Overvågning af bæver Castor fiber i Flynder Å, 1999-2003

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12 Bævernes påvirkning af produktionsarealer har været meget begrænset og der har kun været få skader hos private lodsejere. Enkelte steder har der dog været så store problemer, at det var nødvendigt at fjerne dæmninger og regulere vandstanden i bæverdammene ved at sætte rør gennem dæmninger. Antallet af bævere er steget siden udsætningen. De udsatte bævere kommer imidlertid fra et relativt begrænset område i Tyskland og har dermed en begrænset genpulje. For at opnå en større genetisk variation i bestanden i Danmark i forhold til eventuelle miljømæssige ændringer, er det hensigtsmæssigt at supplere med yderligere udsætninger af individer fra bestanden i Elben-området. Samlet vurderes genindførslen af bæverne i relation til en øget biodiversitet overvejende at være positiv. Lodsejere, som har fået bævere på deres arealer, har en overvejende positiv indstilling til dyrene. Siden genudsætningen af bævere har Klosterheden Statsskovdistrikt oplevet en stigende publikumsinteresse for at opleve bævere.
Beaver population development Food of beavers Impact on production areas English 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, beaver-human conflicts and the beavers’ influence on flora and fauna. 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. Reproduction was observed each year and the population size has increased from the original 18 to minimum 51 individuals in autumn 2003. Seven kits of the year were registered in the autumn of 2003. Five territories are situated at KLS and eight territories on privately owned lands outside the reintroduction area. Beaver activity on private lands was concentrated in undisturbed semi-natural bogs and fens. Beavers have had considerable impact on areas adjacent to streams creating wetland areas up to 2 ha but only 5% of the watercourses in the catchment area were affected by damming. By 2003 beavers occupied most of the Flynder stream catchment and had started to disperse to neighbouring river systems. In 2003 beaver activity was observed in the river Storå in a neighbouring catchment area at a site 25-30 km downstream from the reintroduction area. 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. 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 13
Page 1 and 2: Danmarks Miljøundersøgelser Milj
Page 3 and 4: Danmarks Miljøundersøgelser Milj
Page 5 and 6: Indhold Forord 5 Resumé 7 English
Page 7 and 8: Forord Skov- og Naturstyrelsen (SNS
Page 9 and 10: Bæverbestandens udvikling Bæverne
Page 11 and 12: Undersøgelser af dødtved og dødt
Page 13: Samlet vurdering Erfaringer fra lan
Page 17 and 18: Deadwood and deadwood insect survey
Page 19 and 20: Experiences from other countries Ge
Page 21 and 22: 1 Indledning Den europæiske bæver
Page 23 and 24: Tabel 1: Indhold og tidspunkter for
Page 25 and 26: Tabel 2: Oversigt over de genudsatt
Page 27 and 28: Resultater Tabel 3: Estimeret besta
Page 29 and 30: Tabel 5: Etablering og opretholdels
Page 31 and 32: Metode Resultater Figur 3: Månedli
Page 33 and 34: Konklusion Tabel 7: Præferenceinde
Page 35 and 36: Hestbæk Figur 5: Vandstuvning i Fl
Page 37 and 38: Figur 7: Dæmning og dam ved Risbæ
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Page 59 and 60: Tabel 14: Forekomst af dødtvedsmas
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Konklusion Tabel 16: Definitioner a
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Tabel 18: Oversigt over passagemuli
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Figur 21: Områder undersøgt for y
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Hestbæk: Hestbæk blev undersøgt
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Resultater og diskussion Figur 22:
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Tabel 22: Udviklingen i antallet af
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Konklusion Metode Resultater Der ha
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Metode Resultater Tabel 23: Forekom
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Konklusion I foråret 2003 færdede
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Friluftsliv og jagt I de nordlige s
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10 Samlet vurdering For at kunne gi
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Et andet formål med genudsætninge
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Berthelsen, J.P., Madsen, A.B. & Za
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Glüsing, H. 2003b: Overvågningspr
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Nolet, B.A., Van der Veer, P.J., Ev
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Danmarks Miljøundersøgelser Danma
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Denne rapport beskriver overvågnin
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Overvågning af bæver Castor fiber i Flynder Å, 1999-2003

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