Habitat of the beaver
Table of contents
Beavers inhabit almost any type of waters from large lakes and smaller ponds to small brooks and rivers up to large streams like the river Elbe. The animals can occur at sea level as well as at altitudes up to 3000 m, where mountain rivers can serve as a suitable habitat (Djoshkin & Safonow 1972).
Typical habitat of beavers
at the river Hase. Several publications have investigated, which characteristics are used by beavers searching for a suitable habitat and several authors have tried to attribute the animals selection behaviour to a single criterion (for instance the availability of winter food or the flow speed). Thereby, some investigations came to the conclusion, that the food availability plays only a minor role when the animals are establishing a new territory. Those authors think that the beavers are able to create their own food basis by altering their habitat (Beir & Barret 1987). However, these authors do not take into account, that such a habitat alteration will take some time (the growth of suitable trees as an important source of winter food may take years) and that the animals have to live on something in the meantime. The food availability most certainly does have a significant effect on the habitat choice of the beaver. Most likely, when choosing their habitat, beavers (like many other animals) use a set of criteria and in the end their choice will represent a compromise between all of these habitat variables. Moreover, beavers are very flexible regarding their habitat choice and a restriction on a few "main criteria" to describe the selection of a suitable habitat, will most certainly not reflect the broad ecological plasticity of these animals.
Food availability [Back to the top]
When choosing a habitat not only the availability of suitable winter food is of importance, but also the existence of terrestrial grasses and herbs, as well as aquatic plants, because beavers feed mainly on these plants during the summer (the proportion of grasses, herbs, and aquatic plants can account up to 90% of the ingested food, Gurnell 1998; see the chapter Diet as well). For the long term colonisation of a body of water, woody plants as the only food source are not sufficient (Halley et al. 2009).
A decline in the availability of grasses and herbs can occur for instance, if grazing animals (cattle, sheep) have direct access to waters
inhabited by beavers (Dieter & McGabe 1989). But grazing animals not only influence the herbaceous vegetation. In sections
of the waters, which are grazed up to the bank, the stem diameter of the tress is shifted towards larger diameters, because trees with small
stem diameters (which are preferred by beavers, see the chapter Diet) are damaged
by livestock due to over-browsing (Jungwirthet al. 2005). In such a case, the only possibility the beavers have to meet
their food requirements is the felling of larger trees.
Besides the direct competition for food, also the grazing of the vegetation around a bank dens or a bank lodge can have negative effects on the beaver, because burrows are preferably build at places with a high vegetation cover (normally, the vegetation around a burrow is not used by the beavers, Dieter & McGabe 1989). If the grazing livestock has free access to the water, the aquatic vegetation can be damaged by trampling of livestock and the accompanying devastation of the bank (this can partly also been observed at the river Hase).
Hydrological characteristics [Back to the top]
As a further factor, a body of water permanently inhabited by beavers must have a perennial, stable water level and a water depth of at least
30 - 50 cm (70 - 80 cm after some authors) should not be fallen short of (Allen 1983; Djoshkin & Safonow 1972;
Gurnell 1998; Heidecke 1989). This ensures, that the burrow entrance always lies underneath the water surface and
that the animals are safe from predators inside their burrow (see the chapter
Building behaviour of the beaver as
well). As long as sufficient food is available, because of their abilities to actively alter their environment by building dams, beavers
can inhabit waters, which have a lower water depth than mentioned above. Furthermore, the flow gradient and the corresponding velocity of
flow are regarded as limiting for a colonisation. For instance, C. canadensis in North-America does not inhabit waterways with a
gradient of more than 10 - 15% (this equals an altitude difference of 10 to 15 m respectively over a length of 100 m) and more than half
of all settlements investigated in this respect were situated at waterways with a gradient of less than 6% (Allen 1983;
Suzuki & McComb 1998). After Gurnell (1998) clearly preferred are waterways with a gradient of less than 2%
and a velocity of flow of less than 0.3 m/s and Halley et al. (2013) specify, that C. fiber in Norway does not
colonise waterways with a gradient of more than 2%.
