Fungi are key to the ecosystem because they act as major decomposers, plant symbiotes and parasites. Decomposing fungi are extremely well-adapted to breaking down plant tissues, particularly wood, and return large amounts of carbon to the atmosphere by doing so. They are thus major parts of the carbon cycle.
Though decomposing wood is one of their most important roles, fungi decompose other organic materials as well. