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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。让我们看一个个案研究，这会让事情看起来更简单。
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.
几年前，我的一些同事在栎树橡木林进行一项实验，其中包括三种不同的物种：白足鼠、舞毒蛾和橡树。
OK. Now let me explain what the situation is in this forest.
好的，让我解释一下这个森林里的情况。
Oak trees produce acorns, and acorns are a primary food source for white-footed mice.
橡树产生橡实，而橡实是白足鼠主要的食物来源。
Another food source for the white-footed mice is the gypsy moth.
白足鼠的另一个食物来源是舞毒蛾。
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.
所以舞毒蛾的物种大小受到白足鼠的控制，这是一件好事，因为舞毒蛾的毛虫被认为是害虫。
They strip away the leaves from the oak trees every ten years or so.
每十年左右，它们会剥去橡树的叶子。
So the mice eat both acorns from the oak trees and gypsy moths. And the gypsy moth caterpillars eat oak tree leaves.
所以鼠吃橡树上的橡实和舞毒蛾，而舞毒蛾的毛虫吃像树叶。
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。现在使这一套关系特别有趣的是只有每隔几年橡树才会产出一次大量的橡实。
So during the years with fewer acorns, the white-footed mice have to deal with a smaller food supply.
所以在橡实很少的年份里，白足鼠不得不应对食物供应不足的状况。
Yes. But in the years with large amounts of acorns, the mice have more food, which leads to...
是的，但是在有大量橡实的年份里，鼠会有更多的食物，这会导致......
The white-footed mice population growing.
白足鼠的种群壮大。
And the gypsy moth population decreasing.
而且舞毒蛾的种群会下降。
How can we know that for sure? It seems like a big jump from more acorns to fewer gypsy moths.
我们怎么能确定？从更多的橡实到舞毒蛾的减少，中间似乎有着大步飞跃。
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.
我们可以确定，因为在栎树橡木林里，研究者决定测试橡实和两个物种的联系。
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，他们让志愿者将大量多余的橡实放在地上。
And in another section of the forest, they removed a number of white-footed mice.
在森林的另一个地方，他们移走了许多白足鼠。
In the forest areas where extra acorns had been dropped, the gypsy moth population soon went into a significant decline.
在放上额外的橡实的森林地带，舞毒蛾的种群不久就大幅下降。
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带，舞毒蛾的种群暴涨。
