Mammal Rev
. 2005, Volume 35, No. ••, ••–••.
Printed in Singapore
.
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,
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Blackwell Publishing LtdOxford, UKMAMMammal Review0305-1838Blackwell Publishing Ltd, 2005
? 2005
35
••••••
review article
Ecological impact of beaversF. Rosell
et al.
Correspondence: F. Rosell. E-mail: frank.rosell@hit.no
Ecological impact of beavers
Castor fibre
and
Castor canadensis
and their ability to modify ecosystems
FRANK ROSELL*
,
ORSOLYA BOZSÉR†
,
PETER COLLEN‡
and
HOWARD PARKER*
*
Faculty of Arts and Sciences, Department of Environmental and Health Studies, Telemark
University College, N-3800 Bø i Telemark, Norway,
†
Institute of Wildlife Management,
University of West Hungary, 9400 Sopron, Ady Endre u. 5., Hungary,
‡
Fisheries Research
Services, Freshwater Laboratory, Faskally, Pitlochry, Perthshire PH16 5LB, Scotland, UK
ABSTRACT
1.
The genus
Castor
comprises two species: the Eurasian beaver
Castor fibre
, and the North
American beaver
Castor canadensis
. Both species suffered from overexploitation, but have
seen a revival since the 1920s due to increased protection and reintroduction programmes.
Increases in the populations and distributions of species that are able to modify ecosystems
have generated much scientific interest. Here we review the available literature concerning the
possible ecological impact of beaver species in the Old and New World.
2.
Beavers, being ecosystem engineers, are among the few species besides humans that can
significantly change the geomorphology, and consequently the hydrological characteristics
and biotic properties of the landscape. In so doing, beavers increase heterogeneity, and habitat
and species diversity at the landscape scale. Beaver foraging also has a considerable impact
on the course of ecological succession, species composition and structure of plant commu-
nities, making them a good example of ecologically dominant species (e.g. keystone species).
3.
Nevertheless, the strength of beavers’ impact varies from site to site, depending on the
geographical location, relief and the impounded habitat type. Consequently, they may not be
significant controlling agents of the ecosystem in all parts of their distribution, but have
strong interactions only under certain circumstances. We suggest that beavers can create
important management opportunities in the Holarctic, and this review will help land man-
agers determine the likely outcome of beaver activity.
Keywords
: biodiversity,
Castor canadensis
,
Castor fibre
, conservation, ecosystem engineers,
keystone species
INTRODUCTION
Beavers as ecosystem engineers
Beavers are classified as ecosystem engineers, because their building activities can change,
maintain or create habitats by modulating the availability of resources of both biotic and
abiotic materials for themselves and for other species (Jones, Lawton & Shachak, 1994;
Gurney & Lawton, 1996). Similarly, their foraging activity also alters organic material, thus
creating habitat for other species, because tree felling by beavers for feeding purposes rarely
entails the consumption of the whole plant material. Furthermore, beavers represent a small
impasse of evolutionary history and have a unique foraging strategy. As the sole member, i.e.
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the biomass-dominant species, of their occupied functional groups (see Fox & Brown, 1993
and Bellwood & Hughes, 2001 for a definition of functional group), they could also be
considered as a candidate keystone species (Davic, 2003). Although beavers have been fre-
quently described as a keystone species (e.g. Naiman, Melillo & Hobbie, 1986), some studies
have challenged this role (Nolet, Hoekstra & Ottenheim, 1994; Donkor & Fryxell, 1999),
because beavers may decrease the species diversity in lower trophic levels. This can be due to
their disproportional selection of less abundant species for complementary mineral sources
(Nolet
et al
., 1994), or the increased importance of dominant plant species, as beavers do not
completely shift the plant community composition towards non-preferred species (Donkor
& Fryxell, 1999). Nevertheless, beavers probably have a key role in ecosystem processes,
because their foraging has a considerable impact on the course of succession, species com-
position and structure of plant communities (e.g. Huntly, 1995).
Following a period of devastating overexploitation, beavers are once again abundant in
North America and are continuing to expand their range in Eurasia. The increase in the
population and distribution of a species which is able significantly to modify ecosystems
clearly generates considerable scientific and management interest. The aim of this paper is to
review the extent of the beavers’ ability to modify the physicochemical components and
biological properties of ecosystems throughout the Holarctic.
