Read the passage. Cooperative Trees We often think of trees as simple objects that provide us with things we need, such as lumber, shade, or fruit. Yet trees are far more complex—and social—than we recognize. An underground network of fungi, which some have called the "wood-wide web," connects the roots of numerous trees in a forest. Using the network, trees can communicate with and support one another. They may share water, carbon, nutrients, and protective compounds with one another. Scientists believe that one explanation for this phenomenon may be a "source-sink model." According to this theory, trees growing in excellent conditions with many resources transfer these resources to trees growing in less favorable conditions. In this way, trees in a forest ensure the health of the entire community by working cooperatively. What is the main idea of the passage?