If it weren't for mushrooms, there would be so many un-decayed dead trees that there would be no room on Earth for anything else to grow. But mushrooms don't just recycle dead trees. They help make the trees grow in the first place.

Ref: Discover, 2005, pp. 48-53, Greg Mueller, "Dr. Mushroom.". Photo:Mycelium by James Lindsey CC BY SA 3.0