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OK. I think I get it. Well, let's look at a case study. That should make things clear.
好的，我想我明白了。讓我們看一個個案研究，這會讓事情看起來更簡單。
Some years ago, some of my colleagues conducted an experiment in an oak forest involving three different species: white-footed mice, gypsy moths and oak trees.
幾年前，我的一些同事在櫟樹橡木林進行一項實驗，其中包括三種不同的物種：白足鼠、舞毒蛾和橡樹。
OK. Now let me explain what the situation is in this forest.
好的，讓我解釋一下這個森林里的情況。
Oak trees produce acorns, and acorns are a primary food source for white-footed mice.
橡樹產生橡實，而橡實是白足鼠主要的食物來源。
Another food source for the white-footed mice is the gypsy moth.
白足鼠的另一個食物來源是舞毒蛾。
So the size of the gypsy moth population is controlled by the white-footed mice, which is a good thing because gypsy moth caterpillars are considered pests.
所以舞毒蛾的物種大小受到白足鼠的控制，這是一件好事，因為舞毒蛾的毛蟲被認為是害蟲。
They strip away the leaves from the oak trees every ten years or so.
每十年左右，它們會剝去橡樹的葉子。
So the mice eat both acorns from the oak trees and gypsy moths. And the gypsy moth caterpillars eat oak tree leaves.
所以鼠吃橡樹上的橡實和舞毒蛾，而舞毒蛾的毛蟲吃像樹葉。
Right. Now, what makes this set of relationships particularly interesting is that oak trees only produce a large number of acorns once every few years.
是的?，F在使這一套關系特別有趣的是只有每隔幾年橡樹才會產出一次大量的橡實。
So during the years with fewer acorns, the white-footed mice have to deal with a smaller food supply.
所以在橡實很少的年份里，白足鼠不得不應對食物供應不足的狀況。
Yes. But in the years with large amounts of acorns, the mice have more food, which leads to...
是的，但是在有大量橡實的年份里，鼠會有更多的食物，這會導致......
The white-footed mice population growing.
白足鼠的種群壯大。
And the gypsy moth population decreasing.
而且舞毒蛾的種群會下降。
How can we know that for sure? It seems like a big jump from more acorns to fewer gypsy moths.
我們怎么能確定？從更多的橡實到舞毒蛾的減少，中間似乎有著大步飛躍。
Well, we can know for sure because in this oak forest, the researchers decided to test the links between acorns and the two animal species.
我們可以確定，因為在櫟樹橡木林里，研究者決定測試橡實和兩個物種的聯系。
In some parts of the forest, they had volunteers drop a large number of extra acorns on the forest floor.
在森林的一些部分，他們讓志愿者將大量多余的橡實放在地上。
And in another section of the forest, they removed a number of white-footed mice.
在森林的另一個地方，他們移走了許多白足鼠。
In the forest areas where extra acorns had been dropped, the gypsy moth population soon went into a significant decline.
在放上額外的橡實的森林地帶，舞毒蛾的種群不久就大幅下降。
But in the section of the forest where the white-footed mice had been removed, the gypsy moth population exploded.
但是在移走白足鼠的森林地帶，舞毒蛾的種群暴漲。
