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Now listen to part of a lecture in a biology class.
请听生物课上的部分内容。
OK, so let's talk about what happened to a certain type of insect, a moth, a red-and-black moth that lives in Europe.
好了，我们来讲讲某种昆虫的遭遇，这是一种蛾子，一种生活在欧洲的红黑色飞蛾。
These moths eat a plant called Ragwort and they live in fields where the Ragwort plants grow.
这种蛾子以狗舌草为食，生活在狗舌草生长的田野里。
Now, there was a group of moth that lived in one of these fields and, for many years, there was a lot of Ragwort growing there.
现在，有一群飞蛾就生活在其中一片这样的田野里，多年以来，那里的狗舌草都生长得十分旺盛。
So the moth had plenty to eat and the total number of moth in the field stayed pretty much the same.
所以飞蛾有大量的食物可以食用，那片田野里的飞蛾总数也基本保持不变。
But then one year it rained a lot less than usual and the Ragwort didn't grow as well.
然而，有一年降雨较之往年少了许多，狗舌草的长势也不好。
The result was that the moth didn't get enough to eat and many didn't survive but even the ones that did survive didn't lay as many eggs as before.
结果导致飞蛾食物不足，许多蛾子无法存活，即使有些飞蛾得以幸存，它们所产的卵也不如往年多。
So that year the moth population in the field was quite a bit smaller.
所以那一年那片田野的飞蛾总数减少了许多。
The next year, though, the amount of rainfall returned to normal again, many more Ragwort plants grew and, once again, there was a lot available for the moth to eat.
不过，下一年降雨量还是恢复到了正常水平，狗舌草也长势喜人，飞蛾也再次有了足够的食物。
So that year the moth population increased and the female moth laid many more eggs than the year before.
于是那一年飞蛾总数有所增加，母蛾产卵量也比前一年多了许多。
And now, after all that rainfall and plant growth, there were just as many moth in the Ragwort field as there were before.
现在，多亏了这些降雨和茂盛的狗舌草，那片狗舌草地里的飞蛾数量恢复到了从前的水平。