After the opinion of some authors, there is a correlation between the gradient of a waterway and the duration of settlement of that waterway by beavers: the higher the gradient, the shorter the duration of settlement (Beier & Barrett 1987). In long established beaver populations, which have almost reached the ecological carrying capacity of the populated area, the flow gradient within long established family territories is lower than in recently established ones (DeStefano et al. 2006). This suggests that all the optimal habitats are already colonised and that the dispersing pups are increasingly forced to colonise more and more suboptimal habitats (in this case areas with a higher flow gradient).
However, the data on gradients have to be regarded somewhat critical because the different investigations have judged the suitability of a watercourse with respect to the possibility of building a dam within this waterway and not the general suitability of that waterway for the colonisation by beavers. Of course it is true that building a dam above a certain flow velocity will not be possible because the material piled up by the beavers will be torn away by the current. However, building a dam is not a necessary prerequisite for the colonisation of a body of water, because the animals will not build a dam if the water has a sufficient depth the whole year round. For instance, also Jakes et al. (2007) came to the conclusion that for flowing waters in level grounds with a gradient of less than 1.5% the gradient does not influence the habitat selection.
Structure of the riverbank [Back to the top]
Furthermore, the beavers habitat choice is also influenced by the quality of the ground and of the bank especially. On the one hand, the
ground has to be soft to allow the digging of burrows (Gurnell 1998), on the other hand, it has to be stable enough so that
the burrows do not collapse. Additionally, the inclination and the height of the bank is of importance. Therefore, some authors are of
the opinion, that waters with predominantly shallow banks (inclination of less than 60°) are preferred by beavers, whereas waters with
steeper bank sections are avoided or only inhabited for a short time by the beaver (Beier & Barrett 1987;
Gurnell 1998). However, other authors contrarily claim that beavers predominantly inhabit sections with steep banks. This
is explained by the fact, that steep banks are better suited to dig bank dens (Dieter & McGabe 1989). After
McComb et al. (1999), this discrepancy can be solved by the observation, that beavers - when creating a burrow - either
prefer sections with very steep banks or sections with relatively shallow ones. In steep banks primarily bank dens are build, whereas at
shallow banks brook lodges are created.
Embankments, that are banks with an inclination of approximately 45°, are not very well suited neither for the digging of a bank den nor for the construction of a brook lodge (when digging a subterranean burrow the ceiling tends to collapse frequently and there is not enough space for the construction of a brook lodge along the bank). Unfortunately, in Germany almost every river has been technically altered by humans - among others this also includes the shaping of the natural cross-section of the waterway towards a trapezoid standard cross-section, i.e. the reshaping of a natural bank towards an embankment with an inclination of 45°. In case of higher water leves, this should insure a fast discharge of the water without larger drainage obstructions. By this, the affected bank will be depreciated for semi-aquatic mammals like the beaver and other bank-dwelling animals, because these animals can not make use of such embankments (see for instance Klenner-Fringes 2001; Schröpfer & Paliocha 1989).
Width of the water body [Back to the top]
At flowing waters also the width of the river is of importance for the habitat selection. In general, flowing waters of a width between
8 and 40 m are preferred by beavers. Very small and very broad waterways are thought to be avoided by beavers (Gurnell 19998).
However, in eastern Germany for instance, also small brooks or ditches with less than 1 m width are permanently inhabited by beavers and a
river with a width of 5 m is regarded as optimal for beavers. Furthermore, when migrating the animals are using all kinds of flowing
waters - even those which have no riparian woody vegetation at all (Heidecke 1989). The same can be observed in the district
Emsland, where the beavers are using small rivers like the Nord-, the Mittel- und the Südradde for their migration, although large sections
of these waterways are extremely altered by humans and look like mere drainage pipes without any woody vegetation at their banks. Besides,
in the meantime also relatively small waters are inhabited by beavers in the district Emsland (for instance the family at the Fullener Wald
or the animal at the Hakengraben near Sögel), which is probably a sign for the inhabitation of suboptimal habitats caused by the increasing
Even if a sufficient food supply is given, larger lakes are only permanently inhabited if the shore shows a certain kind of structuring with small bays at which the lodge will be build (as far as the lake does not have steep banks suitable for the digging of bank dens). Larger lakes with predominantly linear and shallow shores are inappropriate because the pronounced wash of the wash of the waves of such lakes would cause a frequent destruction of the lodge (Allen 1983; Halley et al. 2013). If beavers have the opportunity, they will inhabit flowing waters first, whereas standing waters will only be used as a second choice (Collen & Gibson 2001).
