Table of contents
Food composition [Back to table of contents]
A beaver cutting a branch
(photo: Christoph Elbert). Even if woody plants are an important component of the diet, beavers also feed on a large quantity of herbaceous plant species and they are by no means restricted to the consumption of bark alone. For the Eurasian beaver the consumption of approximately 230 plant species has been proved so far, whereas 80 of these food plants are woody plants and the remaining 150 species are herbaceous plants (terrestrial and aquatic grasses and herbs; Kitchener 2001; Recker 1997; Zahner et al. 2005). However, so far only a limited amount of research has been conducted on the food range of the Eurasian beaver and these investigations mainly focus on woody food plants, because this is relatively easy to examine. Contrastingly, studies on the consumption of herbs and grasses by the animals are hard to find (Krojerová-Prokesová et al. 2010). On the one hand, this is due to the fact that based on the feeding pattern it is very difficult to conclude, if a herbaceous plant has been consumed by a beaver or any other animal. On the other hand, analysis of faeces - an investigation method frequently used with other mammals to determine their food range - is difficult in beavers, because usually they discard their faeces and urine directly into the water, which makes the collection of a sufficient quantity of faecal pellets very laborious.
The number of plant species consumed by beavers strongly depends on the availability and can locally differ significantly (the already mentioned Rhône-beavers for instance feed on grasses almost exclusively). Besides, preferences and aversions of individual species or families respectively can strongly influence the food composition (Djoshkin & Safonow 1972; Krojerová-Prokesová et al. 2010). Because the pubs observe their parents when eating and because they receive food from their parents as well, the young beavers learn what is edible and what is not. Regarding the plant species preferably consumed, by this type of social learning individual families can develop some kind of nutritional traditions. This forming of traditions refers first of all to the consumption of woody plants, because beavers are quite conservative when selecting woody food plants. Traditions in the herbs or grasses preferentially eaten have been observed only rarely (it is also considerably more difficult to discover), because - as some authors think - the animals are far more flexible when choosing this kind of food (Kitchener 2001).
The large number of different food plants identifies the beaver as a food generalist, whereas this certainly does not mean, that all food plants known so far are consumed in equal quantities. Independent of the preferences of individual animals already mentioned, there are some plant species, which are preferentially chosen by all beavers.
A poplar felled and debranched by
beavers at the Schulenriedengraben. Regarding the woody plants, mainly members of the willow family (Salicaceae), i.e. willows (genus Salix) and poplar (genus Populus) are used by the Eurasian beaver (Haarberg & Rosell 2006; Kitchener 2001; Recker 1997; O'Connell et al. 2008; Zahner et al. 2005).
Animals of the subspecies Elbe beaver (Castor fiber albicus), to which also the beavers in the district Emsland belong, are known to prefer willows (mainly almond willow - Salix triandra, wicker - S. viminalis, sharp-leaf willow - S. acutifolia, purple willow - S. purpurea, and to a lesser extend grey willow - S. cinerea, eared willow - S. aurita, as well as white willow - S. alba, sallow - S. caprea, daphne willow - S. daphnoides, crack willow - S. fragilis, bay willow - S. pentandra), poplar (mainly European aspen - Populus tremula, white poplar - P. alba, black poplar - P. nigra, as well as hybrid black poplar - P. x canadensis), and European rowan - Sorbus aucuparia, as well as common whitebeam - Sorbus aria (Recker 1997; Stocker 1983).
Less frequently eaten by Elbe beavers are birch (downy birch - Betula pubescens, silver birch - B. pendula), oak (common oak - Quercus robur, sessile oak - Q. petrea), maple (European maple - Acer platanoides, sycamore - A. pseudoplatanus, field maple - A. campestre), elm (European white elm - Ulmus laevis, field elm - Ulmus minor), common beach - Fagus sylvatica, European hornbeam - Carpinus betulus, hazelnut - Corylus avellana (in areas without willows and poplars, after the opinion of Kitchener (2001) the hazelnut can replace these woody species), wild cherry - Prunus avium, hackberry - Prunus padus, European ash - Fraxinus excelsior, as well as spruce (Norway spruce - Picea abies, Colorado spruce - P. pungens) and pine (Scots pine - Pinus sylvestris, European black pine - P. nigra)(Recker 1997).
Very unpopular are above all alder (European black alder - Alnus glutinosa, grey alder - A. incana), but also lime (small-leaved lime - Tilia cordata, large-leaved lime - T. platyphyllos), horse-chestnut - Aesculus hippocastanum, European spindle tree - Euonymus europaeus, alder buckthorn - Frangula alnus, as well as spiny or thorny woody plants like common hawthorn - Crataegus monogyna, blackthorn - Prunus spinosa, dog rose - Rosa canina, European barberry - Berberis vulgaris or common sea-buckthorn - Hippophae rhamnoides (Haarberg & Rosell 2006; Recker 1997).
