A few facts, and some background information
European beavers consume woody vegetation, grasses, forbs, and aquatic vegetation. They depend largely on woody food sources in winter, but can spend up to 90% of feeding time consuming grasses, forbs and aquatic vegetation in summer.
Trails and paths such as this one, made by a mature, resident beaver are apparently easy to follow and can be used to set traps along if the animals need to be removed.
Tree guards, and exclusion fences may be used where necessary to protect specific trees or whole areas from potential beaver damage.
Beavers are able to occupyand/or modify a wide range of water-marginal environments, but in general prime beaver habitat would include easy access to grasses, forbs and riparian tree species; low flow water depths, at least near their lodge and burrow sites, of >0.6m; river channel gradients of lessthan 0.15 and preferably as low as 0.06 and ‘soft’ or finer calibre bed and bank materials.
The European beaver usually digs burrows in riverbanks in which to nest. However, they cut wood to build dams if the water is not deep enough to cover the entrance to the burrow. They will also build lodges from wood if burrowing in banks is difficult.
Beavers are strictly herbivorous; they do not eat fish or any other animals. In spring and summer they mainly eat aquatic and riparian plants, at other times of the year they depend heavily on woody species for food, such as willow, alder, aspen and birch, particularly those that are found in veryclose proximity to the water’s edge.
Beavers are able to live in a wide range of wetland environments, but in general there appear to be four broad characteristics of prime beaver habitat:
they like easy access to grasses, non-woody herbaceous plants and riparian tree species, especially poplars and willows
water at least 60cm deep when at its lowest near lodges and burrow sites
river channel gradients typical of lowland rivers
‘soft’ or ine bed and bank materials
Beavers normally live as a family unit or colony of four or five individuals, consisting of two parental adults, the
yearlings born the previous year and the young of the current year. A 2km length of river with suitable habitat
should be sufficient to support a colony, where habitat is assessed within 20m of the water’s edge.
Densities of beaver populations at particular sites vary depending on how long the sites have been occupied.
Population growth may continue for 25 years or more until all available sites have been occupied. Where data are available, over half of introductions of beavers in Europe have been successful. Failed introductions have been attributed to release into unsuitable habitat and too few individuals being released with subsequent poor population growth.
Beavers often excavate canals to allow more extensive access to the riparian zone.
Water levels in beaver ponds reflect the balance between river flow, precipitation and groundwater seepage into the pond versus river flow and groundwater seepage out of the pond and surface evaporation.
Order of dietary preference willow > rowan > birch > prunus > alder > conifers
In Sweden, European beavers mainly feed on fresh bark, shoots, buds, twigs and leaves of aspen Populus tremula, birch Betula pubescens and willow species (Salix caprea, S. aurita, S. cinerea, S. glauca, S. lapponum, S. pentandra), as well as roots, stems and leaves of many grasses and flowering plants (Curry-Lindahl 1967). In the willow-dominated Biebosch, The Netherlands, Nolet et al. (1994) demonstrated that although willows formed their staple diet, beavers only removed 1.4% of the standing crop and positively selected Alnus, Corylus, Fraxinus, Populus and Prunus species, which were very uncommon, suggesting an attempt to eat a mixed diet.
The maximum distance that beavers travel from water to obtain food is thought to be approximately 100 m and beavers commonly browse within 10 m. However, beaver can change proximity by impounding ponds and constructing canals. The resultant change in the position of the water shoreline can extend considerably the access of beaver to food in areas of low relief (see Chapter 3).
On the River Sverkestaanin, south central Sweden, Hartman (1996) identified tortuosity of the shoreline and a dominant cover of grasses and forbs as the strongest discriminators of European beaver occupancy (although both may result from, rather than be responsible for, beaver occupancy). Softer soils, a higher cover of deciduous tree species and narrower sections of river were also important, although, since the mean river width was 114 m, these preferred narrower sections were of the order of 40 m wide. There was insufficient variability in river gradient or depth to identify their importance in habitat selection.
MacDonald et al. (1995) presented criteria for gauging the quality of release sites for the European beaver, suggesting a very broad range in the characteristics of a 'good' site: 2-4 m water depth, 10-100 m river width, <0.3 m s-1 flow velocity, bank materials of peat-loam soil, bank height >1.5 m, bank slope <60 degrees, woody species predominantly aspen and willow which are <8 cm
diameter, and a good herb cover.
The construction of beaver dams and ponds introduces many additional habitats to river reaches (ponds; complex stream-riparian aquifer flow paths leading to zones of different water table and soil moisture levels including wetlands; browsed openings within riparian woodland; areas of differing tree density and canopy height), resulting in a substantial increase in habitat diversity, the spatial complexity of the habitat mosaic, and the overall resilience of river and riparian ecosystems to disturbances. Beneficial biogeochemical changes are also induced, particularly an increase in carbon, nutrient and fine sediment retention and their gradual
incorporation into the riparian zone through beaver meadow development.