Size of the territory [Back to the top]
The size of the territory defended by the animals depends on the food availability within the populated habitat. With increasing
population density, more and more less suited habitats will be inhabited by the animals and the size of the territories or the home
range respectively are larger in such habitats compared to waters, which are optimally suited for beavers, because the animals need
more space to acquire sufficient food (Wheatley 1997 for C. canadensis). Because of that, to give a definite,
general size for a beaver territory is difficult.
As the re-introduction of the beaver to the river Hase at the district Emsland has shown, even in suitable habitats beavers can have relatively large territories or home ranges respectively, namely in cases when the population density is still low and the animals do not have any neighbouring animals, which might restrict them in their spatial use of the habitat. That is why - despite the good food availability - the animals of the family of six used a section of the river Hase at least 6 km in length and the pair used an section at least 3.5 km in length at the first year after their re-introduction. After the birth of the pubs in the second year after the re-introduction, the two year old pubs of the family have to leave their parental territory. However, they established two new territories within the parental territory, so that the territory of the parental animals now only covered a section 2.5 km in length, whereas the pubs used two sections of the river Hase each approx. 2 km in length (see also the information on the re-introduction as well as Klenner-Fringes 2001).
Beavers as ecosystem engineers [Back to the top]
Because of the ability to actively alter its habitat, beavers are sometimes referred to as "ecosystem engineers" in the literature
(Bruner 1989; Ciechanowski et al. 2011; Cunningham et al. 2006; Dalbeck
et al. 2007; DeStefano et al. 2006; Gurnell 1998; Hyvönen & Nummi 2008;
Jakes et al. 2007; Jones et al. 1994; Obidzinski et al. 2011; Rosell
et al. 2005; Ruys et al. 2011; Stoffyn-Egli & Willison 2011).
After the definition of Jones et al. (1994), "ecosystem engineers" are organisms, "which directly or indirectly influence the availability of specific resources (apart from themselves) for other organisms by altering the physical condition of biotic or abiotic material" (cited after Jones et al. 1994). This "modification of the state of organic or inorganic material" evokes, that ecosystem engineers create completely new habitats or maintain or change already existing habitats.
Beavers act as ecosystem engineers by felling trees and by the use of their wood for the construction of dams, which alters the hydrological characteristics of the body of water. The beaver dam causes the retention of sediments and organic material (Gurnell 1998; Rosell et al. 2005), which in return influences the nutrient cycle and the decomposition dynamic of the organic substance. By this, also the chemical and physical characteristics of the water (Collen & Gibson 2001; Gurnell 1998; Obidzinski et al. 2011; Perkins 2001; Rosell et al. 2005) as well as the amount and type of the material transported by the water below the dam are being influenced. Additionally, by building the dam the structure and dynamic of the bank above the dam is being changed and thereby, finally, the beavers can significantly influence the composition and diversity of the plant and animal communities within the area affected by the dam (Gurnell 1998; Naimann 1988 in Jones et al. 1994).
Effect of the beaver dam on the water regime [Back to the top]
By the construction of the dam and the damming of the water the ground water level is raised and the dam causes a stabilisation in the
amount of water drainage over the year (a beaver dam is not always completely locked and there is always a certain amount of void water
leaking through the river bed, which is cleaner and cooler than the surrounding water; Collen & Gibson 2001;
Rosell et al. 2005). If beavers are able to build and maintain their dams unaffected by humans, than the floodplains
of appropriate rivers are transformed into a succession of non-linear wetlands, which can influence the geomorphology of a landscape over
long periods of time even after the beavers have already disappeared (Cunningham et al. 2006; Worsley
2009). In Europe, the influence of the beaver on the formation of the landscape of water systems may be restricted on the early phase
after the last ice age because the riparian landscape was subsequently shaped mainly by humans. However, in North-America the influence
of the beaver reached over a significantly longer period of time. Both, in North-America as well as in Europe, this interference of the
beaver can partly still be detected (Worsley 2009). Therefore, the beaver is known - with an exception to man - to be the
only organism, which can actively shape its environment to such an extend (Cook 1942 in Bruner 1989;
Grasse 1951 in Bruner 1989). However, beavers are not the only organisms, which can be classified as ecosystem
engineers (a list of organisms, which act as ecosystem engineer can be found in Jones et al. 1994).