Completely avoided by the beavers are the different elder species (European red elder - Sambucus racemosa, European black elder - S. nigra), probably because they contain cyanogenic glycosides like Sambunigrin for instance (Haarberg & Rosell 2006; Nolet et al. 1994).
Proportion of willows, other decidous
trees and conifers of all woddy species
used by beavers at the river Hase
(source: Klenner-Fringes 2001). For the beavers in the district Emsland on a river section of 10 km length (between the road bridge near Bokeloh at river kilometer 8 and the mouth of the river Mittelradde at river kilometer 18) the use of 23 tree species out of a total of 43 discovered woody plant species was observed (Klenner-Fringes 2001). The woddy plant species felled were great sallow, grey willow, lavender willow - Salix elaeagnos, crack willow, almond willow, wicker, white poplar, European aspen, European rowan, sweet cherry, black cherry - Prunus serotina, common broom - Cytisus scoparius, common sea-buckthorn, tree of heaven - Ailanthus altissima, silver birch, European black alder, European beach, common oak, as well a European ash and from the conifers European larch - Larix decidua, Norway spruce, Scots pine, as well as red fir - Pseudotsuga menziesii (Klenner-Fringes 2001).
After Klenner-Fringes (2001), preferentially consumed were the willows mentioned above, which were used most frequently by far with approximately 92% of all felled trees or cuttings respectively. From the remaining 8% of the other woody plants, preferentially used by the beavers were Norway spruce, tree of heaven, European beach, European aspen, silver birch, European rowan, common broom, European ash, sweet cherry and European larch. It has to be noticed, that Norway spruce, common broom, tree of heaven, and European beach only occur with some single individual trees along the river Hase, but that these trees were deliberately used by the beavers. Black cherry and Scots pine were eaten according to their offer. Disproportionately felled were common oak, white poplar, and - above all - European black alder (Klenner-Fringes 2001).
It has to be mentioned that most of the willows were mainly shrubby willows unimportant for forestry and the other woody plants contained also some species, which were of no or only limited forestry use (e.g. black cherry, common broom, common sea-buckthorn silver birch, European black alder). At waters with softwood floodplain forest functional to a certain extend or at least with a strip riparian softwoods, for instance shrubby willows, which are not completely sacrificed to axe or chain saw during "tending strategies", there is only a limited risk, that beavers cause larger damages to forestry.
Herbs & grasses
After Djoshkin & Safonow (1972) Eurasian beavers prefer to eat the following terrestrial herbs: meadowsweet - Filipendula ulmaria, ground elder - Aegopodium podagraria, boneset - Symphysium officinale, amphibious bistort - Persicaria amphibia, marsh woundwort - Stachys palustris, corn thistle - Cirsium arvense, field and common sowthistle - Sonchus arvense and S. oleraceus, as well as different sorrel species - Rumex sp. and stinging nettle species - Urtica sp.
Regarding aquatic grasses and herbs common reed - Phragmites australis, common bulrush and lesser bulrush - Typha latifolia and T. angustifolia, water
lily and yellow water lily - Nymphea alba and Nuphar lutea (mainly the rhizome and the leaf stalk), branched bur-reed - Sparganium erectum,
fine-leaved water-dropwort - Oenanthe aquatica, buckbean - Menyanthes trifoliata, common water-plantain - Alisma plantago-aquatica, as well as different
sedge species - Carex sp. are eaten.
Besides the plant species already mentioned, Elbe beavers are known to consume wild chervil - Anthriscus sylvestris, cabbage thistle - Cirsium oleraceum, rosebay willowherb - Epilobium angustifolium, common hogweed - Heracleum sphondylium, common bistort - Polygonum bistorta, creeping buttercup - Ranunculus repens, yellow rattle - Rhinanthus minor, great burnet - Sanguisorba officinalis, yellow iris - Iris pseudacorus (preferentially the rhizomes), and candle rush - Juncus effusus (Ganzhorn & Harthun 2000).
Among others, the beavers in the district Emsland are eating the following terrestrial herbs and grasses: common wormwood - Artemisia vulgaris, tickseed -
Bidens sp., willowherb - Epilobium sp., common tansy - Tanacetum vulgare, canary grass - Phalaris arundinacea as well as the aquatic plants
calamus - Acorus calamus, yellow iris, yellow water lily, common reed, water pepper - Polygonum hydropiper, great yellow cress - Rorippa amphibia,
as well as common arrowhead - Sagittaria sagittifolia (Klenner-Fringes 2001).
