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利用者:加藤勝憲/ビーバーダム

Beaver dam at Algonquin Park in Ontario, Canada

ビーバーダムまたはビーバー貯水池は、ビーバーがコヨーテ、オオカミ、クマなどの捕食者から身を守るための池や、冬場の食料を蓄えるために築くダムである。これらの構造物は、生態系全体がその変化の上に構築されるように自然環境を変化させるため、ビーバーは生態系の要となる種であり、生態系エンジニアである。彼らは前足で泥や石を運び、歯で木材をくわえて、夜間に盛んに建設を行う。

A beaver dam or beaver impoundment is a dam built by beavers to create a pond which protects against predators such as coyotes, wolves and bears, and holds their food during winter. These structures modify the natural environment in such a way that the overall ecosystem builds upon the change, making beavers a keystone species and ecosystem engineers. They build prolifically at night, carrying mud and stones with their forepaws and timber between their teeth.

ダム造り

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North American beaver (Castor canadensis), one of two species of beaver

冬の間、ビーバーの巣の水中の入り口が氷で塞がれないようにするには、最低でも0.6~0.9メートル(2.0~3.0フィート)の水位が必要である。[要出典] 湖や川、大きな流れのある 水深が十分にある湖や川、大きな流れでは、ビーバーはダムを建設する必要がなく、代わりに土手の巣穴やロッジで暮らすこともある。[1] 水深が十分でないため、ビーバーを捕食者から守り、ロッジの入り口を氷から守ることができない場合、ビーバーはダムを建設する。

A minimum water level of 0.6 - 0.9メートル (2.0 - 3.0 ft) is required to keep the underwater entrance to beaver lodges from being blocked by ice during the winter.[要出典] In lakes, rivers and large streams with deep enough water, beavers may not even need to build dams, and instead simply live in bank burrows and lodges.[1] If the water is not deep enough to keep beavers safe from predators and their lodge entrances ice-free, beavers build dams.

Beaver dam in Lassen Volcanic National Park

ビーバーは、水流の圧力を弱めるために流れを迂回させることで建設を開始する。 その後、枝や丸太を川底の泥に打ち込んで土台を作る。[1] そして、小枝、樹皮(落葉樹のもの)、石、泥、草、葉、植物の塊など、利用できるものは何でも使って上部構造を築く。ビーバーは、自分の体重分の材料を運ぶことができる[要出典]。彼らは丸太を泥流に沿って引きずり、運河に浮かべて所定の位置に配置する。[2] ダムがロッジの周囲に堀を形成するのに十分な面積と深さに水没すると(多くの場合、数エーカーにわたって)、ビーバーはロッジの建設を開始する。[3]

Beavers start construction by diverting the stream to lessen the water's flow pressure. Branches and logs are then driven into the mud of the stream bed to form a base.[1] Then sticks, bark (from deciduous trees), rocks, mud, grass, leaves, masses of plants, and anything else available are used to build the superstructure. Beavers can transport their own weight in material;[要出典] they drag logs along mudslides and float them through canals to get them in place.[2] Once the dam has flooded enough area to the proper depth to form a protective moat for the lodge (often covering many acres), beavers begin construction on the lodge.[3]

Beavers use rocks for their dam when mud and branches are less available as seen on Bear Creek, a tributary to the Truckee River, in Alpine Meadows, California

直径90センチメートル(3.0フィート)に達する木はダムの建設に使用できるが、平均は10~30センチメートル(3.9~11.8インチ)である。長さは、木の直径とビーバーの大きさによって決まる。ビーバーが直径115センチメートル(45インチ)、高さ45メートル(148フィート)の丸太を倒す例も記録されている。このサイズの丸太はダムの構造材として使用されることはなく、樹皮は食料として、また時には上部の枝に届くために使用される。ビーバーは、幹の周囲に砂時計形の溝を噛みながら作って、幅15センチ(5.9インチ)のポプラの木を倒すのに約20分しかかからない。ビーバーの顎は強力で、1.5センチ(0.59インチ)の苗木を一噛みで切断できる。[3]

