams (74 mi) within the Chauga River drainage in the mountains of South Carolina was conducted during 1991-1992 to determine the level of beaver activity within the drainage and the amount of timber damaged by beaver activities. Thirty-six streams had evidence of significant beaver activity with a total of 5.3 mi (7.2%) affected by beaver. Twenty-six streams (17.3 mi), primarily those with steep gradients and no flood plains, had no evidence of beaver activity. On beaver impacted areas, values of beaver damaged timber averaged $781.27/ac for sawtimber and $36.01/ac for pulpwood While high in terms of volume/acre within impacted riparian areas, timber damage was relatively minor for the entire drainage because of the small area (49.2 ac) affected.
Book
This book is designed to satisfy the curiosity and answer the questions of anyone with an interest in these animals, from students who enjoy watching beaver ponds at nature centers to homeowners and land managers. Color and black-and-white photographs document every aspect of beaver behavior and biology, the variety of their constructions, and the habitats that depend on their presence. A second edition of The Beaver: Ecology and Behavior of a Wetland Engineer, published by Cornell University Press under its Comstock Publishing Associates imprint in 2003, this book has been revised throughout and includes a new section on population genetics and features updated data about the beaver’s range in North America, reintroduction efforts in Europe, and information about the world’s largest beaver dam, discovered in northern Alberta in 2010 and visible from space, as well as the most current bibliography on the subject. As this book shows, the beaver is a keystone species-their skills as foresters and engineers create and maintain ponds and wetlands that increase biodiversity, purify water, and prevent large-scale flooding. Biologists have long studied their daily and seasonal routines, family structures, and dispersal patterns. As human development encroaches into formerly wild areas, property owners and government authorities need new, nonlethal strategies for dealing with so-called nuisance beavers. At the same time, the complex behavior of beavers intrigues visitors at parks and other wildlife viewing sites because it is relatively easy to observe. © 2003 by Dietland Müller-Schwarze and 2011 by Cornell University. All rights reserved.
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1. Theory suggests that territorial species should share many of the same dynamical characteristics as metapopulations, including asynchronous local dynamics, potential for stochastic extinction of the population when rates of successful dispersal fall below mortality risk, and critical importance of the ratio of suitable to unsuitable habitat for long-term persistence. These propositions were tested on a population of beavers (Castor canadensis Kuhl) in Algonquin Provincial Park, Ontario, which has been continuously monitored over 11 years. 2. Results showed that the total population was considerably less variable than local abundance at 14 beaver colonies, due to asynchrony among local populations. This suggests that local ecological interactions were more important in determining year-to-year variation in beaver numbers than broad-scale environmental processes, such as weather. 3. Of the local colonies, 20% were never abandoned over ii years, although there was considerable turnover among adults. Offspring production exceeded adult abundance at five source colonies, which did not quite compensate for negative net production at nine sink colonies. These observations were consistent with predictions of spatially structured models of territoriality incorporating local variation in habitat suitability. Mean colony size and probability of recurrence from year-to-year were associated with local food availability, indicating that trophic interactions were important in determining local population dynamics. 4. The beaver population in Algonquin declined steadily over the study period, however, suggesting that spatial and demographic processes were insufficient to stabilize abundance over time. This is consistent with predictions of spatially structured models of territoriality in which suitable and unsuitable habitats are interspersed. It is hypothesized that long-term decline in habitat suitability is induced by acceleration of woody plant succession due to selective foraging by beavers.
Article
Beaver ponds are a widely distributed and abundant wetland resource in the southeastern United States, but their value as avian habitat is poorly documented. We sampled bird populations at six South Carolina Piedmont beaver ponds from April 1993 through June 1994 to examine seasonal community composition and discern microhabitat variables that are associated with bird group abundance and richness. The resident/short-distance migrant group and the neotropical migrant group were most abundant in the spring seasons and waterbirds were least abundant. In fall and winter the resident/short-distance migrant group was most abundant at all ponds with the exception of one pond in winter, which had very high use by waterfowl. Seasons were generally most important in predicting bird species richness and abundance of the resident/short-distance migrant group. Vegetation interspersion, patch evenness, plant richness and total area were most important in explaining abundance of waterfowl, waterbirds, neotropical migrants and woodpeckers, respectively.
Article
Food resources and diet composition of Sitka Mice, Peromyscus keeni sitkensis, were studied over a four-year period in four floodplain and upland forest habitats: old-growth Sitka Spruce (Picea sitchensis) floodplain; Red Alder (Alnus rubra) floodplain; Beaver (Castor canadensis)-pond floodplain; and nearby old-growth Sitka Spruce-Western Hemlock (Tsuga heterophylla) upland forest. Food resources in each habitat were quantified in terms of understory biomass and species richness, fruit production, tree seedfall, and relative abundance of arthropods. Diet composition was analyzed from stomach contents. Between-year differences in the availability of food resources were substantial, but between-habitat differences were minor. Diet composition differed between years and between months within years but did not differ between habitat types or age and sex classes of mice. We conclude that floodplain habitats do not provide unique food resources for Sitka Mice in comparison to upland old-growth forests. However, spatial and temporal complexity within habitats is an important feature of habitat quality in floodplain forests for Peromyscus mice.