After the opinion of some authors, true grasses only play a minor role in the diet of beavers (Krojerová-Prokesová et al. 2010). On the other hand, other authors claim that canary grass (Ganzhorn & Harthun 2000; Heidecke 1977; Kitchener 2001) or purple small-reed - Calamagrostis canescens are preferred food plants of the beaver (Kitchener 2001).
Besides the plant species listed above, the beaver consumes several other species - also some, which are known to be poisonous like lily of the valley - Convallaria majalis or poison hemlock - Conium maculatum. As a rule, of these plants only small amounts are consumed at one time by the animals (Djoshkin & Safonow 1972).
Seasonal alteration in food composition [Back to the top]
Depending on the season, the food composition of the beaver differs distinctively. During the spring and the summer, mainly terrestrial and aquatic herbs and grasses and only a limited amount of woody food (mainly the bark) are consumed (Jenkins & Busher 1979). For North-American beavers (C. canadensis) a ratio of lignified to non-lignified dietary constituents of 1 : 15 was found during the summer. Contrary, during the autumn and fall mainly the bark and young branches of the woody species mentioned above are consumed. Additional food sources during the winter, the rhizomes of water lily, yellow water lily, or yellow iris are frequently dug up (partially, the bottom of the water is systematically searched for theses plants). If sever frost does not prevent it, the fleshy roots of terrestrial plants are also dug up and consumed (according to own observations, beaver in the district Emsland are willingly eat the roots of wild chervil for instance). In North-American beavers the ratio of lignified to non-lignified food amounts to approximately 4 : 1 during the winter (Müller-Schwarze 2011; Zahner et al. 2005). This difference is also reflected in the time the beavers spend for the consumption of woody or herbaceous plants during the different seasons. During the summer, more than 90% of the time budget for food intake is spend on the consumption of non-lignified, herbaceous plants, whereas during the winter, the animals spend between 60 to 90% of their time of food intake by consuming bark (Müller-Schwarze 2011). These proportions, discovered in the North-American beaver may also be applied to the Eurasian beaver.
Feeding site of a beaver. During the cold season, beavers - which do not show hibernation or dormancy but are active the whole year round - are predominantly feeding on woody plant species. Thereby not the whole wood is consumed, but the bark (to be more precise the secondary phloem and the underlying cambium) is peeled from the branches or twigs with the help of the incisors and subsequently eaten. The actual wood (the secondary xylem), which is very hard to digest, will not be eaten by the beavers. Because for eating, the animals frequently visit the same sites within their territory, the accumulation of peeled branches and twigs at such feeding sites is an unambiguous sign for the presence of beavers at a body of water.
An exception in this respect are the French Rhône beavers (the subspecies Castor fiber galliae), which also populate waters, which lack any riparian trees or shrubs. The animals exclusively feed on grasses and herbs at such habitats and are therefore sometime termed "grass beavers". Something similar is known from the Mongolian beaver (the subspecies Castor fiber birulai), which have to be satisfied with a narrow strip of woody plants along the rivers of the Mongolian arid regions and which have to feed to a larger extend on herbaceous plants over there (Djoshkin & Safonow 1972).
Creation of food rafts as winter stock [Back to the top]
Food storage (food raft) of a beaver
(river Ohre near Brome, district Gifhorn). With the beginning of autumn, approximately from September onwards, the beavers increase their efforts to cut and fell woody plants. A portion of these branches are not eaten immediately, but put aside as a stock for the winter. For this purpose, the animals stack branches in the water somewhere near their burrow to build a "food raft" (Djoshkin & Safonow 1972). The food storage contains mainly woody species preferentially eaten by the animals like willows or poplars, however, also less fancied woody plants like alders are incorporated into the food raft. It is assumed, that the alder branches are not eaten, but serve to stabilise the food raft (Djoshkin & Safonow 1972; Müller-Schwarze 2011; Zahner et al. 2005). Partly it was observed, that the alder branches were put on top of the food storage and that these branches eventually add some weight to the food raft to keep the preferred woody branches under water (Jenkins & Busher 1979; Zahner et al. 2005) so that they can be reached even if the water surface freezes up (the beavers approach the food raft by diving).
Apart from woody plants, sometimes also grasses and herbs are placed in the food raft, especially different aquatic plants (Djoshkin & Safonow 1972), the rhizomes of water lilies and yellow water lilies, as well as buckbean, cattail, and true grasses like, for instance, great water grass - Glyceria maxima (Kitchener 2001). Sometimes herbaceous plants can reach between 40 - 50% of the total mass of the food raft (Djoshkin & Safonow 1972).