Trees approaching a diameter of 90センチメートル (3.0 ft) may be used to construct a dam, although the average is 10 - 30センチメートル (3.9 - 11.8 in). The length depends on the diameter of the tree and the size of the beaver. There are recorded cases of beavers felling logs of as much 45メートル (148 ft) tall and 115センチメートル (45 in) in diameter. Logs of this size are not intended to be used as structural members of the dam; rather, the bark is used for food, and sometimes to get to upper branches. It takes a beaver about 20 minutes to cut down a 15センチメートル (5.9 in) wide aspen, by gnawing a groove around the trunk in an hourglass shape. A beaver's jaws are powerful enough to cut a 1.5センチメートル (0.59 in) sapling in one bite.[3]

ダムやロッジのメンテナンス作業は、秋に行われることが多い。

Maintenance work on the dam and lodges is often done in autumn. さらに、ビーバーが中心地での採食者であると考えられるならば、 、ビーバーの運河は、ビーバーの池と切り株をマッピングした2004年から2012年の研究によれば、山小屋をはるかに超えた「中心地」の延長線上にあると考えられるかもしれない。引用エラー: 冒頭の <ref> タグは正しくない形式であるか、不適切な名前ですダムを作ることによって、ビーバーは道具使用行動を表現していると主張する者もいる引用エラー: 冒頭の <ref> タグは正しくない形式であるか、不適切な名前です。

Size

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ビーバーダムは通常、長さが数メートルから約100メートル(330フィート)の範囲である。[6] さらに運河は、長さが0.5キロメートル(1,600フィート)を超えることもある。[7] 知られている中で最大のビーバーダムは、 存在が確認されているものは、カナダのアルバータ州にあるウッド・バッファロー国立公園内の長さ2,790フィート(850m)のものである。[8] NASA WorldWindが提供する衛星写真では、1975年にはダムは存在していなかったが、その後の画像では現れている。2つ以上のロッジがあり、2つの元のダムを組み合わせたものである。Google Earthの画像では、最終的に本ダムとつながり、今後10年間でさらに50~100メートル(160~330フィート)延長される可能性がある新たなダムが建設されているのが確認できる。[9] 座標: 58°16'15「N 112°15'6」W

Beaver dams typically range in length from a few meters to about 100メートル (330 ft).[4] Additionally, canals can be over 0.5キロメートル (1,600 ft) in length.[5] The largest beaver dam known to exist is in Wood Buffalo National Park in Alberta, Canada and measures 2,790フィート (850 m) in length.[6] Satellite photos provided by NASA WorldWind show the dam did not exist in 1975 but it appeared in subsequent images. It has two or more lodges and is a combination of two original dams. Google Earth images show new dams being built which could ultimately join the main dam and increase the overall length by another 50 - 100メートル (160 - 330 ft) during the next decade.[7] Coordinates: 58°16'15"N 112°15'6"W

モンタナ州スリーフォークスでは、長さ2,139フィート(650 m)、高さ14フィート(4.3 m)、底の厚さ23フィート(7.0 m)を測る、もう一つの大きなビーバーダムが発見された。[3]

Another large beaver dam measuring 2,139フィート (650 m) long,[6] 14フィート (4.3 m) high and 23フィート (7.0 m) thick at the base was found in Three Forks, Montana.[3]

Effects

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Beaver dam in winter in Mont Mégantic

ダムの建設は湿地帯の復元に役立つ。湿地帯の利点には、下流の洪水防止、生物多様性(さまざまな種に生息地を提供すること)、水質浄化(農薬などの毒素の分解とビーバーダムによる沈泥の保持の両方による)などがある。ビーバーダムは浸食を減らすとともに、一部の水生生物の生育を妨げる要因となる濁度を減少させる。その利点は、流域が綿密に監視されない限り、長期的かつほとんど気づかれないままになる可能性がある。北米では、絶滅危惧種および絶滅危惧種のほぼ半数が湿地帯に依存している。[10]

Dam building can be helpful in restoring wetlands. Wetland benefits include flood control downstream, biodiversity (by providing habitat for different species), and water cleansing, both by the breakdown of toxins such as pesticides and the retention of silt by beaver dams. Beaver dams reduce erosion as well as decrease the turbidity that can be a limiting factor for some aquatic life. The benefits may be long-term and largely unnoticed unless a catchment is monitored closely. Almost half of endangered and threatened species in North America rely upon wetlands.[8]

2012年には、ビーバーダムが魚類および魚類の生息地に与える影響に関する系統的レビューが実施された(北米に偏っている(88%))。ビーバーダムの利点として最も多く挙げられたのは、生息地の不均質性の増加、飼育および越冬生息地、流水の避難場所、無脊椎動物の生産である。ダムによる魚類の移動の妨げ、産卵生息地の沈泥化、池の低酸素レベルが、最も多く挙げられた負の影響であった。利点(184)は、コスト(119)よりも多く挙げられた。[11]

In 2012, a systematic review was conducted on the impacts of beaver dams on fishes and fish habitat (biased to North America (88%)). The most frequently cited benefits of beaver dams were increased habitat heterogeneity, rearing and overwintering habitat as well as flow refuge, and invertebrate production. Impeded fish movement because of dams, siltation of spawning habitat and low oxygen levels in ponds were the most often cited negative impacts. Benefits (184) were cited more frequently than costs (119).[9]

Flood control

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Beaver dam as depicted in Frances Fuller Victor's 1887 book Eleven years in the Rocky Mountains and a life on the frontier.

A beaver dam may have a freeboard above the water level. When heavy rains occur, the river or lake fills up. Afterwards the dam gradually releases the extra stored water, thus somewhat reducing the height of the flood wave moving down the river.[10][11]

The surface of any stream intersects the surrounding water table. By raising the stream level, the gradient of the surface of the water table above the beaver dam is reduced, and water near the beaver dam flows more slowly into the stream. This may also help in reducing flood waves, and increasing water flow when there is no rain. In other words, beaver dams smooth out water flow by increasing the area wetted by the stream. This allows more water to seep into the ground where its flow is slowed. This water eventually finds its way back to the stream. Rivers with beaver dams in their head waters have lower high water and higher low water levels.

By raising the water table in wetlands such as peatlands, they can stabilize a fluctuating water table, which influences the levels of both carbon and water. In a 2017 study of beaver dam hydrology, monitored beaver dams in a Rocky Mountain peatland were found to increase groundwater storage and regional water balance, which can be beneficial for preventing drought. The study also suggested potential to improve carbon sequestration.[12]

Excess nutrient removal

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Beavers that work on top of heavy snowfall make cuts that are high above ground

Beaver ponds can cause the removal of nutrients from the stream flow. Farming along the banks of rivers often increases the loads of phosphates, nitrates and other nutrients, which can cause eutrophication and may contaminate drinking water. Besides silt, the beaver dam collects twigs and branches from the beavers' activity as well as leaves, notably in the autumn. The main component of this material is cellulose, a polymer of β-glucose monomers. (This creates a more crystalline structure than is found in starch, which is composed of α-glucose monomers. Cellulose is a type of polysaccharide.) Many bacteria produce cellulase which can split off the glucose and use it for energy. Just as algae receive energy from sunlight, these bacteria derive energy from cellulose, and form the base of a very similar food chain.

Additionally, bacterial populations absorb nitrogen and phosphorus compounds as they pass by in the water stream and keep these and other nutrients in the beaver pond and the surrounding ecology. [13]

Pesticide and herbicide removal

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Agriculture introduces herbicides and pesticides into streams. Some of these toxicants are metabolized and decomposed by the bacteria in the cellulose-rich bottom of a beaver dam.

Denitrification

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Some scientists believe that the nitrogen cascade, the production of more fixed nitrogen than the natural cycles can turn back into nitrogen gas, may be as much of a problem to Earth's ecology as carbon dioxide production. [14] Studies have shown that beaver dams along a stream contribute to denitrification (the conversion of nitrogen compounds back into nitrogen). Bacteria in the dirt and the plant debris, which collects at the dams, turns nitrates into nitrogen gas. The gas bubbles to the surface and mixes with the atmosphere once more.[15]

Salmon and trout

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Large European beaver dam near Olden, Jämtland, Sweden
An experimental pipe through a beaver dam, placed to allow migratory fish to cross through the dam during their spawning season

Beaver dams and the associated ponds can provide nurseries for salmon and trout.[16] An early indication of this was seen following the 1818 agreement between the British government of Canada and the government of America allowing Americans access to the Columbia watershed. The Hudson's Bay Company, in a fit of pique, instructed its trappers to extirpate the fur-bearing animals in the area. The beaver was the first to be made locally extinct. Salmon runs fell precipitously in the following years, even though none of the factors associated with the decline of salmon runs were extant at that time.[17]

There are several reasons why beaver dams increase salmon runs.[18][19][20] They produce ponds that are deep enough for juvenile salmon to hide from predatory wading birds. They trap nutrients in their ecology and notably the nutrient pulse represented by the migration of the adult salmon upstream. These nutrients help feed the juveniles after the yolk sac has been digested. The dams provide calm water which means that the young salmon can use energy for growth rather than for navigating currents; larger smolts with a food reserve have a better rate of survival when they reach the sea. Finally, beaver dams keep the water clear which favours all salmonoids.Template:Pond

Smaller beaver dam on a creek near Fife Lake, Michigan

Frogs

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Beaver dams have been shown to be beneficial to frog and toad populations, likely because they provide protected areas for larvae to mature in warmer, well-oxygenated water.[21] A study in Alberta, Canada, showed that "Pitfall traps on beaver ponds captured 5.7 times more newly metamorphosed wood frogs, 29 times more western toads and 24 times more boreal chorus frogs than on nearby free-flowing streams."[22]

Birds

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Beaver dams help migrating songbirds. By stimulating the growth of species of plants that are critical to populations of songbirds in decline, beaver dams help create food and habitat. The presence of beaver dams has been shown to be associated with an increased diversity of songbirds.[23] They can also have positive effects on local waterfowl, such as ducks, that are in need of standing water habitats.[24]

Disruption

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Canoeists try to run a beaver dam in Algonquin Park. The dam is about 1 m (3 ft 3 in) high.

Beaver dams can be disruptive; the flooding can cause extensive property damage, and, when the flooding occurs next to a railroad roadbed, it can cause derailments by washing out the tracks. When a beaver dam bursts, the resulting flash flood may overwhelm a culvert.

Traditional solutions to beaver problems have been focused on the trapping and removal of all the beavers in the area. While this is sometimes necessary, it is typically a short-lived solution, as beaver populations have made a remarkable comeback in the United States (after near extirpation in the nineteenth century) and are likely to continually recolonize suitable habitat.[25] Modern solutions include relatively cost-effective and low maintenance flow devices.

Introduced to an area without its natural predators, as in Tierra del Fuego, beavers have flooded thousands of acres of land and are considered a plague. One notable difference in Tierra del Fuego from most of North America is that the trees in Tierra del Fuego cannot be coppiced[要出典] as can willows, poplars, aspens, and other North American trees. Thus the damage by the beavers seems more severe. The beaver's disruption is not limited to human geography; beavers can destroy nesting habitat for endangered species.

Warming temperatures in the Arctic allow beavers to extend their habitat further north, where their dams impair boat travel, impact access to food, affect water quality, and endanger downstream fish populations.[26] Pools formed by the dams store heat, thus changing local hydrology and causing localized thawing of permafrost that in turn contributes to global warming.[26]

Stream life cycle

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Drained beaver pond in Adirondack State Park
Drained beaver pond in Allegany State Park

Wetland creation

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If a beaver pond becomes too shallow due to sediment accumulation, or the tree supply is depleted, beavers will abandon the site. Eventually the dam will be breached and the water will drain out. The rich thick layer of silt, branches, and dead leaves behind the old dam is an ideal habitat for some wetland species.

Meadow creation

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As the wetland fills up with plant debris and dries out, pasture species colonize it and the wetland may eventually become a meadow suitable for grazing in a previously forested area. This provides a valuable niche for many animals which otherwise would be excluded. Beaver dam creation also increases the plants the dams were made from (such as willows) to reproduce by cutting, encouraging the growth of adventitious roots.

Riverine forest

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Finally the meadow will be colonized by riverine trees, typically aspens, willows and such species which are favoured by the beaver. Beavers are then likely to recolonize the area, and the cycle begins again.

