Fungi are neither plants nor animals but yet they play an extremely important role in the forest ecosystem. A total of about 6,000 different species can be found in Germany, 500 to 700 of them in the Kottenforst area. When the mushroom hats sprout from the ground or on trees, only a small part of the mushroom, the fruiting body, becomes visible. The largest part, the so-called mycelium, is a dense network beneath the surface.

Many fungal species form close partnerships with forest trees, so-called symbioses, which are beneficial to both sides. The fungus grows its network around the root tips of the tree, supplies it with nutrients and protects it from pathogens. In return, it receives carbohydrates, which the tree produces during photosynthesis. Without root fungi, trees would grow much worse. This symbiosis between fungi and trees is also called mycorrhiza.

Other fungi live as parasites, mainly on dying and dead trees. Parasites, like the honey fungus, penetrate the living tree through existing wounds and can cause it to die. Without fungi, the nutrient cycle in the forest would actually collapse. For  - in contrast to a lot of other living organisms - many fungus species  can decompose wood.

Fungi are also an important source of food for snails, maggots, dung beetles, mice and many other animal species.