Not all beavers build a winter storage. Families living in waterways which do not freeze up during the winter, usually renounce building a food raft. Also single animals and small family groups do not gather a food stock (Djoshkin & Safonow 1972) - perhaps because the team work of a larger group is necessary to collect a sufficient quantity of branches. Apart from that, the geographic position is of importance when creating a winter food supply or not, because animals in warmer climates have to rely on it to a lesser extend (Zahner et al. 2005). Rhône beavers in their original range, for instance, do not collect branches to build a winter food supply. However, the animals are surely capable to do this, as animals re-introduced in Switzerland have proven (the winters in Switzerland are colder and the Rhône beavers build food rafts after they became established over there; Kitchener 2001).
Regarding the construction and the size of food rafts, moreover personal experiences of individual beavers plays a role as well. Even neighbouring families can clearly differ in this respect (Zahner et al. 2005).
Quite different observations were made, regarding the size of food rafts. In the Eurasian beaver, Djoshkin & Safonow (1972) found food rafts consisting out of some few branches up to those with a maximum size of 70 m3, whereas the average size was calculated at 4.6 m3 (this as well illustrates the large variability regarding the construction of food rafts). After Zahner et al. (2005), the size varies between 22 and 154 m3. Müller-Schwarze (2011) gives weight values for food rafts build by beavers in Alberta, Canada (relatively cold winters). Accordingly, the weight of the collected branches varied between 39.5 and 56 kg.
After nutritionally calculations most of the food rafts examined in Alberta, Canada were not sufficient to feed all of the family members, which collected the respective food stock, over the whole winter time (Müller-Schwarze 2011). Therefore, even the beavers which collect a winter food supply are still dependent on leaving the water from time to time to search for food on land, i.e. to cut or fell woody plants.
Effect of plant materials on the diet [Back to the top]
Regrowth of a willow after being
browsed by a beaver (Neetze canal
near St. Dionys, district Lüneburg). The preference of willows and poplars as food plants by beavers is explained by the very low amount of secondary plant materials (e.g. tannic acids) of these tree species. Tannic acids (also called tannins) serve several plants as defensive materials against herbivorous animals, because they have a negative effect on the digestive processes by inhibiting the activity of the animal digestive enzymes (Sitte et al. 1981). However, young poplar and willow shoots developing out of a tree stump after the felling of the tree contain a higher amount of bitter substances (different phenolic substances, tannins), which disappear by and by with the increasing age of the shoots (Gallant et al. 2004; Müller-Schwarze 2011; Zahner et al. 2005). Consequently, these young shoots are not being eaten by beavers during this time. However, the influence of a certain group of tannins (the so called "hydrolysabel tannins" or pyrogallol-type tannins) on the beavers keeps within limits, because the beaver's saliva contains enzymes, which are able to decompose these tannins (contrastingly, this is not the case for the second group of tannins, the "condensed tannins" or catechins). It has been proved, that beavers are able to distinguish between these two tannin-types (they prefer trees which mainly contains pyrogallol-type tannins) and that they are even able to determine the amount of the tannin content of a tree (they choose trees with the lowest tannin content; Müller-Schwarze 2011). Besides the olfactory sense, perhaps also the sense of taste is of importance in evaluating if a tree is palatable or not because inside beaver territories gnawed trees can often be found, which are not further worked on by the animals (Kitchener 2001; Müller-Schwarze 2011).
Therefore, willows and poplar are not preferred by the beavers because of their high nutritional value but rather because of their easy digestibility. As a comparison of the retention time - that is the time which is necessary for a certain kind of food to pass the alimentary tract - of different kind of bark food shows, depending on the tree species there are pronounced differences in this respect. For the North-American beaver consuming the bark of American aspen (Populus tremuloides) - the preferred woddy plant food of the beaver in North-America - retention times between 10 and 20 hours have been determined (Ganzhorn & Harthun 2000; Müller-Schwarze 2011), whereas the bark of alder for instance takes between 40 to maximum 60 hours to pass the alimentary tract (Ganzhorn & Harthun 2000) and red maple (Acer rubrum) 30 - 50 hours (Müller-Schwarze 2011). The long retention time and therefore the low digestibility of alder bark is at least partly caused by the relatively high fibre content (above all hemicellulose, cellulose, and lignin, subsumed as NDF = neutral detergent fibre) and the resulting low amount of easy digestible, organic substance. Although alders have a comparable high energetic value, this energy is hardly usable by the beaver (Nolet et al. 1994). Similar findings were made for the maple bark. For this reasons, the different alder and maple species are not among the preferred woody food plants of the beaver.