Bottomland

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Each time the stream life cycle repeats itself another layer of organic soil is added to the bottom of the valley. The valley slowly fills and the flat area at the bottom widens. Research is sparse, but it seems likely that parts of the bottomland in North America was created, or at least added to, by the efforts of the generations of beavers that lived there. [27]

Beaver dam analogue

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Humans sometimes build structures similar to beaver dams in streams, either to get the benefits of beaver dams in places without beavers, or to encourage beavers to settle in a particular area.[28] These are often called "beaver dam analogs" (BDA) although other names are also used.[29] When the goal is to attract beavers, sometimes the site is unsuitable in its present condition, such as being too eroded for beavers to build a dam in their usual way. BDA builders may use construction techniques beyond the beaver's capabilities, such as driving wooden posts into the stream bed to brace horizontal branches that would otherwise be washed away. The hope is that beavers who wander by or are brought in will choose to live there and take over construction and maintenance of the dam.

See also

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  • Environmental impacts of beavers
  • Logjam, an accumulation of wood debris on a river or stream

References

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[[Category:ダムの型式]] [[Category:池]] [[Category:ビーバー科]] [[Category:未査読の翻訳があるページ]]

  1. ^ a b Baker, B.W.; Hill, E.P. (2003). Feldhammer; Thompson; Chapman. eds. “Beaver (Castor canadensis)”. Wild Mammals of North America: Biology, Management, and Conservation (Baltimore, Maryland: The Johns Hopkins University Press): 288–310. https://www.aphis.usda.gov/wildlife_damage/beaver_damage/downloads/Baker%20and%20Hill%20Beaver%20Chapter.pdf 2020年3月8日閲覧。. 
  2. ^ “Comparative Mammalian Brain Collections: North American beaver (Castor canadensis)”. neurosciencelibrary.org. 2020年3月8日閲覧。
  3. ^ a b c Fall, S. (2007年). “Beaver pictures & facts”. August 11, 2013時点のオリジナルよりアーカイブ。September 2, 2013閲覧。
  4. ^ “World's biggest beaver dam discovered in northern Canada” (英語). phys.org. 2020年3月8日閲覧。
  5. ^ Grudzinski, Bartosz P.; Cummins, Hays; Vang, Teng Keng (2019-09-15). “Beaver canals and their environmental effects” (英語). Progress in Physical Geography: Earth and Environment 44 (2): 189–211. doi:10.1177/0309133319873116. ISSN 0309-1333. 
  6. ^ a b Strege. “Explorer first to reach world's largest beaver dam”. 24 September 2014時点のオリジナルよりアーカイブ。24 September 2014閲覧。
  7. ^ Thie, J.. “Exploring beaver habitat and distribution with Google Earth: The longest beaver dam in the World”. September 16, 2013時点のオリジナルよりアーカイブ。September 2, 2013閲覧。
  8. ^ “About beavers”. Beavers: Wetlands and Wildlife. August 13, 2013時点のオリジナルよりアーカイブ。September 3, 2013閲覧。
  9. ^ Kemp, P.S., Worthington, T.A., Langford, E.L., Tree, A.R.J. and Gaywood, M.J., (2012). "Qualitative and quantitative effects of reintroduced beavers on stream fish". Fish and Fisheries, 13(2): 158–181 doi:10.1111/j.1467-2979.2011.00421.x
  10. ^ “Beavers cut flooding and pollution and boost wildlife populations” (英語). The Guardian (2020年2月17日). 2021年5月31日閲覧。
  11. ^ “River Otter Beaver Trial | University of Exeter”. www.exeter.ac.uk. 2021年5月31日閲覧。
  12. ^ Karran, Daniel J.; Westbrook, Cherie J.; Bedard‐Haughn, Angela (2018). “Beaver-mediated water table dynamics in a Rocky Mountain fen” (英語). Ecohydrology 11 (2): e1923. doi:10.1002/eco.1923. ISSN 1936-0592. 