The regulatory effect of the beaver ponds caused by the beaver dams causes the retention of precipitation water, the slow release during the drier month of the year and a long lasting water supply of the riparian areas adjacent to the beaver ponds (Gurnell 1998). At a beaver inhabited river system in Oregon for instance, approximately 30% of the water can be found in beaver ponds during the dry season (Collen & Gibson 2001). Moreover, in case of heavy rainfalls the flood wave will be temporarily delayed, the hight of flood crest will be reduced and by that the danger of potential flooding will be diminished (because the velocity of flow of the discharged flood will be reduced as well, the enhanced erosion during flood events will be counteracted; Collen & Gibson 2001; Gurnell 1998; Rosell et al. 2005). However, during times of enhanced water flow (for instance during the snow melt) the dams are no considerable drainage obstruction (Collen & Gibson 2001). On the other hand, if a beaver dam should break, the flood event caused by this can be very serious (eventually significantly higher than the normal, annual flooding events; Collen & Gibson 2001; Rosell et al. 2005). However, because reports of breaking beaver dams and the consequences are scarce in the literature, Gurnell (1998) assumes, that such events only occur relatively seldom. Within a functional beaver settlement, the animals are frequently busy with repairing the dam. From time to time, the dam will be partly opened in case of larger flooding events (for instance heavy rainfalls or during the snow melt), to reduce the water pressure and to prevent the uncontrolled breaking of the dam (Gurnell 1998).
However, because of the beaver ponds there might be a higher evaporation of water during the warm season because of the enlargement of the water surface as well as through the vegetation surrounding the beaver pond. This in return can cause a reduction in the amount of water drainage during the warm season (Collen & Gibson 2001; Rosell et al. 2005).
Conflicts to human utilisation claims (frequently affected are agriculture and forestry) primarily arise in cases when - due to the construction of a dam - the surrounding areas are waterlogged so that crops or forest trees die or machines can not drive across or process the land. Such a water logging close to the soil surface is mainly probable in areas with a weakly developed topography (i.e. in areas with low differences in altitude; Gurnell 1998).
Effect of the beaver dam on the water temperature [Back to the top]
The impact of beaver dams on the water temperature is also discussed. Inside the wide and mostly shallow &qout;beaver ponds&qout; caused by damming a river, the water temperature is usually increased especially during the warmer season (Collen & Gibson 2001; Rosell et al. 2005). However, the extend of this warming up (in comparison to the water temperature in undisturbed flowing waters) highly depends on the current location. Of importance is, among others, the initial size of the dammed waterway (small, deep, and rapid flowing rivers are normally cooler than broad, shallow, and slow flowing rivers), the size and depth of the beaver ponds, as well as the extend of shadowing of the pond by the surrounding trees (Rosell et al. 2005). Over a certain length, the higher water temperature inside the beaver pond can have an effect upon the water downstream. However, in general the water is cooling down rapidly because the warm surface water will merge with the interstitial water, which passes underneath the foot of the dam (Collen & Gibson 2001; Rosell et al. 2005). A further effect of the beaver ponds created by the construction of a dam is the stabilising of the temperature regime of the affected waterway. Mainly larger beaver ponds can function as a thermal buffer, because larger bodies of water are both heated up as well as cooled down more slowly. However, until now there is no assured correlation between the number and the size of beaver ponds along a waterway and the effect on the water temperature. For this, the role of local factors like the shadowing (caused by the vegetation or the topography), the extend of the ground water inflow, as well as the volume of the flowing water are much too important (Collen & Gibson 2001).
Effect of the beaver dam on the oxygen content of the water [Back to the top]
Because of the increased water temperature and because of oxygen-consuming processes taking place in the increasingly deposited sediments inside the beaver ponds, the oxygen concentration of the pond water can be decreased especially in spring and early summer, because during this time the decomposition of the organic material deposited in the ponds will be significantly intensified (Collen & Gibson 2001; Gurnell 1998; Rosell et al. 2005). In addition also during the winter, if the pond is covered by a closed ice shield over a longer time, the oxygen concentration of the water can be decreased (Collen & Gibson 2001). The decreased oxygen content of the water inside the beaver ponds can also have an effect on the area downstream of the dam. However, because at this part the water is flowing again and there will be some turbulences when the water passes or overflows the dam, the deterioration is normally restricted to a distance of approximately 200 to 300 m below the dam (Collen & Gibson 2001; Rosell et al. 2005).