As already stated above, besides willows and poplars several other plants are consumed, because beavers can not feed on a single tree species alone without this leading to deficiency symptoms. At feeding trials with a single tree species at a time, all experimental animals were loosing weight. When being fed American aspen or bigtooth aspen (Populus grandidentata) the beavers were loosing approximately 0.1% of their body weight per day, when being fed birch, maple or alder the weight losses of 0.3 - 0.6% per day were even more pronounced (Müller-Schwarze 2011): Besides the impact on the weight development the quality of the diet ingested is also reflected in the littler size of the beaver. With decreasing food quality the females give birth to increasingly smaller litters (Jenkins & Busher 1979). Therefore, beavers are dependent to complement their preferred food (willows and poplars) with other plant species. Plants like hazelnut, ash and also most of the aquatic plants serve the beaver as a sodium source, whereas bird cherry and poplars are mainly consumed because of their relatively high phosphorous content (Nolet et al. 1994).
Required food quantity [Back to the top]
Because the consumption of woody plants by the beaver is very conspicuous and a even the felling of just a few larger trees are hard to overlook, frequently the question
arises, how much food a beaver needs per day (maybe because out of fear, that the beavers may fell all the trees along a river or a lake shore).
The basal metabolic rate - that is the energy required to maintain the metabolism of a resting animal organism - is said to be approximately 265 - 296 kJ per kg body mass per day for the North-American beaver (Brenner 1967; Jenkins & Busher 1979), whereas, however, no distinction between summer and winter is been made. More detailed data can be found at Zahner et al. (2005), who estimate the basal metabolic rate of grown Eurasian beavers at 180 kJ/kg/day during the summer and 210 kJ/kg/day during the winter. According to that the data of Brenner (1967) and Jenkins & Busher (1979) were estimated for the winter and the comparable higher values are due to the colder winters in North-America.
The required amount of food needed per animal varied depending on the season, the size of the animal, and its activity between 0.63 and 0.99 kg per day (for North-American beavers). In single cases up to 2 kg per animal per day has been detected as well (Brenner 1967). Regarding woody food plants (bark and young twigs), grown beavers require about 0.9 kg per day during the winter (Zahner et al. 2005). However, on has to take into account, that the animals even during the winter time do not feed and woody plants alone (see above). During the summer month beavers consume between 1.5 to 2 kg fresh herbages (mainly terrestrial an aquatic herbs and grasses; Zahner et al. 2005).
Studies have been conducted in North-America, which evaluated the amount of food a beaver can use of a tree with a certain stem diameter, because beavers only consume the bark and the young twigs (see above). According to theses studies, the 0.9 kg fresh bark corresponds to a poplar with a diameter at breast hight (DBH) of approximately 3.1 cm (Aldous 1938). Other authors calculated a somewhat more favourable ratio, because a poplar of a DBH of 2.5 cm should already provide approximately 1.3 kg of usable food for the beaver (Müller-Schwarze 2011). Thus, a relatively small tree of no more than 3 cm stem diameter would already be sufficient to feed a beaver for one day.
Because beavers only consume the bark and the young twigs of the felled trees and the timber is used only for the construction of their lodge or a dam, after the opinion of some authors every single beaver needs up to 4,000 kg of timber per year (Freye 1978; Kitchener 2001). However, very large trees are relatively rarely felled by beavers (see also the next section).
Stem diameter of the woody plants used [Back to the top]
Approximately 65% of the trees used beavers in Bavaria have a diameter at breast hight of less than 5 cm (Zahner et al. 2005). During studies at the
river Elbe it was noted, that 90% of the trees felled by beavers had a stem diameter of less than 10 cm (DBH), whereby trees with a diameter of less than 5 cm represent
a high proportion (Recker 1997). In Norway, stems of a diameter of less than 5 cm even represents 95% of all trees felled by beavers
(Haarberg & Rosell 2006).
Distribution of the stem diameters of
woody pants used by beavers at the
river Hase (source: Klenner-Fringes
2001). At the river Hase, district Emsland the proportion of felled or cut willows with a stem diameter of less the 5 cm (measures taken below the cut surface) amounted to approximately 90% of all willows used (all in all 6,354 willow cuttings were measured; Klenner-Fringes 2001). Of the felled woody plants not belonging to the willows (all in all 574 fellings were surveyed) approximately 84% showed a stem diameter of less than 5 cm (measured directly below the felling cone; the majority of cuttings were between 2 and 4 cm in diameter), approximately 12% of all non-willows had a stem diameter between 6 - 10 cm and only about 4% of all stems had a diameter of more than 10 cm (Klenner-Fringes 2001). All in all, the stem or branch diameter respectively of 89% of all felled or cut woody plants (willows and non-willows) was less than 5 cm (measured below the cutting, the DBH would have been respectively lower).
