051 爱干净的海狸

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One day some scientists looked around and saw that the rivers were very dirty, and that there were less fish to catch

有一天，一些科学家环顾四周，发现河流非常脏，能捕到的鱼也变少了。

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They also noticed that there were fewer birds and dragonflies, and that the trees were starting to die

他们还注意到，鸟类和蜻蜓也变少了，树木也开始枯死。

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They couldn't find what was causing the problem

他们找不出问题的原因。

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But they noticed something else was missing — the beaver

但他们注意到还有一样东西不见了——海狸。

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So scientists decided to study how beavers live so that they can bring them back to the rivers

因此，科学家们决定研究海狸是如何生活的，以便把它们重新带回河流。

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Wherever beavers live, they make big changes to nature

只要有海狸生活的地方，它们就会对自然环境造成巨大的改变。

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In fact, the only creatures who affect nature more are humans!

事实上，比它们对自然影响更大的生物，只有人类！

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However, unlike humans, the changes that beavers make are good for the other animals and plants around them

然而，和人类不同的是，海狸带来的改变对周围的其他动植物是有益的。

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Once a beaver finds an empty spot on a stream or river, it builds a dam

一旦海狸在小溪或河流上找到一个空旷的地方，它就会开始筑坝。

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As soon as the dam is built, the land upstream begins to fill with water

水坝一建成，上游的土地就开始蓄满水。

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The extra water changes the land behind the dam into a wetland

多余的水把水坝后面的土地变成了湿地。

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A wetland is a wide area of mud and water, and wetlands are very important habitats

湿地是一片广阔的泥水地带，是十分重要的栖息地。

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In fact, over half of North America's endangered species need wetlands to survive

事实上，北美超过一半的濒危物种需要湿地才能生存。

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Once a beaver makes its dam, it takes care to make sure the water level around the dam stays high

海狸筑好坝后，会细心保持水坝周围的水位在高处。

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The beaver does this to protect its home, but it also protects nature

海狸这么做是为了保护自己的家，但同时也保护了自然。

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The dam slows down a river, which helps make the water cleaner

水坝减缓了河流的流速，有助于让水变得更干净。

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And instead of the river carrying good soil away to the ocean, the dam keeps the soil in the forest

而且，水坝没让河流把肥沃的土壤冲进大海，而是把土壤留在了森林里。

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Even though beavers cut down trees to make their dams, they actually help the trees to grow

尽管海狸为了筑坝会砍倒树木，但它们其实有助于树木生长。

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A beaver's favorite trees for eating and building are cottonwoods and aspens

海狸最爱吃、也最爱用来筑坝的树是棉白杨和白杨。

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Both of these trees make hundreds of baby trees every year

这两种树每年都会长出好几百棵小树。

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However, if all the baby trees lived, they would eventually get too crowded, and they all would die

然而，如果所有小树都活下来，最终会过于拥挤，全都活不成。

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But when the beaver takes out some of the trees, it gives the rest more room to grow

但是，当海狸移除一些树木后，就给了剩下的树更多生长空间。

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This helps keep the forests healthy

这有助于森林保持健康。

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Beavers eat only plants, and this makes its other animal friends happy

海狸只吃植物，这让它的其他动物朋友很高兴。

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The wetland created by a beaver dam gives otter, mink, muskrats, deer, bears, dragonflies, turtles, frogs, and ducks —

海狸坝形成的湿地为水獭、水貂、麝鼠、鹿、熊、蜻蜓、乌龟、青蛙和鸭子——

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and lots of other birds — a place to grow and hunt

还有好多别的鸟——提供了生长和捕食的场所。

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The reason scientists couldn't find fish or dragonflies or birds is that the beavers weren't there building their dams

科学家们找不到鱼、蜻蜓或鸟类的原因是，海狸不在那里筑坝。

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Even though beavers keep the rivers clean, the trees growing, and the other animals alive, many people get angry with beavers

尽管海狸能让河流保持清洁、树木生长、其他动物存活，很多人还是对海狸很生气。

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When dams make wetlands, sometimes they ruin a farmer's field or flood a road

当水坝形成湿地时，有时会毁坏农民的田地，或者淹没道路。

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Some dams make it hard for fish to travel in streams

有些水坝让鱼很难在溪流中洄游。

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But when people try to trap beavers to take them away, a new beaver always moves in

可是，人们想诱捕海狸把它们弄走时，总会有新的海狸搬进来。

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So scientists are studying ways to help beavers and humans live together so we can all make changes that will be good for nature

所以，科学家们正在研究办法，让海狸和人类能共同生活，这样大家都能做出对自然有益的转变。