OK. I think I get it. Well, let's look at a case study. That should make things clear.
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.
OK. Now let me explain what the situation is in this forest.
Oak trees produce acorns, and acorns are a primary food source for white-footed mice.
Another food source for the white-footed mice is the gypsy moth.
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.
They strip away the leaves from the oak trees every ten years or so.
So the mice eat both acorns from the oak trees and gypsy moths. And the gypsy moth caterpillars eat oak tree leaves.
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.
So during the years with fewer acorns, the white-footed mice have to deal with a smaller food supply.
Yes. But in the years with large amounts of acorns, the mice have more food, which leads to...
The white-footed mice population growing.
And the gypsy moth population decreasing.
How can we know that for sure? It seems like a big jump from more acorns to fewer gypsy moths.
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.
In some parts of the forest, they had volunteers drop a large number of extra acorns on the forest floor.
And in another section of the forest, they removed a number of white-footed mice.
In the forest areas where extra acorns had been dropped, the gypsy moth population soon went into a significant decline.
But in the section of the forest where the white-footed mice had been removed, the gypsy moth population exploded.