As already mentioned, the felling of larger trees by beavers is rare. However, this does not mean, that they do fell such large trees from time to time. They will not even back away from very large trees. For Norway, for instance, the felling of a poplar of 68 cm, a birch of 58 cm, and an oak of 38 cm stem diameter was reported and in Sweden the felling of birches up to 100 cm thick was observed. Likewise, at the Voronezh area in Russia, willows and poplars of more than 100 cm stem diameter were felled by the beaver. Furthermore, at the Ukraine an oak with 90 cm diameter was used in the past (Kitchener 2001).
At the river Hase, the stem diameter of the largest tree, felled between the years 1990 and 1995 - an European aspen - was 34 cm (Klenner-Fringes 2001). However, during the survey of Klenner-Fringes (2001) over a time period of 5 years only 7 trees of more than 20 cm stem diameter were felled (1 wild cherry, 6 European aspens).
The preferred utilisation of woddy plants with small stem diameters can be explained by the fact, that larger trees (not regarding some exceptions) are not very attractive for beavers, because they have to spend relatively large amounts of time and energy for the felling for such trees, but the nutritional value does not match the expenditure for time and energy (Nolet et al. 1994; Zahner et al. 2005). Some authors even guess, that in areas where beavers fell larger trees the food supply is insufficient for the beaver (the amount of woody plants with smaller stem or branch diameter respectively is insufficient). In such areas the population density is thought to be lower, because to supply a family with a sufficient amount of food the territories have to be correspondingly large (Recker 1997).
Distance to the riverbank during foraging [Back to the top]
Beavers are so-called "central place foragers", which means, that the animals start foraging from a central place within their territory (normally
their lodge), that they carry the collected food items towards this central place and that they consume them over there. However, beavers do not transport all the
food to their lodge, but they have several feeding sites scattered over their territory, which are as a rule situated near the bank or shore of the water bodies
(the animals place themselves half way inside the water when eating), because the animals use the water to escape in case of danger
(Haarberg & Rosell 2006; Kitchener 2001; Krojerová-Prokesová et al. 2010;
Müller-Schwarze 2011; Zahner et al. 2005). Due to the fact that beavers are endangered by terrestrial predators when on land
and that they try to escape by seeking the water when in danger, the animals do not move far from the next water body when foraging.
For the North-American beaver it has been proofed, that the time spend for foraging and feeding, as well as the number of cut or felled woody plants linearly decreases with increasing distance from the bank or shore. Normally, Eurasian beavers move about 10 to 20 m away from the bank or shore when foraging; only the occurrence of preferred food can cause the animals to move further distances away from the water (Zahner et al. 2005). During studies in Norway, 40% of all trees felled by the beaver were found within a distance of 10 m away from the bank (Haarberg & Rosell 2006). Only in rare cases, beaver are utilising food plants, which grow more than 60 m away from the bank (Krojerová-Prokesová et al. 2010). With increasing distance from the bank or shore, beavers tend to get more and more selective when choosing their food, whereby literature references partly contradict each other with regards to this selectivity. In some studies it has been described, that with increasing distance from the water more and more trees with smaller stem diameters are felled (Jenkins 1980; Müller-Schwarze 2011), whereas the animals in other studies felled more and more larger trees with increasing distance from the water (Gallant et al. 2004). Also with respect to the choice of the tree species used in dependence of the distance to the bank, contradictory data exists. After the opinion of some authors, with increasing distance from the water only tree species which are preferred by the beavers (e.g. poplars) are being felled (Müller-Schwarze 2011), whereas other authors were not able to show any selectivity dependent on the tree species (Haarberg & Rosell 2006; Gallant et al. 2004). After the theory of "optimal foraging" it would to be expected, that beavers choose smaller trees of selected species with increasing distance from the water, in order to keep the effort (time and energy for felling and transport) in a favourable relation to the benefit (energy and nutrient content of the trees felled).
Distribution of woody plants used by
beavers at the river Hase compared
to the supply (source: Klenner-Fringes
2001). A similar situation can be found at the river Hase at the district Emsland. Almost all willows (97% of the 6,354 cuttings) were cut within a distance of 5 m from the bank, because the willows used were predominantly shrubby willows growing near the bank. Of the non-willow trees felled, more than half of them were utilised in a distance of not more than 10 m from the water, although the supply of woody plants within a distance of 10 to 25 m from the bank has been substantially higher (81% of all trees were growing within this distance) than at a distance of 0 to 10 m (only 19% of all trees were growing at this distance). Over the distance of 25 m from the bank, the number of fellings and cuttings was almost exponentially declining with increasing distance to the water (Klenner-Fringes 2001), i.e. with increasing distance from the water less and less trees were felled.
For single tree species, a significant correlation between the distance from the bank and the stem diameter could have been proven. According to that, with increasing distance from the water common ash trees with a larger stem diameter were used, whereas of the Scots pines and common oaks trees with smaller stem diameters were felled with increasing distance from the bank. However, if all tree species were analysed together, no correlation between the distance from the bank and the stem diameter was detectable (Klenner-Fringes 2001). These tree species dependent differences in the utilisation of trees with different thickness in dependence of the distance from the bank may be the reason for the inconsistent results mentioned above is the studies of the same phenomenon, because this studies did not differentiate between the single tree species.
