Summary

Eighteen European beavers (Castor fiber) were reintroduced in 1999 in Klosterheden State Forest District (KLS) in the northwestern part of Denmark. The reintroduction area covers the upstream parts of the Flynder stream catchment area. The KLS comprises coniferous forests, heath lands and wetlands with artificial lakes, brooks and streams. Valleys surrounding the watercourses and lakes were dominated by purple moor grass, bog myrtle and willow scrubs. Prior to the reintroduction a monitoring programme was initiated to describe population and distribution development of beavers, beaverhuman conflicts and the beavers' influence on flora and fauna.

Beaver population development
Distribution and population development of beavers was determined from daily observations and identification of beaver lodges and core areas, and annual counts in spring and autumn. Observers counted simultaneously at dawn and dusk at all active lodges.

Food of beavers
Beaver diet was investigated by analysis of 400 excrements collected at three sites in 2001 and 2002. Beavers' seasonal exploitation of herbaceous and ligneous plants reflected seasonal variations in availability. During the winter months the beavers forage almost exclusively on ligneous plants. In June the food changes and is dominated by herbaceous plants. Willow is the most important woody species constituting 90% of ligneous plants. Birch was the most important alternative food species (4%). Spatial variations in the diet and utilisation of ligneous plants reflected differences in availability between study sites. Willow was the only species that was positively selected by beavers in all study areas compared to the availability of willow.

Impact on production areas
Most beaver activities were recorded less than 5 m from the waters. A few deciduous trees were cut more than 25 m from streams inside the coniferous stands. The flooding and gnawing by the beavers have only affected undisturbed areas with no forest interest or production. Beavers have not exploited resources on production areas in private forest or farmlands but minor management problems have been encountered. At two private sites the beavers have cut significant numbers of trees and willow scrubs. A few trees have been protected by chicken wire. A grassing meadow has been flooded and pipes have been installed in dams at three sites to adjust the water level in the dams and reduce the flooding of adjacent areas. Clogged culverts under roads have been cleared to prevent flooding at a few locations. At two other locations dams in a small brook have been repeatedly removed to prevent flooding of arable fields and gardens in a village. Premorsed sticks and aquatic plants have blocked the inlet gate on a fish farm at several occasions.

Vegetation in streams and stream valleys
The vegetation in streams and adjacent meadows were described at 40 sites in 1999 and at 18 sites affected by beavers at various levels in 2003. At each site the vegetation was divided into different habitat types and vascular plants were listed and their abundance assessed.

Vegetation and abiotic parameters in lakes and beaver ponds
Surveys in 1999 and 2003 described the vegetation in the reintroduction lakes and in 2003 in two new beaver ponds. Water samples were collected to determine abiotic parameters (total N, NO2+NO3-N, Total P, PO4-P (filtered), alkalinity, and pH). Environmental state was assessed according to Danish quality standards for lakes. Only negligible changes were recorded in the lakes. The beavers abandoned the original release sites in the lakes. Thus the recorded changes were attributed to natural variations. The new beaver ponds did not meet the quality standards as diversity and abundance of the aquatic vegetation was too low. The insufficient development of the aquatic vegetation is probably related to the young age of the ponds.

Surveys of freshwater invertebrates in streams
The invertebrate fauna in streams was described according to the Danish Freshwater Fauna Index in 1999 and 2003. In 2003 a total of 81 species/taxa were found on six locations compared to 71 in 1999. The richest groups were dipterans, caddis flies and beetles representing 14, 14, and 11 different taxa. Between 20 and 44 taxa were collected at each site. Two species listed on the Danish Redlist were not recorded again in 2003 but they are assumed to live on unaffected stretches of stream in the KLS. Overall the beavers are assumed to increase the diversity of stream invertebrates. The beaver primarily affects the freshwater invertebrates by changing parts of the running waters to pond habitats. Species that disappeared due to the beaver induced changes of the stream habitats are assumed to exist in unaffected streams elsewhere in the reintroduction area. The increased variability of the wetlands also enables new species to disperse to the former stream habitats. Beaver dams may act as barriers to the fauna in the stream. However, this is not regarded a problem as most species have a state in the lifecycle as flying insects. Other species may disperse upstream with birds or migrate actively through the beaver dams.

Deadwood and deadwood insect surveys
The presence and volume of deadwood on the riverbanks were recorded in 2000 and 2003. Approximately 4.5 tones of beaver felled deadwood masses were recorded in 2003. Most of the deadwood masses left on the ground were debarked and relatively dry which hampered their decomposition by deadwood insects and fungi. However, attack by caterpillars and death watches were detected. Ascent of sap in many stumps has created a new important niche for many insects. Large amounts of wood chips from the beavers felling of trees were attacked by cellulose decomposing oribatids and fungi. The deadwood insect fauna has only developed little but the deadwood masses create a large potential food and habitat resource for decomposing organisms.

Surveys of fish populations
Fish stocks were surveyed by electrofishing in 1999 before the reintroduction of beavers and again in 2003. Trout, threespined stickleback, and eel were registered in both surveys. In 1999 roach, ninespined stickleback and rainbow trout were caught, whereas dace, pike and brook lamprey were only registered in 2003. Trout only occurred in significant numbers at two localities in the largest watercourses. Anglers regularly restock trout populations in the rivers. The different occurrences of fish species are assumed to result from natural variations in small populations and marginal habitats.

