海狸如何恢复湿地
2月5日发表
作者：Navin Singh Khadka
英国广播公司（BBC）世界服务部环境记者
根据《拉姆萨尔湿地公约》，我们失去湿地的速度是森林的三倍。当谈到将它们恢复到自然状态时，有一位具有非凡力量的英雄——海狸。
湿地储存水，作为碳汇，并且是食物的来源。《拉姆萨尔湿地公约》称，湿地对人类的贡献超过了其他陆地生态系统，然而，它们正在以惊人的速度消失。
主要问题是农业和城市扩张，以及气候变化带来的干旱和气温升高。但是，如果你有一条河和一只海狸，就有可能阻止这一进程。
这些毛茸茸的尖牙啮齿类动物在水道上筑坝，形成一个池塘，在池塘里面建一个“小屋”，在那里它们可以保护自己免受捕食者的袭击。
它们的技术是咀嚼树干直到树干倒下，然后用树干和树枝作为建筑材料，再加上底部的石头以及泥土和植物来密封大坝的上游墙。大坝造成了洪水，减缓了水流的速度，使其在景观上停留的时间更长。加州州立大学的生态水文学家艾米莉·费尔法克斯说：“这种行为将简单的溪流转变为繁荣的湿地生态系统。”
“湿地中可用的食物和水的数量使之成为许多不同物种的理想栖息地。这就是为什么海狸是众所周知的关键物种的部分原因。”
海狸啃食所选树木的根部，直到它们倒下
在过去的50年里，加拿大和美国的几个州重新引进了海狸。最初，这样做是为了恢复海狸的数量，因为在19世纪，海狸因皮毛和肉类而被猎杀，几近灭绝。湿地生态系统的恢复也带来了巨大的生物多样性效益，包括许多种类的青蛙、鱼类和无脊椎动物的回归。芬兰研究人员在2018年的一项研究发现，海狸建造的池塘中，哺乳动物种类比其他池塘多出近两倍。黄鼠狼、水獭甚至驼鹿都更为常见。斯特林大学淡水科学教授奈杰尔·威尔比说：“海狸湿地是相当独特的。”“任何人都可以建造池塘，但海狸建造的池塘为生物多样性创造了令人惊叹的好池塘，部分原因是它们很浅，到处都是枯木，而且一般都被以植物为食的海狸搞得一团糟，挖掘水渠、修理水坝、建造住所等。”“基本上，海狸擅长创造复杂的湿地栖息地，这是我们无法比拟的。”- 海狸建造的水坝最高可达5米，迄今记录的海狸建造的最大水坝，位于加拿大阿尔伯塔省，长达850米。
- 海狸砍伐过的树木，树桩往往会冒出新芽，而不是死亡。
- 北美海狸和欧亚海狸在20世纪70年代被确认为单独的物种。
科学家们表示，一个健康的湿地生态系统还可以吸收大量的碳，并且通过充当海绵吸收洪水，它可以减轻气候变化的影响。费尔法克斯博士说：“当你进入干旱期时，所有生活在洪泛区的植物都依靠土壤中储存的水分来保持绿色和健康。如果它们没有太多的水分可以使用，它们就会开始枯萎、凋谢和干枯。”
美国怀俄明州的一个海狸坝
她和她的团队研究了2000年至2021年间美国五个州的10场不同的野火，发现即使在特大火灾事件期间，海狸及其生态系统工程也能可靠地创造和保护湿地栖息地。“海狸湿地储存了大量的水，所以其中的植物不会真正感觉到干旱，它们保持绿色和茂盛。而当野火来袭时，它们没有被烧毁，我们发现它们保持着良好的水份。”但专家表示，海狸只是恢复湿地解决方案的一部分。威尔比教授表示，其他必要措施包括在湖泊和河流两岸种植林地，以及恢复泥炭地和盐碱地。将它们引入到不合适的地方可能会适得其反。这一点在阿根廷和智利得到了证明，20世纪40年代从北美引进的海狸在没有捕食者的情况下成倍增长，导致了严重的森林损失。《拉姆萨尔湿地公约》在2021年发表的《全球湿地展望》发现，非洲、拉丁美洲及加勒比地区的湿地退化最为普遍。
乍得湖是它以前的一个影子
靠近西非乍得、喀麦隆和尼日利亚边境的乍得湖，急剧萎缩是最引人注目的例子之一。自20世纪60年代以来，它已经缩小了90%，主要原因是人口迅速增长对水需求量的急剧上升、无计划的灌溉以及现在气候变化引起的干旱。尼日利亚湿地保护活动家阿德尼克·奥拉多苏说：“为了争夺有限的湖水，主要是农民和养牛人之间的冲突已经出现，现在干旱正在进一步使湖水干涸，争夺水资源的情况变得更糟。”
里约内格罗河是1971年《拉姆萨尔湿地公约》所保护的最大湿地
联合国防治荒漠化公约的首席科学家巴伦·约瑟夫·奥尔表示，湿地通常是有弹性的生态系统，但现在长期的干旱构成了越来越大的威胁。他说：“气候变化的预测显示，旱地的干旱严重程度增加，可能会影响湿地的恢复力，减少重要的栖息地服务。”在其他地区，干旱也会破坏湿地，但海狸可以帮助保护湿地。在北美和北欧，已经有超过100个成功的重新引入项目。威尔比教授表示，在欧洲，海狸的数量在过去20年里被认为增加了两倍，现在大多数欧洲国家都重新建立了海狸数据库。根据自然历史博物馆的数据，瑞典、德国和奥地利引领了这一潮流，英国在21世纪初紧随其后。威尔比教授说：“将海狸带回英国的早期动机主要是为了在将一个衰退的物种恢复到其原生范围内发挥作用。”“但它作为其他生物多样性和自然洪水管理的关键物种，价值正在获得更多的关注，这些是现在提出的论据，以支持在许多地方进行的释放迁移动物或围栏试验。"
How beavers are reviving wetlandsEnvironment correspondent, BBC World Service
We are losing wetlands three times faster than forests, according to the Ramsar Convention on Wetlands. When it comes to restoring them to their natural state there is one hero with remarkable powers - the beaver.