Agricultural use of the land right up
to the waters edge (river Alte Leine,
district Hannover). The studies cited over here show, that beavers only use a relatively limited strip of the bank of the water bodies populated by them. Therefore, to prevent most damages due to beavers feeding on cultivated plants or trees used for forestry, it would be sufficient to allow natural succession to take place on a 10 to 20 m broad riparian part of a river or a lake (Krojerová-Prokesová et al. 2010). However, not only at the district Emsland common practice appears to be much different from this, because at most water bodies the soil is being cultivated right up to the bank. Due to such a management problems with beavers are very often inevitable.
Unfortunately with the revision of the water resource act (WHG as of 07/31/2009) the § 38 specifies the breadth of the riparian strip to 5 m nationwide, which not only with regards to the beaver as a semi-aquatic mammal may lead to increasing problems in the near future. Originally, in the water act of Lower Saxony (NWG) a breadth of the riparian strip of 10 m was provided for waters of first order and 5 m for waters of second order. However, even this not substantially better regulation of the NWG has been abandoned in favour of the nationwide regulation of the WHG (as an example see this statement of the Landvolk Niedersachsen at archive.org). Many nature conservation associations regard this general breadth of the riparian strip as too small for more than one reason, as this position statement of the BUND Lower Saxony exemplary illustrates.
Felling of a tree [Back to the top]
A pine double conically gnawed at the
river Lotter Beeke. If beavers feed on woody plants, trees of a stem diameter of more than 8 cm are generally felled by a typical double-conically cut (resulting in a shape of an hourglass; Freye 1978). After Göhre (1954), the workload of felling amounts to approximately 30 J for every single bite (for illustration: 30 J corresponds to the work necessary to lift a 3 kg weighing object to a height of 1 m). After the same author, beavers have to spend a force of 785 N for every single bite.
Generally, trees are felled by the beaver at a height of approximately 20 cm (maximum up to 50 cm; Haarberg & Rosell 2006). For felling a tree, the animal raises to its rear extremities and supports himself with his hands on the trunk. Thereby, one hand is always set higher than the other, so the torso is bend sideways. The tail is being used for support. In most cases, the part of the stem at which the felling wedge will be applied will be cleaned of the bark beforehand (Djoshkin & Safonow 1972).
Next, the lower cone will be gnawed by the beaver by bending the head sideways in an angle of approximately 45° (Kitchener 2001; Wilson 1971)and by gnawing woody chips 3 to 10 cm long with its incisivi (Göhre 1954). If the "lower cone" is finished to a certain degree, the beaver starts to work on the "upper cone" by bending its head sideways at an angle of approximately 135° based on the normal head posture and gnawing single chips from the stem again (Kitchener 2001; Wilson 1971). Thinner stems and branches are cut by beavers with a simple, oblique cutting using the incisivi, resulting in a cut surface, which is bent at an angle of approximately 45° regarding the perpendicular axis of the stem or the branch respectively (Kitchener 2001).
During gnawing the upper, broader incisivi are used as a counter bearing or anchor point, whereas the smaller incisivi of the lower jaw are doing the actual cutting (Wilson 1971). In some papers - like Göhre (1954) for instance - this is stated wrong, because it is claimed that the lower incisivi are used as an anchor point whereas the upper incisivi are doing the actual cutting.
Link to a video showing the
felling of a tree by a beaver
(source: www.arkive.org). During the felling of a tree, the animals gnaw on the wood for about 5 to 10 minutes followed by a short break, which is used for sharpening the incisivi. The upper incisivi are being sharpened by grinding the external side of the lower incisivi against the inner surface of the upper incisivi with a rapid up and down movement of the lower jaw. In contrast, the lower incisivi are sharpened by grinding the external surface of the upper incisivi against the internal side of the lower incisivi (like in all other rodents, the outer surface of the beaver incisivi are covered by the hard enamel whereas the inner surface consist mainly of the relatively softer dentin). After about an hour the animals take an extended break generally moving towards the water to drink or to eat (Wilson 1971).
Usually, not more than one animal is working at a tree selected for felling at the same time; but occasionally a tree is processed by different family members in succession (Djoshkin & Safonow 1972; Kitchener 2001; Wilson 1971). The ARKive-Video shows to beavers working on the same tree, which is somewhat unusual (probably the video has been filmed inside an enclosure).
