下面就让小编来为大家介绍一下托福听力TPO13原文中Lecture 2的文本内容吧，大家要好好把握，这些都是非常有价值的材料，同时，大家也可以登录前程百利论坛进行TPO练习辅导，希望能够给准备托福听力的同学带来帮助。
TPO13 Lecture 2
Professor：So, continuing our discussion of ecological systems--- whole systems. The main thing to keep in mind here is the interrelationships. The species in the system err…. and even the landscape itself, they are interdependent. Let’s take what you’ve read for this weekend and see if we can apply this interdependence idea. Mike?
Student：Well, um…, how about beavers--- ecosystems with beavers in waterways.
Professor：Good, good, go on.
Student：Like, well, you can see how it's so important, cause if you go back before European settled in north America, like before the 1600s, back when native Americans were the only people living here, well, back then there were a lot of beavers, but later on, after Europeans…
Professor：OK, 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? Think about what it was like before the Europeans settlers came, we’ll come back to where you were headed.
Student：OK, well, beavers live near streams and rivers and they block up the streams and rivers with like logs and sticks and mud. 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.
Professor：And that creates wetlands. OK, tell me more.
Student：Well with wetlands, it's like there is more standing water, more Stillwater around, and that water is a lot cleaner than swiftly flowing water, because the dirt and settlement and stuff has the chance to sink to the bottom.
Professor：More important for our discussion, wetland areas support a lot more variety of life than swiftly flowing water. For example, there are more varieties of fish or insects, lots of frog spices, and then species that rely on those species start to live near the wetlands too.
Student：Yes, like birds and mammals that eat the fish and insects, and you can get trees and plants that begin to grow near the standing water, that can't grow near the running water. Oh, and there's something about wetland, and ground water too.
Professor：OK, good. Wetlands have a big affect on ground water, the amount of water below the surface of the land. Think of wetlands as, Umm, like a giant sponge,the earth soaks up a lot of this water that's continually flooding the surface,which increases the amount of water below. So where is there a wetland, you get a lot of ground water, and ground water happens to be a big source of our own drinking water today.All right… So, back to the beavers, what if the beavers weren't there?
Student：You just have a regular running stream, because there is no dam, so the ecosystem would be completely different, there would be fewer wetlands.
Professor：Exactly, so, now let's go back to where you were headed before, Mike. You mentioned the change that occurred after Europeans came to North America.
Student：Yeah, well, there used to be beavers all over the place, something like 200 million beavers, just in the continental United States. But when Europeans came, they started hunting the beavers for their fur, because beaver fur is really warm, and it was really popular for ** hats in Europe. So the beavers were hunted a lot, overhunted, they are almost extinct by the 1800s, so… that meant fewer wetlands, less standing water.
Professor：And what does that mean for the ecosystem? Kate?
Student：Well if there is less standing water than the ecosystem can support its many species, because a lot of insects and fish and frogs can't live in running water,and then the birds and animals that eat them, lose their foods supply.
Professor：Precisely, so the beaver in this ecosystem is what we call a keystone species.The term keystone kind of explains itself. In architecture, a keystone in an archway or doorway is the stone that holds the whole thing together, and keeps it from collapsing. Well, that's what a keystone species does in an ecosystem. It's the critical species that keeps the system going. Now, beaver populations are on the rise again, but there is something to think about.Consider humans as part of these ecosystems, you've probably heard about water shortages or restrictions on how much water you can use, especially in the summer time, in recent years. And remember what I said about groundwater; imagine if we still have all those beavers around, all those wetlands. What would our water supply be like then?
《生态学》
教授：那么，继续讨论生态系统的问题，整个系统。这里我们要记住的一点是相互关系。生态系统中的各种类生物，呃，甚至是地形地貌本身，都是相互依赖的。现在大家回顾的文章，看看是否可以运用这种相互依赖的理念，Mike?
学生：嗯，海狸的例子怎样，水道中生长的海狸的生态系统。
教授：很好很好，继续。
学生：像，嗯，你可以明白为什么这如此重要，因为如果你回到欧洲人在北美定居下来之前，大概十七世纪的时候，当美洲当地人是居住在那里的唯一居民时，嗯，当时又很有海狸，而之后，在欧洲人……
教授：好的，等等。我知道你要讲什么了，但在我们探讨欧洲人是怎么影响生态系统之前，告诉我，海狸的居住环境是怎样的?想想在欧洲人定居之前，这里环境是怎样的，我们再回到你刚刚说的话题上。
学生：好的，嗯，海狸住的地方靠近溪流和河流，他们会用木块，树枝和泥土将溪流堵起来。你知道的，他们建起的堤坝确实能减低溪流的流速。所以水位便会上涨，形成一个池塘，水会流入附近的土地上。
教授：这就造成了湿地，好的，在跟我说点。
学生：有了湿地，就有了更多静水，这样的水要比那些迅速流动的水显得更加干净，因为泥土和各种沉淀物会沉到水底。
教授：我们要探讨的更重要的一点是，湿地区域比起流动的水域能供养更多各种各样的生命。比如，这些区域会有更多各种鱼类或昆虫，各种青蛙，这样，依赖这些物种的其他物种就开始在湿地周边生活。
学生：是的，像以鱼类和昆虫为生的鱼类和哺乳动物，还有在静水旁生长而不能在流动水域生长的树木和各种植物。哦，还有湿地的其他相关，还有地下水。
教授：好的，湿地对地下水会产生很大影响，地表下的水量。嗯，把湿地想成是一个巨大的海面，地球将很多水分吸收上来，这些水持续不断流到地表上，这增加了地下水的流量。因此，有湿地的地方，就会有大量地下水，而地下水便是我们今天最大的饮水来源。好的。那么，回到海狸的问题，如果海狸不在那里生长的话呢?
学生：只有规律的流动溪流，因为那里没有堤坝，所以生态系统会完全不同，湿地数量也就更少。
教授：确实是，所以，现在我们回到刚刚你想说的，Mike。你提到了欧洲人到底北美洲后发生的改变。
学生：是的，嗯，大概2亿年前，仅在美国大陆，到处就有海狸。但当欧洲人到来时，他们开始捕猎海狸，制作毛皮产品，因为海狸毛皮确实很暖，而在欧洲，人们青睐于毛皮做的帽子。所以，海狸遭受大量捕杀，过度捕杀，19 世纪几乎面临灭绝，这意味着湿地越来越少，静水越来越少。
教授：那么这对于生态系统意味着什么呢?Kate?
学生：嗯，如果静水的量比其生态系统能支撑很多物种的量要少的话，因为很多昆虫鱼类和蛙类都不能在流动水中生存，而以这些生物为食的鸟类和其他动物便会在此失去其食物供给。
教授：准确的说，那么在这种生态系统中的海狸就是我们所说的拱顶石物种了。所谓拱顶石物种，名副其实。在建筑领域，拱道或门口的拱顶石就是支撑整栋建筑物的基石，以防其倒塌。嗯，这就是拱顶石物种在生态系统中的功能。这是保持系统运转的关键物种。现在，海狸数量再一次处于增长趋势，但我们还要考虑某些东西。将人类当做这些生态系统的一部分，你可能听过水资源稀缺或对你所能使用的水资源进行限制，特别是近些年夏天的时候。记住我说过的关于地下水的问题。想象一下如果现在这些海狸还生活在这些湿地上。那么水资源供给会是怎样的一种情况呢?
以上就是托福听力TPO13原文中Lecture 2的文本内容，希望大家能够用心体会，更多TPO文本内容小编稍后为您呈现。最后，小编祝大家在托福考试中取得好成绩！
您还可能关注：
