海狸筑水坝，鸟筑巢。
作为一个例子，让我们看看海狸的觅食行为。
As an example, let's look at foraging behavior among beavers.
海狸把树皮剥下来吃，它们也用树枝来筑巢。
Beavers peel off the bark to eat and they also use the branches for building their shelters.
去年秋天，一只海狸搬进了我家旁边的小溪里。
Last fall a beaver moved into the small stream beside our house.
土狼杀死了公园里大部分的红狐，还赶走了所有的海狸。
The coyotes killed a large percentage of the park's red foxes, and completely drove away the park's beavers.
没人会相信一只海狸能有意识地去权衡每个要素的利弊。
No one believes a beaver consciously weighs the pros and cons of each of these elements.
海狸住在小溪和河流附近，它们用木头、树枝和泥巴堵住小溪和河流。
Beavers live near streams and rivers and they block up the streams and rivers with like logs and sticks and mud.
如果像北美大部分地区一样夏季水位下降，那么海狸的小屋入口就会暴露出来。
If water levels fall in summer, as they do in most of North America, then beavers' lodge entrances may be exposed.
如果夏季水位下降，就像在北美大部分地区所发生的那样，那么海狸居所入口可能会暴露。
If water levels fall in summer, as they do in most of North America, then beavers lodge entrances may be exposed.
海狸被大量捕杀、过度捕杀，到19世纪它们几乎灭绝了，这意味着湿地和静水变得更少了。
The beavers were hunted a lot, overhunted, they are almost extinct by the 1800s, that meant fewer wetlands, less standing water.
它是由长时间的约束形成的，都指向一个事实，就是对海狸来说最好的觅食策略不是产出最多食物或木材。
It's been shaped by constraints over vast stretches of time, all of which comes down to the fact that the best foraging strategy for beavers isn't the one that yields the most food or wood.
等等，我明白你的意思了，但在我们讨论欧洲人的定居如何影响生态系统之前，告诉我，海狸生活在怎样的环境中？
Wait, I see where you are heading with this, but before we go into how European settlement affected the ecosystem, tell me this—what kind of environment do beavers live in?
这些海狸知道如何建造水坝，而且在有需要的时候就会这么做，比如当它们在砍伐和消耗了附近所有的树木后被迫迁移的时候。
These beavers do know how to build dams, and do so if the need arises, as may occur if they are forced to relocate after felling and consuming all nearby trees.
海狸鼠会严重破坏生态系统。
许多植物生长在海狸群附近。
为什么科学家关心海狸鼠？
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