Hydrogeomorphological effects of dams and ponds - The construction of beaver dams provides water stores (ponds) which can sustain low river flows, seepage to the riparian zone, and areas of stored water within the channel network far longer during dry periods than would occur in river channels without beaver dams.
Beaver dams introduce steps along the river‟s long profile where energy is dissipated and, as a result, both sediment and organic matter are deposited within beaver ponds. This important retention of sediment and organic matter, coupled with management of sediment sources, has the potential to lead to a reduction in catchment yields as well as an increase in the incorporation of sediment and organic matter into the riparian zone (beaver meadows). Sorting of mineral sediment particles occurs within the ponds, with coarser sediments deposited in pond heads and finer sediments within the main pond body. Within free flowing river sections between ponds, flushing of finer sediment and sorting of coarser bed sediment may also occur, which may be highly beneficial for fish spawning areas. The storage, decomposition and processing of organic matter and sediment within the beaver ponds may also improve water quality and turbidity downstream. These highly beneficial effects of beaver construction on sediment and organic matter retention (e.g. see chapter 11) have provided a rationale for beaver reintroduction into degraded, incising river systems within the USA.
Beavers and British rivers - The majority of British rivers have relatively low gradients and narrow widths, making them physically suitable for beaver colonisation. However, the presence of bedrock or very coarse sediment banks, relatively shallow water depths, and limited presence of riparian trees (particularly Salicacaea) make some river types less than ideal. The most suitable habitat conditions for European beavers are found along the least degraded lowland rivers that have the lowest river gradients, the finest bed and bank sediment with occasionally deep pool habitats, and plentiful riparian tree cover
Beaver dams consist of tree trunks, branches, twigs, earth, mud and sometimes stones. They are constructed to create beaver ponds, which maintain access to lodges and burrows below water level and extend food foraging areas. Several dams may be built by the same colony to achieve this (Richard 1983, Baker & Hill 2003).Dugmore 1914, Warren 1927) indicate occasional very large structures, several hundred metres long and several metres high, most dams were less than 1.5 m high (Dugmore 1914, Townsend 1953). Butler (1995) suggests typical dam sizes of 15-70 m long and 1-2 m wide for Canadian beavers with widely varying heights.
The planform of dams also varies widely, but upstream-orientated arc-forms (with convex of arc upstream) are common, particularly for the largest structures. Small within-channel dams can be progressively extended into long channel-floodplain dams (Richard 1967), producing large shallow ponds on floodplains where the width of dammed channel may be quite small.
Richard (1955) described a similar structure for dams constructed by European beavers on the River Tave, France. Dams were up to 8-10 m in length and were constructed of wood pieces, typically 1-2 m long, and sometimes stones, which were then sealed with mud on the upstream face. The wood pieces were aligned across the river, parallel to the banks or near vertical. The near-vertical pieces and those parallel to the bank formed the key pieces of the structure, which were then filled out upstream with transverse
pieces. Three basic types of structure were found: (1) built around a fallen or inclined tree to give support to the structure; (2) based upon vertical key pieces of wood driven into the stream bed; and (3) counterbalanced by wooden props which support the downstream face of the dam against upstream water pressure (types 2 and 3 are combined in the largest dams). Once a dam and pond are constructed in a low-gradient area, the zone of floodplain accessible to beaver can be further extended by constructing canals, which are used as route-ways to access food and transport timber to the beaver pond.
In addition to dams, beavers construct shelters or dens. Erome (1984), working on the River Rhone, France, found that natural holes in the bank are often used and that European beavers also excavate burrows where the bank is sufficiently high and the bank material is appropriate. Elsewhere, intermediate structures between burrows and free-standing lodges, called bank lodges, use wood to conceal the burrow entrance, or to increase soil cohesiveness, effective bank height or effective soil depth. The nest chamber is typically 0.3-0.7 m above the upper edge of the burrow entrance, defining the upper limit to desirable water-level fluctuations, and nest chambers are typically 0.4-0.5 m in height. Thus, bank heights of 1.5-2.0 m above the burrow entrance or 1-1.5 m above normal water level are needed for burrow construction. Elsewhere, the addition of wood to create bank lodges achieves the required height. Erome (1984), Pupininkas (1999) and Danilov (1995, cited in Collen & Gibson 2001) found true lodge (free-standing) construction by European beavers to be relatively rare. For example, Pupininkas (1999) found only semi-lodges on lakes in east Lithuania, whereas the proportion of lodges to semi-lodges to burrows on rivers and streams was 1 : 11.9 : 2.3 respectively. Danilov (1995, cited in Collen & Gibson 2001) found that in northwest Russia, 75% Canadian beaver colonies but only 34% European beaver colonies had lodges, whereas 64% European beaver colonies and only 25% Canadian beaver colonies had bank burrows. Individual colonies often have secondary lodges, semi-lodges or burrows, which may be occupied when the water level is low, near the main lodge, or when the beavers suffer disturbance.