Effect of the beaver dam on the water chemistry [Back to the top]
The effect of the beaver dams upon the chemical characteristics of the water within the developing beaver ponds are also strongly influenced by the local, initial water chemistry of the body of water prior to the construction of the dam, as well as the geology, the soil type, the type of land use, and the climate. The amount of nutrients tends to rise and the pH-value of the water inside the beaver ponds tends to be elevated (Collen & Gibson 2001; Rosell et al. 2005). Mainly nitrogen (primarily as NO3-) becomes enriched within the beaver ponds, whereas part of the nitrogen is produced by nitrogen-fixing bacteria, which are accumulated in the sediments (Collen & Gibson 2001; Gurnell 1998; Rosell et al. 2005). The activity of nitrogen-fixing bacteria in the sediments of beaver ponds can be increased by the 9 to 44fold (Rosell et al. 2005). Apart from that, also the concentration of organic carbon (Gurnell 1998) and and phosphor (Rosell et al. 2005) can be increased in beaver ponds. For the concentration of nitrogen and phosphor within the water of beaver ponds, an annual periodicity has been discovered. Mainly in times of higher water levels, nitrogen and phosphor are leached out, whereas in the drier season and in times of low water levels, the concentration of these two elements increases in the water of the beaver ponds (Rosell et al. 2005) Other authors are of the opinion, that there is no increase in the concentration of nutrients (except for calcium and magnesium) in the drain water of beaver ponds (Gurnell 1998), whereas this discrepancy may be explained by the timing of the investigation (i.e. if the nutrient content was investigated during the summer and not during the winter). Parts of the nutrients are delivered from the water of the beaver ponds directly to the air, for example carbon (as CO2) or nitrogen (mainly as N2), which adds to the reduction of their concentration within the drainage water of the beaver ponds (Rosell et al. 2005).
Temporal development of a beaver pond [Back to the top]
|Succession stages of a beaver pond afterWelch 1935|
|young||approx. 3 years|
|adolescent||approx. 4 - 10 years;
water depth > 0.3 m;
40 - 50% water surface
|senescent||> 10 years;
water depth mostly < 0.3 m;
abandoned by the beavers
A beaver pond passes several different development stages (after Welch 1935 in Collen & Gibson 2001 there are 6 of such development or succession stages; see the adjoining table), which are characterised by a increasing aggradation and - after some time (usually after more than 10 years) - ended up by the complete loss of water and the development of "beaver meadows". In coniferous forests, these beaver meadows can be long lasting, because the resettlement of the meadows by coniferous trees like spruce (genus Picea) and fir (genus Abies) is inhibited by the lack of certain "root fungi" (mycorrhiza-fungi; in this case especially ectomycorrhiza-fungi), which the coniferous trees need for growth (most of the woody plants form a symbiosis with root fungi and can not sprout without them). In extreme cases, the resettlement of such beaver meadows by coniferous trees can take up to 70 years (Rosell et al. 2005).
Influence of the beaver on other species [Back to the top]
By the construction of a dam and the successive formation of a beaver pond, the increasing aggradation of the pond and the resulting development of a beaver meadow, the beavers not only have an pronounced impact on their own habitat but their habitat shaping has a large effect on many other animal and plant species as well. Thereby, the impact on single species can either be positive (the species can profit) as well as negative (the species will be ousted locally). Because of the great impact of the beaver on several other animal and plant species, the beaver is also regarded as an ecological "keystone species" (Beck et al. 2010; Bruner 1989; Ciechanowski et al. 2011; Collen & Gibson 2001; DeStefano et al. 2006; Gurnell 1998; Rosell et al. 2005; Ruys et al. 2011; Stoffyn-Egli & Willison 2011), because by the management of a single species (is. the beaver) several other species can be influenced without the need to manage each of these other species directly.