THE GENUS CASTOR
Comparison of the two species
The genus
Castor
comprises two species: the Eurasian beaver
Castor fibre
, and the North
American beaver
C. canadensis
. The two species are similar both morphologically and behav-
iourally (e.g. Novak, 1987), and were originally classified as one species. However, distinct
karyotypes have been demonstrated (Lavrov & Orlov, 1973). This difference in karyotype
probably explains why they do not interbreed successfully (Lahti & Helminen, 1974). The
genus of true beavers originated in Eurasia and penetrated into North America during the
course of the Pliocene epoch (Lavrov, 1983).
During the 1920s and 1930s, before confirmation that there were two beaver species, North
American beavers were introduced to Poland and Finland (Lahti & Helminen, 1974; Ermala,
Helminen & Lahti, 1989). They thrived in Finland and spread into parts of Russian Karelia
in the 1950s (Danilov, 1992, 1995). Introductions were also made in Russia (Safonov &
Saveliev, 1999), along the Seine in France (Richard, 1985), Hungary (Bozsér, 2001a) and
Austria (Sieber, 1989). However, North American beavers are probably extinct now in
Austria, Hungary, Poland and France (Bozsér, 2001b; Halley & Rosell, 2002). These intro-
ductions have provided opportunities for comparative studies on the two beaver species.
Although comparative studies have generally found that the Eurasian beaver demonstrates
less building activity than the North American species (Danilov & Kan’shiev, 1982; Danilov,
1995), the effects of their dams on the environment may not differ much (Nolet, 1996).
However, two important differences between the species are that the North American beaver
may mature earlier and has larger litters than the Eurasian beaver (Lahti & Helminen, 1974;
Danilov & Kan’shiev, 1983). In an extensive review, Rosell & Parker (1995), found a mean
colony size of 5.2
±
1.4 S.D. for the North American and 3.8
±
1.0 S.D. for the Eurasian
beaver. Where the two species have been introduced at the same place, the North American
beaver often dominates and displaces the Eurasian species (Lahti & Helminen, 1974). This
could be associated with the higher reproductive rate of the North American beaver (Danilov
& Kan’shiev, 1983). This is supported by studies in Finland, where differences observed in
the population growth rates of the two species could not be linked to differences in habitat
Ecological impact of beavers
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or food (Lahti & Helminen, 1974). Nevertheless, some Russian evidence suggests that the low
litter size in Eurasian beaver may be due to inbreeding depression (Saveljev & Milishnikov,
2002; Saveljev
et al
., 2002). However, Milishnikov (2004) concluded that the effective repro-
ductive size N
e
of the minimum Eurasian beaver population in tat study was estimated to be
equal to three animals. This extremely low value of effective reproductive population size
largely explains the high tolerance of Eurasian beaver to inbreeding and striking viability of
the species.
MacDonald
et al
. (1995) concluded that studies on North American beavers can provide
important clues about the potential effects of Eurasian beavers. This is important for man-
agement because the majority of information on the beavers’ impact on the environment
originates from studies in North America (MacDonald
et al
., 1995; Nolet, 1996). Where
Eurasian beavers construct dams and fell trees, they are likely to have similar environmental
effects to the North American beavers (MacDonald
et al
., 1995).
Historical and present distribution
Beavers were once found throughout the northern forest belt from Canada and the United
States to Europe and Asia. Along this extensive northern range, they are able to occupy a
broad spectrum of ecoregions from subtropical to subarctic. Both species suffered from
overexploitation, as great value was placed on their pelt and castoreum. In North America,
a large proportion of the beaver population at middle and southern latitudes was depleted
before 1900 (Hill, 1982). Similarly, it has been estimated that only 1200 Eurasian beavers
remained by the early 20th century (Nolet & Rosell, 1998).
To reverse the effects of this overexploitation, beavers on both continents were protected
and, from the 1920s, reintroduction programmes were initiated. In North America, after
almost four centuries of commercial exploitation, the beaver is once again abundant, and the
population in USA is currently estimated to be 6–12 million (Naiman
et al
., 1986). Although
the recovery of the Eurasian beaver has been slower, it is continually expanding its range and
the current minimum population estimate is 639 000 (Halley & Rosell, 2003).
