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

旁白：听一段环境科学课程的讲座。

Female professor: Today we're going to begin discussing ecosystems.

女教授：今天我们要开始讲生态系统了。

One important point I wanna emphasize in the reading is that there're many interactions that take place within an ecosystem…interactions between animals, interactions between living and non-living things and so on.

这篇阅读里我要强调的重要的一点是，在一个生态系统里存在很多的相互作用，比如说动物之间的相互作用、生物和非生物间的相互作用等等。

Now these interactions can be fairly simple and straightforward. Ah, there are certain species of ants and rodents sharing a desert ecosystem in Arizona, and they compete for the same plant seeds.

这些相互作用可以相当简单直接。啊，在亚利桑那州，某些种类的蚂蚁和啮齿类动物共享一个沙漠生态系统，它们争夺同样的植物种子。

And the competition influences is not only the size of the ant and rodent populations, but also the number of eventual plants.

这种竞争不仅会影响蚂蚁和啮齿动物的种群规模，也会影响最终植物的数量。

Now this interaction is easy to see, right?

我们很容易看出来这种相互作用，对吗？

However, there're many other interactions within ecosystems that are not so apparent and require closer examination.

然而，生态系统中还有许多其他的相互作用并不明显，需要更仔细的研究。

And the example from your reading was the forest ecosystem along the Pacific coast of North America…um…specifically the role of salmon.

你们读到的例子是北美太平洋沿岸的森林生态系统……嗯……特别是鲑鱼的作用。

OK. As you probably know, salmon are born in fresh water streams, they migrate to oceans where they spend most of their lives, and then they return to the same streams where they were born to reproduce or spawn.

你可能知道，鲑鱼出生在淡水溪流中，它们迁徙到海洋中，在那里度过了它们生命中的大部分时间，然后它们回到它们出生的小溪中繁殖或产卵。

In order to spawn, salmon need cold, clear streams to ensure the survival of their eggs and trees in the surrounding forest play an important role here.

为了产卵，鲑鱼需要寒冷、清澈的溪流，以确保其卵的存活，而周围森林中的树木起到重要的作用。

Their leaves provide shade from the sun. When logging removes the trees, the streams are open to the sun and the water becomes warmer.

它们的叶子可以遮挡阳光。当人们把树木砍伐掉以后，溪水就会直接暴露于日晒下，水温就会上升。

When the water warms up, the concentration of dissolved oxygen in the water decreases and this reduces the chance that the salmon eggs will survive.

当水温上升时，水里溶解的氧气量会下降。这就降低了鲑鱼鱼卵的存活率。

And the trees also help keep the soil on the banks of the stream in place.

同时，树木也有助于将土壤保持在河流的河岸上。

Salmon cannot spawn in streambeds clogged with sediment, dirt, from the surrounding area, they need a clean, gravel streambed. Bred?

鲑鱼不能在泥沙淤塞的河床上产卵，它们需要一个干净的砾石河床。布拉德，怎么了？

Male student: I've read that salmon also help keep streams healthy.

男学生:我读到过鲑鱼也有助于保持河流的良好状态。

Female professor: Right. Salmon contribute important nutrients like carbon and phosphorous, and these nutrients promote diversity in the stream environment.

教授：对的。鲑鱼提供重要的营养物质，如碳和磷，这些营养物质促进河流环境的多样性。

OK. Um, so salmon need trees to successfully reproduce.

好的，嗯，所以鲑鱼需要树木才能成功繁殖。

But surprisingly, trees also need salmon and bears play an important intermediary role.

但是令人惊奇的是树木也同样需要鲑鱼。熊在这里同样起到了一个重要的媒介作用。

So in the autumn, bears are busy putting on extra weight as they prepare to hibernate.

所以在秋天，熊在准备冬眠的时候会忙于使自己增肥。

Each bear catches an estimated 700 fish during the 45 days that the salmon are spawning.

在鲑鱼产卵的45天里，每只熊估计能捕到700条鱼。

The bears catch the salmon in the streams, and then they carry them back into the forest to eat… sometimes as much as 800 meters from the streams.

熊在溪流中捕捉鲑鱼，然后把它们带回森林，有时它们会在距溪流800米的地方进食。

And since the bears only eat about half of each fish they catch, other animals like eagles, crows, and insects feed on the leftovers. Maria?

因为熊只吃自己抓到的鱼的一半，其他的动物比如鹰、乌鸦和昆虫会以剩下的部分为食。玛丽亚?

Female student: Why did the bears bring the salmon so far into the forest? Why not just eat the fish near the streams?

女学生：为什么熊把鲑鱼带到这么远的森林里?为什么不直接在小溪边吃鱼呢？

Female professor: Well, imagine several hungry bears looking for salmon.

教授：嗯，想象一下几头熊同时在抓鱼。

When one bear catches a fish, it's not uncommon for another bear to try stealing it.

当一只熊抓到了一条鱼，另一只熊试图偷鱼是很常见的。

These confrontations can be pretty intense, so it's safer to bring it back into the forest…to a place where the bear can eat undisturbed.

这些对抗可能会非常激烈，所以把鱼带回森林里，才能让熊不受干扰地更安全地进食。

Male student: Um, you said that the bears only eat half of each fish they catch?

男学生:嗯，您刚才说熊只吃一半的鱼？

I mean if I were a bear preparing to hibernate, I probably eat everything I could catch.

我的意思是，如果我是一头准备冬眠的熊，我可能会把我抓到的食物全吃掉。

Female professor: Well, certain parts of salmon are more nourishing…fattier than others.

女教授：嗯，鲑鱼的某些部分比其他部分更有营养，脂肪含量更高。

It's actually more efficient for a bear to only eat some parts of the fish, and then try catching another one, instead of eating the whole fish.

事实上，熊只吃鱼的一部分，然后试着抓另一条鱼，而不是吃掉整条鱼，效率更高。

OK. So after the scavengers have eaten the leftovers, only the fish's skeleton remains.

好的。所以食腐动物吃完剩菜后，只剩下鱼的骨架。

Now, salmon contain nitrogen, so their decomposing bodies and skeletons provide a lot of nitrogen to the surrounding forest.

现在，鲑鱼含有氮，所以它们分解的身体和骨架为周围的森林提供了大量的氮。

Plants absorb this nitrogen, which they need to grow, so the transfer of this nitrogen to the forests is important. Forests near streams with salmon actually reach maturity faster than other forests.

植物吸收它们生长所需要的氮，所以把氮转移到森林中是很重要的。溪流附近有鲑鱼的森林实际上比其他森林成熟得更快。

OK. So, why's all this important? Well, salmon are in trouble.

好的。那么，为什么所有这些都很重要呢? 鲑鱼现在情况并不好。

Some of their populations have gone extinct, and most of the remaining populations have been significantly reduced by overfishing and environmental challenges.

它们的一些种群已经灭绝，剩下的大多数种群也由于过度捕捞和环境挑战而大大减少。

Now, conservationists can try to prevent overfishing, but, well, I mean you can see the interconnections within this ecosystem.

现在，自然资源保护主义者可以尝试防止过度捕捞，但是，我的意思是，你可以看到这个生态系统内部的相互联系。

We've already talked about the importance of trees to salmon, and the negative effect that something like logging can have.

我们已经讨论过树木对鲑鱼的重要性，以及伐木业的负面影响。

So you can see that protecting this ecosystem is going to take a broad effort.

所以你可以看到，保护这个生态系统需要付出更多的努力。