Barnes & Mallick (1997) also found that the density of small (1.5-4.4 cm diameter) woody stems at the active and abandoned sites was significantly greater than at sites with no dams. Finally, McComb et al. (1990, cited in Collen 1995) found that reaches with dams were shallower, had a lower gradient, a greater tree canopy cover and gentler bank slopes than reaches without dams, and that dams were not built at sites with a rock substrate.
Hydrological changes include:
increased storage of precipitation, reduced flow velocities and reduced variability in the river‟s discharge regime;
an enormous increase in water surface area, particularly in low relief environments;
increased water depth;
an increase in the level of the local water table.
Biogeochemical changes include:
a great increase in the amount and availability of organic carbon, nitrogen and other nutrients in the channel;
an increase in carbon turnover time;
an increase in nitrogen fixation by sediment microbes;
amelioration of stream acidity; increased trapping of sediment and a decrease in turbidity downstream;
an increase in aerobic respiration as a result of increased water-surface area;
a substantial shift to anaerobic biogeochemical cycles in sediments beneath ponds;
an increase in the amount of organic matter suitable for methane-producing microorganisms and increased carbon output by methanogenesis;
reduced oxygen levels in the water in spring and early summer due to decomposition of the augmented organic matter.
Ecological changes include:
an increase in the extent of open canopy in wooded areas;
loss of species dependent on riparian trees as habitat;
more favourable conditions for riparian tree and wetland plant growth;
creation of conditions favourable to species dependent upon ponds, pond edges and/or dead wood;
both enhancement and degradation of conditions for fish, depending on the species;
replacement of running-water invertebrate taxa by pond taxa, an increase in the absolute importance of collectors and predators and a decrease in the relative importance of shredders and scrapers in impounded sites;
a several-fold increase in the mass of insects emerging from the water surface per unit stream length;
increased plankton productivity;
increased ecosystem resistance to perturbation.
Hydraulic gradients between [beaver] pond surfaces, downstream water levels and riparian water table levels give rise to significant surface water-groundwater interactions, including downstream flows beneath beaver dams (White 1990); correlated fluctuations in pond and riparian water table levels (Lowry 1993) that are larger than those adjacent to unponded sections of river; and enhanced recharge of the riparian aquifer adjacent to ponds in comparison with free-flowing reaches.
As a result of the sizeable water storage in large beaver ponds, failure of major beaver dams can result in flooding (Butler & Malanson 2005), although the effects of dam failure is often attenuated by beaver-constructed ponds and meadows downstream. For example, Hillman (1998) reported the sudden release of 7500 m3 of water from a beaver dam failure into a creek in Alberta. The flood wave had a peak flow 3.5 times larger than the maximum discharge recorded over 23 years, destroyed five hydrometric stations, and scoured and deposited large quantities of sediment. However, this large peak flow was reduced to 6% of its estimated
upstream magnitude by flood attenuation through a 90-ha wetland containing a small lake and several beaver ponds. Thus, the aggregate effect of beaver constructions on river flow regimes can be very substantial. They increase the time of rise and decrease the magnitude of the flood flows. They also sustain low river flows, seepage to the riparian zone, and areas of water storage within the channel network far longer during dry periods than would occur in river channels without beaver dams.
Butler and Malanson to estimate that the total sediment storage in the ponds of the unexploited beaver population prior to European settlement was between 7.5 and 125 billion m3 (based on 500 m3 of sediment per pond), and that current sediment storage is 1.9 to 3.9 billion m3. […] Significant water storage and low flow velocities within beaver ponds induce deposition of large quantities of mineral and organic sediment (Butler & Malanson 1995).
[…] They [Naiman et al] showed that anaerobic conditions caused by saturation of soil greatly alters biogeochemical pathways, leading to the accumulation of large quantities of organic matter from surrounding vegetation in both ponds and wet meadows. This leads to preferential, long-term retention of chemical elements associated with organic matter in these deposited organic horizons, rather than their export downstream or return to the atmosphere.
The presence of beaver dams increases variability in channel width and depth, bed sediment calibre, and in-channel morphological features. Where the beaver dams extend across the floodplain, diffuse seepage may lead to the development of floodplain wetlands, and concentrated flow may excavate additional stream channels.
Although morphological evidence for the historical impacts of European beaver on British rivers is poorly preserved, the impacts of re-introduced populations across Europe suggest that similar landscape scale effects may have occurred. The relatively low gradients, narrow widths and low stream orders of the majority of British streams, suggest that much of the landscape would have been affected in a manner similar to that suggested for middle-order (intermediate slope and width) streams in North America.