INFLUENCE ON THE PHYSICOCHEMICAL CHARACTERISTICS
OF STREAMS
It is widely recognized that there are strong and continuous interactions between hydrology,
geomorphology, water chemistry and temperature (Naiman
et al
., 2000). They are all signif-
icant factors that influence aquatic organisms, and they can all be modified by beaver activity.
Hydrological effects
The effect of beaver dams on stream flow will vary according to their location in the catch-
ment. In upland narrow valleys, beaver ponds are generally small; whereas in flood plain
areas, even a low dam can flood a relatively large surface area (Johnston & Naiman, 1987).
Due to large initial differences in velocity, beaver dams that flood upland areas reduce the
kinetic energy of the stream more than those that flood wetlands (Johnston & Naiman, 1987).
The age and structural characteristics of the dam can be important, and Meentemeyer &
Butler (1999) reported that older beaver dams reduced stream velocity and discharge more
efficiently than young dams in low-order streams in Montana. In a second-order stream in
Maryland, the creation of a 1.25-ha beaver pond reduced the annual discharge of water by
8% (Correll, Jordan & Weller, 2000).
Although a single beaver dam may have little influence on stream flow, a series of dams
can have a significant effect (Grasse, 1951) by moderating the peaks and troughs of the annual
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discharge patterns. During dry periods, Duncan (1984) reported that up to 30% of the water
in an Oregon catchment could be held in beaver ponds. By increasing storage capacity, it has
been suggested that large numbers of beaver dams will lead to greater flows during late
summer (Parker, 1986), which may result in continual flows in previously intermittent streams
(Yeager & Hill, 1954; Rutherford, 1955).
Beaver dams, depending on their number and location, may decrease peak discharge and
stream velocity during a run-off event, thereby reducing the erosion potential (Parker, 1986)
and the possibility of flooding (Bergstrom, 1985; Harthun, 2000). Associated with the reduc-
tion in stream velocity are reductions in the sediment-carrying capacity of the stream and
increases in deposition (Naiman, Johnston & Kelley, 1988). Parker (1986) suggested that
beaver dams can protect areas from erosive perturbations, if these perturbations are not too
great.
Although beaver dams normally reduce the severity of flooding events, they may contribute
to them if dam failure occurs (Butler, 1991). The failure of a beaver dam on a small stream
in Alberta produced an estimated flood wave which was 3.5 times the maximum discharge
recorded over a 23-year period (Hillman, 1998).
Beaver impoundments increase the area of riparian habitat and recharge groundwater by
elevating the water table (Bergstrom, 1985; Johnston & Naiman, 1987). The movements of
water within bed sediments can be influenced by beaver dams. In a typical convective pattern,
stream water penetrates the bed, travels through the sediment along the longitudinal gradient
and then upwells back into the river (White, 1990). As pore water moves beneath a beaver
dam, it is no longer affected by the pressure of the impounded water. The pressure decreases
immediately downstream from the dam, and therefore causes a sharp upwelling of under-
flowing stream water and upwelling of cooler pore water from deeper in the substrate.
Most studies conclude that beaver dams stabilize stream flow. However, Reid (1952)
reported that increased evaporation from beaver ponds in the Adirondacks could reduce the
volume of flow. Woo & Waddington (1990) also found that, in the subarctic wetlands of
Ontario, the enlargement of the open water area by beaver activities enhanced evaporation
in the summer. However, they reported that losses from enhanced evaporation could be offset
by the reduction in water loss through run-off in areas with well-maintained dams. Correll
et al
. (2000) suggested that high evapotranspiration by the riparian forest fringing the pond,
rather than evaporation, could have been responsible for the reduction in water discharge
from a 1.25-ha beaver pond.
Geomorphology
Beavers construct burrows and lodges to enhance their survival (predator avoidance, shelter,
thermoregulation), canals to extend feeding areas, food caches to provide a winter food
supply and dams to improve their habitat by raising water levels (Zurowski, 1992). These
activities affect hydrology, sediment yields and debris accumulation, and therefore have an
overall impact on channel morphology.
