Now listen to part of a lecture in a biology class.
(Professor) 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. These moths eat a plant called Ragwort and they live in fields where the Ragwort plants grow. Now, there was a group of moth that lives in one of these fields and, for many years, there was a lot of Ragwort growing there. So the moth had plenty to eat and the total number of moth in the field stayed pretty much the same.

But then one year it rained a lot less than usual and the Ragwort didn’t grow as well. 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. So that year the moth population in the field was quite a bit smaller. The next year, though, the amount of rainfall returned to normal and again many more Ragwort plants grew and, once again, there was a lot available for the moth to eat. So that year the moth population increased and the female moth laid many more eggs than the year before.
And now, after all that rainfall and plant growth, there were just as many moth in the Ragwort field as there were before.