  13. ^ “Beavers do 'dam' good work cleaning water”. Science Daily (University of Exeter). (2018年5月19日). https://www.sciencedaily.com/releases/2018/05/180509121552.htm 2021年7月9日閲覧。 
  14. ^ Fields, Scott (2004). “Global Nitrogen: Cycling out of Control”. Environmental Health Perspectives 112 (10): A556–A563. doi:10.1289/ehp.112-a556. PMC 1247398. PMID 15238298. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1247398/. 
  15. ^ Lazar, Julia; Addy, Kelly; Gold, Arthur; Groffman, Peter; McKinney, Richard; Kellogg, Dorothy (16 September 2015). “Beaver Ponds: Resurgent Nitrogen Sinks for Rural Watersheds in the Northeastern United States”. Journal of Environmental Quality 44 (5): 1684–1693. doi:10.2134/jeq2014.12.0540. PMID 26436285. 
  16. ^ Grannes, S.G. (2008年). “Beaver dam information site”. September 12, 2013時点のオリジナルよりアーカイブ。August 30, 2013閲覧。
  17. ^ “Beavers”. nwcouncil.org. 14 November 2012時点のオリジナルよりアーカイブ。2 May 2018閲覧。
  18. ^ “When Beaver met Salmon: how fish-friendly flow devices keep their relationship moving forward”. WWF.CA (7 April 2021). 2023年6月18日閲覧。
  19. ^ Smith, Joseph M.; Mather, Martha E. (July 2013). “Beaver dams maintain fish biodiversity by increasing habitat heterogeneity throughout a low-gradient stream network”. Freshwater Biology 58 (7): 1523–1538. doi:10.1111/fwb.12153. 
  20. ^ “Riverscapes Restoration Design Manual”. lowtechpbr.restoration.usu.edu. 2023年6月18日閲覧。
  21. ^ “wildlifeextra.com – wildlifeextra Resources and Information.”. www.wildlifeextra.com. 20 December 2016時点のオリジナルよりアーカイブ。2 May 2018閲覧。
  22. ^ “Beavers Helping Frogs And Toads Survive”. Science Daily. (January 11, 2007). オリジナルのJune 10, 2015時点におけるアーカイブ。. https://web.archive.org/web/20150610142058/http://www.sciencedaily.com/releases/2007/01/070110180828.htm 
  23. ^ “Beavers: Dam Good For Songbirds”. Science Daily. (October 9, 2008). オリジナルのMay 30, 2017時点におけるアーカイブ。. https://web.archive.org/web/20170530154211/https://www.sciencedaily.com/releases/2008/10/081008151316.htm 
  24. ^ Nummi, Petri; Suontakanen, Eeva-Maria; Holopainen, Sari; Väänänen, Veli-Matti (2019). “The effect of beaver facilitation on Common Teal: pairs and broods respond differently at the patch and landscape scales” (英語). Ibis 161 (2): 301–309. doi:10.1111/ibi.12626. hdl:10138/302629. ISSN 1474-919X. 
  25. ^ Pollock, Michael M.; Morgan Heim; Danielle Werner (2003). “Hydrologic and geomorphic effects of beaver dams and their influence on fishes”. American Fisheries Society Symposium 37: 213–233. http://etal.usu.edu/ICRRR/Beaver/Beaver%20dam%20effects%20paper%20final.pdf Feb 20, 2016閲覧。. 
  26. ^ a b Milman, Oliver (January 4, 2022). “Dam it: beavers head north to the Arctic as tundra continues to heat up”. The Guardian. オリジナルのJanuary 4, 2022時点におけるアーカイブ。. https://web.archive.org/web/20220104220623/https://www.theguardian.com/world/2022/jan/04/beavers-arctic-north-climate-crisis 
  27. ^ Levine, Rebekah; Meyer, Grant (2019-05-31). “Beaver-generated disturbance extends beyond active dam sites to enhance stream morphodynamics and riparian plant recruitment”. Scientific Reports 9 (1): 8124. Bibcode: 2019NatSR...9.8124L. doi:10.1038/s41598-019-44381-2. PMC 6544642. PMID 31148552. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6544642/. 
  28. ^ Backhouse, Frances (2021). Beavers - Radical Rodents and Ecosystem Engineers. Orca Book Publishers. pp. 81;100–103. ISBN 9781459824690 
  29. ^ Goldfarb, Ben (2018). Eager: The Surprising, Secret Life of Beavers and Why They Matter (1 ed.). Chelsea Green Publishing. ISBN 9781603587396 
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