Surveys of trout spawning grounds
The substrate of riverbeds was mapped in selected stretches of watercourses in the reintroduction area in the winter of 2000 and 2003 to evaluate occurrence of spawning grounds and spawning activities of trout. Approximately 11 km stream mostly affected by beaver activity was surveyed in 2003. A total of nine gravel banks were registered. As observed in 2000 none of the gravel banks survey showed any indication of trout spawning. The beaver dams are not assessed to cause a significant reduction of the spawning activities of trout in the rivers in KLS. The absence of spawning activities and natural recruitment of trout is attributed to poor physical conditions of the watercourses.

Fish passage at selected beaver dams
A number of the largest beaver dams were examined to assess whether fish passage was possible and to assess the possible effects on various fish populations by the damming of the watercourses.

Amphibian surveys
Surveys of amphibians were made in areas adjacent to the reintroduction sites in the spring of 2000 and 2003. In both surveys only three species were recorded: common frog, moor frog and common toad. Common frog had almost abandoned the breeding areas in the artificial lakes, which were used in 2000. In 2003 numerous egg clusters from common frog were found in the shallow areas of the beaver ponds adjacent to the artificial lakes and in small floodings of previously dry meadows. Some of the ponds have become so large that common toad can use the ponds for breeding. Several common toads were recorded in the watercourses and deeper parts of the beaver ponds. Moor frog was only recorded at one of the original release sites, which was abandoned by the beavers.

Surveys of breeding bird
The occurrence of breeding bird has been surveyed in the lakes and meadows along the watercourses in KLS. Surveys have been performed each spring since 2000. A total of 39 species were recorded during the whole survey period. Some species, which had been recorded in low numbers in previous surveys, had disappeared. Natural variations in small populations are assumed to cause these variations. The development in breeding bird populations differs in each area affected by beavers. Overall there is a tendency for an increasing numbers of species. Some passerines, which were breeding in the now flooded bushes in the meadows near the watercourse, are now breeding in the bushes near the forest edge. The largest beaver ponds have improved the habitat for kingfisher and other water birds fx coot and moorhen. The number of breeding water bird is expected to increase in the future.

Bat surveys
The occurrence and distribution of bats were surveyed by ultrasound detection in KLS and at sites in the vicinity of the forest in the summer of 2000 and 2003. Daubenton's bat was the only species detected in both surveys in Klosterheden forest. However, in 2003 hunting Daubenton's bats were observed on more locations and more individuals were observed leaving roosts in deciduous trees in the centre of KLS compared to the 2000 survey. Serotine, pond bat, noctule and Nathusius' pipistrelle were detected in the vicinity of KLS. The low occurrence of bats in the forest area and other beaver sites outside the reintroduction area is assumed to result from low prey availability and shortage of suitable old trees and buildings for day hides and breeding sites. The landscape changes caused by the beavers have not affected the occurrence of bats significantly yet, but the 2003 survey indicates that the increased area of open ponds inside the forest has resulted in more suitable hunting grounds for Daubenton's bats.

Otter surveys
The occurrence of otters in Flynder stream catchment area has been monitored 1999, 2000 and 2002. Occurrence of otter was established by surveying banks and shores for spraints. In 2003 tracks of otters were surveyed at the larger dams to establish a possible relationship between beaver activity and otter distribution. Otters used most of the catchment area but occurrence on the upstream part in KLS fluctuated. Most of these streams only hold small fish stocks but stable fish stocks are found in the artificial lakes. Otter spraints were recorded near all large beaver dams. The densities of spraints at distances less than 50 m from beaver dams tended to be higher than densities at bank stretches further away from dams. Otters mark at conspicuous sites e.g. dams. The biomass of fish in new beaver ponds is low but eventually the ponds may supply stable food resources for otters. Number of positive locations with otter tracks has increased in the entire catchment area. Increases also occurred at locations without beavers. A similar positive development has occurred throughout the country.

Experiences from other countries
Beaver has been reintroduced in 15 European countries since the early 20th century. Increased dynamics and biodiversity in wetland ecosystems have been recorded in several countries following reintroductions and natural spread of the beavers. Conflicts between human landuse interests and beavers are inevitable. Dam building and foraging on cultivated crops and trees causes nearly all beaverhuman conflicts. Minor problems relate to flooding of roads and excavation of burrows that damage road verges, fields, riverbanks and dikes. Most beaver conflicts occur in a narrow riparian zone. Beaver damage is usually local in scale and very visible but generally the impact of beavers is small compared to wildlife damage caused by other species. Guided beaver safaris are an important management feature in several countries. In the Baltic States, Belarus, Russia, Ukraine and Fenno Scandinavia, which have the most developed populations, hunting has again been allowed and hunting is applied as the principle method to manage beavers. Wildlife tourism and beaver hunting may benefit local economy.

General evaluation
Beavers have modified wetland biotopes at some locations but only small changes in biodiversity were recorded. Probably a longer postreintroduction time period is necessary to see significant changes of flora and fauna. Alterations of the landscape have caused both negative and positive ecological effects on the various groups of flora and fauna. An overall positive effect was recorded for freshwater invertebrates, dead wood insects, amphibians, and breeding birds. Changes in vegetation in wetlands, environmental state of lakes, fish populations and spawning potential in the streams, bats and occurrence of otters were not attributed to the beavers. Reduction in abundance of willow scrubs was only recorded very locally on sites with heavy beaver activity. The most important negative effects were recorded on the migration possibilities of fish in the streams. The beaver dams are assumed to have a negative effect on trout populations that become isolated in the waters upstream beaver dams. Eventually populations of species like roach and sticklebacks may benefit from the beaver ponds.

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