They decompose dead or diseased trees and brush, releasing nutrients for future generations of plants and animals. Without nutrient recycling by fungi, the fertility of the soil would decrease. The forest would become stagnant. Insects feed on the decaying wood and are a critical source of food for small animals. Snags and downed logs also provide homes for many different kinds of animals, such as woodpeckers, salamanders, insects, bears, bats and raccoons.
Mushrooms are food for many woodland critters, including deer and squirrels.
Some mushrooms, called truffles, mature underground. Their rich odors entice flying squirrels to dig down and eat them. Owls, in turn, prey on the flying squirrels, making mushrooms and fungus critical links in the food web. A little known but extremely important role some species of fungi play is in their interdependent relationship with plants. Another reason not to collect mushrooms is that many contain deadly poisons.