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Regarding the fauna, first of all water-dwelling organisms are influenced by a beaver dam of course. After the construction of a beaver dam for instance, for water-dwelling invertebrates or their larvae respectively it has been observed, that the species diversity within the developing beaver pond has been shifted from species preferring flowing waters (lotic species) towards those preferring still waters (lentic species; Ciechanowski et al. 2011; Collen & Gibson 2001; Rosell et al. 2005). Insects, which are dependent on flowing waters for the development of their larvae like mayflies (order Ephemeroptera), stone flies (order Plecoptera), caddies (order Trichoptera) as well as black flies (order Simuliidae) are ousted by the damming of a waterway by the beaver, whereas the number of insect species, whose larvae mainly develop in still waters like dragonflies (order Odonata) or midges (family Chironomidae, order Nematocera), will increase. Additionally, also worms from the order oligochaetes (Oligochaeta) with representatives of the family of enchytraeids (Enchytraeidae), the subfamily of sludge tube worms (Tubificinae, family Naididae), and leeches (subclass Hirudinea), as well as different fresh water mussels (class Bivalvia) will profit from the transformation of a flowing water into a still water. Primarily the enchytraeids and the sludge tube worms on their part serve as food for several fish species (Collen & Gibson 2001; Rosell et al. 2005). In North-America beaver ponds are characterised by a decrease or complete lac of species out of the insect order of mayflies (Ephemeroptera) and an increase of species out of the order of caddies (Trichoptera; Smith et al. 1991). In newly created beaver ponds the number of invertebrate animal species initially decreases, whereas the population density of the remaining species increases (there are many individuals of a few species). However, with increasing age of the beaver pond the number of species increases by and by (Collen & Gibson 2001). Like already mentioned above, there are more still water species to be found, whereas the flowing water species are largely missing.
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Also with regards to the fish fauna, the influence of a dam created by beavers can be positive or negative. This applies not only for species with different habitat demands, but sometimes even for individuals within a species. For instance, different species of salmonids, which need a silt-free, gravelly ground for spawning (in Europe these are sea trouts - Salmo trutta with the brown trout - Salmo trutta m. fario and the lake trout - Salmo trutta m. lacustris, rainbow trouts - Salmo gairdneri or Oncorhynchus mykiss native to North-America, salmons - Salmo salar, chars - genus Savelinus, danube salmons - Hucho hucho, whitefish - genus Coregonus, and graylings - Thymallus thymallus), are profiting from the restraint of sediments and the relieve of the water of fine suspended materials by the dam in river sections downstream of the dam. On the contrary, upstream of the dam the increased deposit of sediments caused by the reduction of the flow rate can have the consequence, that the relevant fish species can not find a suitable place for spawning (Collen & Gibson 2001; Rosell et al. 2005). Beaver dams can either have the effect, that sediment-free spawning grounds are preserved or even newly created or that valuable spawning grounds may possibly be destroyed by an increased deposit of sediments, depending if a river section up- or downstream of the dam is considered. On the other hand, for fish species, which deposit their eggs in a soft substrate or on aquatic plants, a beaver pond can offer valuable spawn grounds (Collen & Gibson 2001).
In addition, beaver dams can have a barrier effect for migrating fish species. Primarily this can be a problem for species, which migrate during the autumn to reach their spawning grounds (e.g. speckled trout - Savelinus fontinalis or brown trout - Salmo trutta m. fario). During late summer and autumn most flowing waters carry only a limited amount of water, so that migrating fishes can barely pass a beaver dam. This is different for instance for the non-native rainbow trout (Oncorhynchus mykiss): similar to all species of pacific salmons (Oncorhynchus spp.) also the rainbow trout is spawning during the spring thereby partially travelling large distances. Generally, during this season the flowing waters carry more water, so that beaver dams (and other obstacles) can be passed more easily (Collen & Gibson 2001; Rosell et al. 2005). For the fry of some fish species, beaver dams represent only a minor migration obstacle, because they can pass the dam through the interstitial space below the dam (Collen & Gibson 2001).
As already stated above, by influencing the invertebrate fauna through a beaver pond, the nutritional condition for different fish species can be altered as well. For instance, brown trouts (Salmo trutta m. fario) of similar age are larger in Swedish beaver ponds compared to those, living in free flowing river sections (Rosell et al. 2005). This is also the case for other fish species.