Wetlands store water, act as a carbon sink, and are a source of food. The Ramsar Convention on Wetlands says they do more for humanity than all other terrestrial ecosystems - and yet they are disappearing at an alarming rate.
The main problems are agricultural and urban expansion, as well as droughts and higher temperatures brought about by climate change.
But if you have a river and a beaver it may be possible to halt this process.
These furry sharp-toothed rodents build dams on waterways to create a pond, inside which they build a "lodge" where they can protect themselves from predators.
Their technique is to chew tree trunks until they fall, and to use the trunk and branches as building materials, along with stones at the base, and mud and plants to seal the dam's upstream wall.
The dam causes flooding, slows down the flow of water and keeps it on the landscape longer.
"This transforms simple streams into thriving wetland ecosystems," says Emily Fairfax, an ecohydrologist at California State University.
"The amount of food and water available in their wetlands makes them ideal habitat for many different species. That's part of why beavers are what's known as a keystone species."
Beavers chew the base of selected trees until they fall
Over the past 50 years, Canada and several states across the US have reintroduced beavers. Initially this was done to restore beaver numbers, after they were hunted nearly to extinction for their fur and meat in the 19th Century.
But the restoration of wetland ecosystems has also brought huge biodiversity benefits, including the return of many species of frogs, fish and invertebrates.
A study by Finnish researchers in 2018 found that ponds engineered by beavers contained nearly twice as many mammal species than other ponds. Weasels, otters and even moose were all more prevalent."Beaver wetlands are pretty unique," says Nigel Willby, professor of freshwater science at University of Stirling."Anyone can make a pond, but beavers make amazingly good ponds for biodiversity, partly because they are shallow, littered with dead wood and generally messed about with by beavers feeding on plants, digging canals, repairing dams, building lodges etc."Basically, beavers excel at creating complex wetland habitats that we'd never match."•Dams built by beavers can be up to 5m high, and the largest one so far recorded - in Alberta, Canada - is 850m long•While beavers chop down trees, the tree stumps often sprout new shoots instead of dying - effectively the beavers carry out coppicing•The North American beaver and the Eurasian beaver were confirmed to be separate species in the 1970sA healthy wetland ecosystem also sequesters large amounts of carbon, and by acting as a sponge and soaking up floodwaters it can soften the impacts of climate change, scientists say.Wetlands store water during wet seasons and release it slowly during drought episodes."When you enter a period of drought, all the plants living in a floodplain rely on stored water in the soil to keep green and stay healthy. If they don't have much water to access they will start to wilt and wither and dry out," says Dr Fairfax.
A beaver dam in Wyoming, USAShe and her team studied 10 different wildfires in five US states between 2000 and 2021 and found in each one beavers and their ecosystem engineering reliably created and preserved wetland habitat, even during megafire events."Beaver wetlands have a lot of stored water, so plants in them don't really feel droughts, they stay green and lush. And when wildfire came through, they were not burnt and we found that they stayed well-watered."But experts say beavers are only part of the solution to restore wetlands. Other necessary measures include planting woodland along the banks of lakes and rivers, and restoration of peatland and saltmarsh, says Prof Willby.And crucially, beavers are only found naturally in North America and Eurasia.Introducing them to inappropriate places can be counter-productive. This was demonstrated in Argentina and Chile, where beavers introduced from North America in the 1940s multiplied exponentially in the absence of predators, resulting in severe forest loss.The Global Wetlands Outlook published in 2021 by the Ramsar Convention on Wetlands found the most widespread wetland deterioration in Africa and Latin America and the Caribbean.
Lake Chad is a shadow of its former selfThe drastic shrinking of Lake Chad, closer to the border of Chad, Cameroon and Nigeria in West Africa is one of the most striking examples.It has shrunk by 90% since the 1960s mainly due to a steep rise in water demand from a rapidly growing population, unplanned irrigation and now climate-change-induced drought."Conflicts, mainly between farmers and cattle-rearers, over the limited remaining water of the lake was already there and now drought is further drying it up and fighting over the water has gone worse" says Adenike Oladosu, a wetland conservation activist in Nigeria.The Rio Negro is the largest wetland protected under the 1971 Ramsar Convention on WetlandsBarron Joseph Orr, lead scientist with the United Nations Convention to Combat Desertification, says wetlands are often resilient ecosystems, but prolonged droughts now pose a growing threat."Climate change projections show increased drought severity in drylands that could compromise wetland resilience and reduce important habitat services," he says.In other areas too, drought can damage wetlands, but the beaver can help protect them. There have already been more than 100 successful reintroduction projects in North America and northern Europe.In Europe the population is believed to have tripled in the last 20 years, according to Prof Willby, with beavers now re-established in most European countries. Sweden, Germany and Austria led the way, according to the Natural History Museum, but the UK followed in the early 2000s."The early motivation for bringing beavers back to the UK was mostly about playing a part in restoring a declining species to its native range," Prof Willby says."But the value it could have as a keystone species for other biodiversity and in natural flood management was gaining a lot more traction, and these are the arguments usually put forward now to support the local releases of translocated animals or fenced trials happening in many places.”原文地址：https://www.bbc.co.uk/news/science-environment-64502365