The intensity with which a beaver is working on a tree changes over the course of the night and is highest around midnight. Contrastingly, both at dusk and at dawn the felling activity is lowest (Wilson 1971). After a tree has been felled, the smaller branches will be cut off and transported towards the bank, where they are consumed. Thicker branches at a diameter of approximately 10 cm and above will be cut into smaller, 1 to 2 m long segments and then transported towards the water as well (partially, after debarking, these segments will be used for the building of a dam, a lodge or a bank lodge respectively). The rest of the stem will be debarked in situ, as far as the bark is suitable for consumption, i.e. if the cork layer (or periderm) is not too thick (Freye 1978; Jenkins & Busher 1979; Wilson 1971). About 98% of the removed branches have a diameter below 16 cm because larger branches can hardly be moved by the beaver (Zahner et al. 2005).
The time a beaver needs for felling a tree largely depends on the tree species (softwood or hardwood) as well as the stem diameter (Djoshkin & Safonow 1972). According to Kitchener (2001) for the felling of a poplar with a diameter of 12 cm beavers spend about half an hour, whereas the felling of a poplar of 25 cm stem diameter will take them about 4 hours. Similar values are given by Müller-Schwarze (2001), who estimates that the felling of a tree with a diameter of 15 cm amounts to 50 minutes and the felling of a stem with 25 cm diameter to 4 hours as well. Because the animals do not always continuously spend time on felling a tree but partially take some extended breaks as well (the time spans given above relate to the pure processing time), the felling of thicker stems of a diameter of 18 cm and more take beavers 2 to 3 nights as a rule, whereas trees with a stem diameter of 8 to 10 cm are felled within a single night (Djoshkin & Safonow 1972). If hardwood like oaks or elms are felled, the beavers will need significantly more time for that (Djoshkin & Safonow 1972). At times, trees with a diameter of more than 10 cm are not felled but only debarked as far as the animals can reach (up to approximately 50 cm; Jenkins & Busher 1979; see also the video below). This can often be observed on conifer trees, for instance.
Video of a beaver eating the bark of a tree. In this video, the peeling of the bark of a larger tree by a beaver is to be seen. The animal takes off a strip of bark with its incisivi, pulls it upwards and bites it off from the tree. The strip of bark is leaded to the mouth with both hands, small pieces are bitten off with the incisivi and this pieces are cut up with the molars (video: hand-held filming by Christoph Elbert, Meppen).
The taking of roots of the decorticated
stick of a beaver. The felling of a tree does not necessarily mean, that it will subsequently die off. Primarily softwoods like willow and poplar have a relatively high potential for regeneration and are able to sprout again out of the remaining parts of the stem (also humans derive benefits from this by cutting back trees and shrubs). Between 60% and 88% of the willows cut back by beavers will sprout again, whereby each cut surface leads to the sprouting of 10 to 35 new shoots (Zahner et al. 2005). Because newly developed shoots of poplars and probably willows as well possess a certain protection against herbivorous animals by the deposition of bitter substances (tannic acids and similar components, see above), it is ascertained that the fresh shoots are not been immediately browsed again by beavers but are able to grow for a certain amount of time uninhibited and gain a certain diameter. Because each cut branch or felled stem results in the regrowth of several new sprouts, which can be used again by the animals after some time, beavers are utilising their available resources on a sustainable level, at least to a certain degree. Partly beavers make sure that the abundance if certain woody species (especially shrubby willows) increases within their territory. Because they peel the bark from the branches and consume the bark at the bank, the decorticated sticks remain in this area. Because not all branches are completely debarked, some of them are able to take roots and develop towards a new shrub or tree (Klenner-Fringes, own observations at the river Hase in the Emsland district). Above all, this is of importance because nowadays willows for instance have hardly an opportunity to reproduce by seeds because of today's management and maintenance of water bodies (in this respect mainly flowing waters). The seeds of willows and to a certain degree those of poplars as well need bare soils for germination, i.e soils, which are largely without any vegetation and not covered by a dense layer of grasses and herbs. Due to the training of rivers towards a standard track cross section (embankment with an inclination of 45°) followed by an inhibition of the natural river dynamic and the promotion of a dense, continuous grass cover due to a frequent mowing of the embankment, such bare soils became very rare at our waterways, which sets narrow limits for the spreading of willows and poplars by seeds. By the sprouting of the decorticated sticks beavers can at least indirectly contribute to the spreading and propagation of riparian softwoods like willows and poplars as natural components of floodplain biotopes.
However, there are also observations of beavers which have overused the available supply of woody species and were therefore forced to leave the area after a certain time. In such cases, very frequently the food supply was limited from the start and the corresponding habitat was not very suitable for the inhabitation by beavers (such suboptimal habitats are mainly inhabited if the beaver population density is relatively high and all the optimally suitable habitats are already occupied by beavers).