Also the rise of the water temperature on beaver ponds (see above) may have a positive effect (increased growth rate) even for representatives of the salmonids, as long as the water temperature stays below a certain maximum temperature critical for these species (in this, the temperature might only play a minor role, whereas the oxygen concentration, which decreases with increasing water temperature, might be of higher importance). However, if the water temperature rises above this critical value (or the oxygen concentration falls below a critical threshold), an increased mortality has to be expected for this fish species (Rosell et al. 2005). Regarding their growth rate, an increased water temperature compared to a river section unaffected by the beaver has also a positive effect on several carp-like fishes (family Cyprinidae) as well as on pikes (Esox lucius) and several small fishes (Collen & Gibson 2001). Mainly the different smaller fish species can serve as food for the larger, predatory fish species.
Apart from that, also the species diversity and the population density of members of the family of carp-like fishes (Cyprinidae) are profiting from beaver ponds (Ciechanowski et al. 2011).
Beaver ponds can function as retreats for several fish species, for instance in times of drought when the rest of the waterway is almost dried out and the only water remaining is that inside of the beaver ponds, or during the winter when the rest of the waterway is frozen to the bottom (both, however, requires a certain minimum depth of the beaver pond; Collen & Gibson 2001; Rosell et al. 2005). In case that the beaver ponds are so shallow, that they freeze to the bottom in winter as well, the population composition of such beaver ponds differ from that of deeper lakes or ponds, because these beaver ponds predominantly contain smaller and younger fish of a given species. The larger and older animals in such ponds show a higher mortality because of the lack of oxygen after freezing-up, whereas the smaller animals can cope better with the lack of oxygen (Collen & Gibson 2001). All in all, mainly fish species with a higher tolerance towards higher water temperatures and lower oxygen concentrations in the water which prefer either a soft substrate or a dense vegetation for spawning, profit from beaver ponds (Rosell et al. 2005).
Amphibians and reptiles
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Also amphibians profit from the activity of the beavers, because by the construction of dams new spawning waters for amphibians will develop and the distance between suitable spawning waters will be reduced. Moreover, potentially suitable spawning waters for amphibians are created by the dam-building activity of the beaver, which meet the different demands of single amphibian species (like waters with perennial water supply; waters which periodically fall dry and therefore do not contain any fish, which is beneficial for the development of the amphibians etc.). As as a consequence, in areas with beaver ponds the density of individuals of some amphibian species will be increased as well as the species diversity of amphibians dependent on still waters for spawning (Cunningham et al. 2006; Dalbeck et al, 2007).
With respect to the amphibian diversity, the number of species in beaver ponds in the Eifel is significantly higher compared to all other water bodies under investigation, and older beaver ponds usually show a higher number of species than younger ones (Dalbeck et al. 2007). Both in Europe as well as in North-America, mainly representatives of frogs (genera Pelophylax or Rana respectively) and toads (genus Bufo) benefit from beaver ponds (Stevens et al. 2007). Besides the lack of fishes in some beaver ponds (fishes prey on spawn and on the larvae of amphibians), amphibians profit from the high amount of deadwood as shelter against predators, the increased food availability in the beaver ponds (amphibian larvae feed on the algae and bacteria covering the deadwood), as wells as from the possibility to attach their spawn cords to the deadwood. Because beavers fell trees close to the bank, sunny spots are created in the beaver ponds, which are preferably visited by the amphibians (Dalbeck et al. 2007).
One of the few amphibian species, which do not profit from a beaver pond, is the fire salamander (Salamandra salamandra), because it prefers small, clear, and rapid flowing waters for spawning (Rosell et al. 2005). Moreover, several other factors (which are in some cases not yet fully understood) play a role in whether and to what extend a beaver pond will be inhabited by amphibians. Some of these factors are the distance to other spawning waters and therefore the probability, that the amphibians can find their way to the newly developed beaver pond, small-scale habitat differences or the occurrence of fishes in the beaver pond (fishes are the main predators on spawn and developing young animals). Therefore, not all beaver ponds will be in any case inhabited by amphibians (Dalbeck et al. 2007).
Regarding the importance of beavers for reptiles, in Europe mainly the European pond terrapin (Emys orbicularis) and the grass snake (Natrix natrix) profit from the construction of a dam by beavers. The shallow, slowly flowing or standing water, the dense submerse vegetation, and the soft, organic substrate meet the habitat demands of these species (Rosell et al. 2005).