The discharge regime, structural elements, sediment volume and accumulation pattern of
sediment have a great role in shaping the river channel (Gurnell, 1998). Beaver dams are
significant structural elements in river channels, greatly influencing discharge regime and
sediment transport. Beaver ponds function as sediment traps, and they also accumulate
organic matter, especially as anaerobic conditions caused by the restrained stream velocity
decrease decay rates (Pollock
et al
., 1995). A 1.25-ha beaver pond in Maryland reduced
annual discharge of total organic carbon by 28% and total suspended solids by 27% (Correll
et al
., 2000). The organic matter and sediment derives from the stream flow, bank failure,
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excavation of burrows and canals, collapse of burrows and lodges, primary production in the
pond, and organic material from riparian vegetation which could be introduced by beavers,
or delivered by other natural means. Although the velocity and discharge of water emerging
from beaver ponds are reduced, areas downstream of the dam may have increased potential
for erosion due to the trapping of sediments and subsequent release of underloaded water.
Beaver impoundments undergo a process of infilling; therefore the older ponds generally
contain more sediment than the recently established ones. Meentemeyer & Butler (1999)
found that the depth of sediment ranged from an average 24.6 cm in younger ponds (
<
6 years
old) to 45 cm in an older pond (
>
10 years old) in Glacier National Park, Montana. Accord-
ingly, the sedimentation volume ranged from 9.4 m
3
in a young pond (38 m
2
) to 267 m
3
in an
older pond (588 m
2
). The amount of sediment accumulated in the ponds will vary, depending
on the stream discharge, slope, upstream surface material and the extent of erosion-prone
areas in the watershed (Meentemeyer & Butler, 1999). For instance, the sediment load within
the Matamek and Moise River watersheds, Quebec, differed largely in magnitude from the
example above with sediment volumes varying from 35 to 6500 m
3
in the beaver ponds
(Naiman
et al
., 1986). They estimated that the beaver dams constructed on the Matamek
River watershed’s 2nd
−
4th-order streams (approx. 322 km) are responsible for the retention
of 3.2 million m
3
sediment. If this mass was evenly distributed over the total area of stream
bed, it would cover the bottom with 42-cm sediment. Thus, beaver dams and resulting ponds
give the channel gradient a stair-step profile, increasing the diversity in channel width and
depth (Naiman
et al
., 1988; Gurnell, 1998). Palaeoecological evidence suggests that, over a
million years, entire valley floors have been raised by the continuous sedimentation process
in beaver ponds (Rudemann & Schoonmaker, 1938; Ives, 1942).
Similarly, but to a lesser degree, the woody debris accumulated by beavers in streams also
increases the patchiness of bed sediment and represents an important in-channel morpholog-
ical feature. This woody debris controls the transport of sediment and particulate organic
matter, and creates conditions for the formation of braided channel, pools and islands. By
importing woody debris into streams, beavers play a considerable role in the stabilization of
low-order (mainly 1st
−
5th order) streams (Gurnell, 1998; Naiman
et al
., 2000).
Beavers can also impact on the drainage network of larger rivers (
≥
5th order), where their
alterations to channel geomorphology occurs mainly along river banks and in wide, relatively
flat flood plains (Naiman
et al
., 1986; Gurnell, 1998). In situations where the water level is
sufficiently high on the upstream side of the beaver dam for flow diversions to occur across
flood plains of very low relief, the river channel might be subdivided into a series of smaller,
interconnected channels occupied for shorter or longer periods (Townsend, 1953; Woo &
Waddington, 1990). These diversion channels, which may be short or reach a few hundred
metres in length, may become permanent routes of water flow if they are utilized enough to
allow sufficient down cutting to occur (Woo & Waddington, 1990). By this means, multi-
thread channel systems might be created, which accommodate flood flows (Gurnell, 1998).
Water chemistry
Beaver activity can have a significant effect on water quality, although the magnitude and
nature of induced changes in water chemistry will be modified by catchment characteristics
such as geology, soil type, land use and climate.
Beavers may exert considerable influence on the productivity of fresh waters by altering
nutrient levels. Naiman & Melillo (1984) found that beaver ponds stored approximately 1000
times more nitrogen (N) in sediments, per linear metre of stream channel, than riffle areas
and that this was solely a function of the amount of sediment accumulated in the different