Narrator: Listen to part of a lecture in an ecology class.

旁白：听一段生态学讲座。

Female Professor: So, um, continuing our discussion of ecological systems, whole systems, the main thing to keep in mind here is the inter-relationships.

教授：那么，继续我们的生态系统---整个系统的讨论，最主要的是要记住这里的相互关系。

The species in the system, um, and even the landscape itself, they are inter-dependent.

该种系统呃……甚至风景本身，他们是相互依存的。

Let's take what you've read for this week and see if we can apply this inter-dependence idea. Mike?

让我们拿你们这星期读过的内容来举例，看看我们是否可以运用这个相互依存的概念。迈克？

Male student: Well, um, how about beavers?

学生：嗯，嗯……你认为海狸怎么样？

Ecosystems with beavers in waterways.

海狸和水道的生态系统。

Professor: Good! Good, go on.

教授：好，好，继续。

Male student: Like, well, you can see how it's so important because if you go back before Europeans settled the North America, like, before the 1600s back when Native Americans were the only people living here.

学生：比如，你可以看到它是如何变得如此重要的，因为如果回到欧洲人定居北美之前，如17世纪以前，回到美洲原住民是居住在那里的唯一的民族的时候。

Well, back then there were a lot of beavers.

当时有很多的海狸。

But later on, after Europeans…

但后来，经过欧洲人……

Professor: OK, wait. I see where you're 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?

教授：OK，等等。我看到你用这个朝什么方向发展，但在我们进入欧洲如何影响生态系统之前，告诉我这个---海狸生活在什么样的环境下？

Think about what it was like before the European settlers came.

想想欧洲人定居之前是什么样子的。

We will come back where you were headed.

我们会回到你方才说的方向的。

Female student: OK. Well, beavers live near streams and rivers and they block up the streams and rivers with, like, logs and sticks and mud.

学生：OK，海狸住在小溪和河流附近，它们用原木、棍棒和泥阻止了溪流和河流。

You know, they build dams that really slow down the flow of the stream, so then the water backs up and creates like a pond that floods the nearby land.

你知道，他们修建大坝，减缓溪流的流动，然后这些水倒退，并且像池塘那样形成，淹没了附近的土地。

Professor: And that creates wetlands.

教授：那个创建了湿地。

OK. Tell me more.

OK，告诉我更多。

Female student: Well, with wetlands, it's like there's more standing water, more still water around.

学生：有了湿地，它就像有更多死水，附近有更多死水。

And that waters are a lot cleaner than swiftly flowing water because the dirt and sediment and stuff has a chance to sink to the bottom.

那水比快速流动的水清澈很多，因为污垢和沉淀物之类的东西有机会沉入水底。

Professor: More important for our discussion, wetland areas support a lot more varieties of life than swiftly flowing water.

教授：对于我们的讨论更重要的是，湿地支持许多类型的生命，比快速流动的水。

For example, there are more varieties of fish, of insects, lots of frog species.

例如，有更多品种的鱼，或昆虫，大量的青蛙种群。

And species that rely on those species start to live near the wetlands, too.

还有依赖于这些物种，开始靠近湿地居住的那些物种。

Female student: Yes, like birds and mammals that eat the fish and insects.

学生：是的，像以鱼类和昆虫为食的鸟类和哺乳动物。

And you get trees and plants that begin to grow near the standing water that can't grow near running water.

并且你知道有树木开始在这些死水附近生长，而他们不能在活水附近生长。

Oh, and there's something about wetlands and groundwater, too.

哦，湿地和地下水也有关。

Professor: OK. Good!

教授：好，好。

Wetlands have a big effect on groundwater, the amount of water below the surface of the land.

湿地对地下水，陆地表面以下的水量有很大的影响。

Think of wetlands as, um, like a giant sponge.

将湿地看作是，嗯，就像一块巨大的海绵。

The earth soaks up a lot of this water that's continually flooding the surface, which increases the amount of water below.

地球吸收了很多这样不断泛滥在表面的水，这增加了陆地表面以下的水量。

So where there are wetlands, you get a lot of groundwater.

所以哪里有湿地，你就能得到很多地下水。

And groundwater happens to be a big source of our own drinking water today.

而地下水恰好是我们今天饮用水一大来源。

All right, so back to the beavers.

好吧……所以，回到海狸。

What if the beavers weren't there?

如果海狸不在那里？

Male student: You'd just have a regular running stream,cause there was no dam.

学生：你只有一个普通的运行流，因为没有大坝。

So the ecosystem would be completely different.

因此生态系统将是完全不同的。

There'd be fewer wetlands.

会有更少的湿地。

Professor: Exactly! So now let's go back to where you were headed before, Mike.

教授：没错，所以，现在让我们回到你之前想要讨论的方向，迈克。

You mentioned a change that occurred after Europeans came to North America.

你提到，欧洲人来到北美后发生的变化。

Male student: Yeah, well, there used to be beavers all over the place, um, something like two hundred million beavers just in the continental United States.

学生：是的，曾经在那里到处都是海狸，大约有2亿海狸，仅在美国大陆。

But when Europeans came, they started hunting the beavers for their fur, because beaver furs were really warm and it was really popular for making hats in Europe, so the beavers were hunted a lot, overhunted; they were almost extinct by the 1800s.

但是，当欧洲人来了，为了它们的毛皮他们开始狩猎海狸，因为海狸毛皮是真的很温暖，在欧洲用这个做帽子是非常受欢迎的，所以，海狸被猎杀了很多，过度捕杀，到19世纪，它们几乎绝迹。

So that meant fewer wetlands, less standing water.

所以……这意味着更少的湿地，更少的死水。

Professor: And what does that mean for the ecosystem? Kate?

教授：而这对生态系统意味着什么？凯特？

Female student: Well if there's less standing water then the ecosystem can't support as many species, because a lot of the insects and fish and frogs can't live in running water and then the birds and animals that eat them lose their food supply.

学生：如果有较少的死水，那么生态系统不能支持许多物种，因为有很多昆虫和鱼类和蛙类不能活在活水中，然后吃它们的鸟和动物失去了它们的食物供应。

Professor: Precisely! So the beaver in this ecosystem is what we called a keystone species.

教授：准确地说，在这个生态系统中的海狸就是我们所说的一个关键种。

The term keystone kind of explains itself.

关键种这个术语解释了它自己。

In architecture, a keystone in an archway or doorway is the stone that holds the whole thing together and keeps it from collapsing.

在建筑学中，在牌楼或门口的楔石是保持整个物体连结在一起，并防止它倒塌的石头。

Well, that's what a keystone species does in an ecosystem.

好了，这就是一个关键种在一个生态系统中起到的作用。

It's the crucial species that keeps the system going.

那是使系统保持运转的关键品种。

Now, beaver populations are on the rise again, but there's something to think about.

现在，海狸种群再次上升，但有一些需要思考。

Consider humans as part of these ecosystems.

考虑到人类是这些生态系统的一部分。

You've probably heard about water shortages or restrictions on how much water you can use, especially in the summer time in recent years.

你可能已经听说过缺水或者你可以使用多少水的限制，尤其是在夏天的时候，在最近几年。

And remember what I said about groundwater, imagine if we still had all those beavers around, all those wetlands, what would our water supply be like then?

请记住我说过的地下水，想象一下，如果我们还有那些海狸在周围，所有的那些湿地，那我们的供水会是怎样的呢？