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Several birds are using beaver ponds for resting, for breeding, and for raising their young as wells as for foraging, whereas the occurrence of invertebrates plays an important role as well. Mainly different duck species quickly accept newly constructed beaver ponds; however, these species generally prefer ponds with a size of 1000 m2 and above (Nummi 1992; Rosell et al. 2005; Stoffyn-Egli & Willison 2011).
Among others, beaver ponds were used for foraging by herons (in western Europe first of all by grey herons - Ardea cinerea), great bittern (Botaurus stellaris), cormorants (Phalacrocorax carbos), mergansers (genus Mergus) or kingfishers (Alcedo atthis). The increased amount of deadwood near beaver ponds (dying trees caused by peeling off the bark or by water logging) can be a valuable resource for woodpeckers when foraging and/or for the construction of breeding holes for raising their young, and they sometimes are used by birds of prey as a preferred lookout perch. In addition, insectivorous birds (for instance swallows) are profiting from beaver ponds, because flying insects are occurring in larger quantities over here as well (Collen & Gibson 2001; Rosell et al. 2005).
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Several other mammals profit from the beaver ponds or the beaver lodges as well. For instance, water voles (Arvicola terrestris, A. sapidus), otters (Lutra lutra), badgers (Meles meles), red foxes (Vulpes vulpes), lynxes (Lynx lynx), as well as European minks (Mustela lutreola) are using still occupied or abandoned beaver burrows (Rosell et al. 2005). Otters and European minks also use beaver ponds as hunting areas (Collen & Gibson 2001).
Roe deer (Capreolus capreolus), red deer (Cervus elaphus), moose (Alces alces), wild boars (Sus scrofa) and brown hares (Lepus europaeus) profit from the beaver meadows resulting from the drying out or the silting up of beaver ponds, because here they can find grasses and herbs for feeding, which might be of importance in densely wooded areas. As already mentioned above (see the chapter on the Temporal development of a beaver pond), the regrowth of trees on a beaver meadow will be inhibited for an extended period of time, so the land mammals can use the beaver meadows for foraging for some time. Moreover, the water plants are readily browsed by moose and wild boars use the shallow edges of beaver ponds as wallows (Rosell et al. 2005).
Because of the increased appearance of flying insects at the beaver ponds, they are readily visited by some bat species for hunting. Inside the dead trees caused by the activity of the beavers, the bats can find appropriate daytime roosting possibilities or nursery roosts as well (Rosell et al. 2005).
The attractiveness of beaver ponds for bat species, which predominantly hunt over open water areas (e.g. the Daubenton's bat - Myotis daubentonii), is influenced by the age of the beaver pond: Mainly in the beginning, the water surface of a beaver pond will be covered by duckweed (genus Lemna) to a large extend, which will disperse the ultrasonic calls, emitted by the bats for orientation and hunting, which will significantly impair the orientation abilities and the location of prey of the bats in return. This is the reason why young beaver ponds (0 - 10 years) are less frequently visited by bats hunting over open water areas. With the ageing of the beaver ponds, by an by large-leafed floating-leaf plants (e.g. water lilies - Nymphaea sp. or yellow water lilies - Nuphar sp.) are dominating, which disperse the ultrasonic calls of the bats to a far lesser extend. Very old beaver ponds (40 years and above) often completely lack floating-leaf plants. With the appearance of large-leafed floating-leaf plants, beaver ponds become very attractive for bat species, which hunt near or above the water (Ciechanowski et al. 2011).
During investigations in northern Poland, along waterways inhabited by beavers the activity of different pipistrelle species (Pipistrellus sp.) as well as the noctule (Nyctalus noctula) was higher in comparison to water sections not inhabited by beavers. Both, pipistrelles as well as noctules preferably hunt in gaps between densely growing trees - such gaps are created by the tree-felling activity of the beaver (Ciechanowski et al. 2011).
Finally, beavers themselves can serve as food for predators like wolves (Canis lupus) or brown bears (Ursus arctos). In Latvia for instance, beavers are thought to be the most important source of food for wolves during the summer, mainly when the population density of ungulates is low (Rosell et al. 2005).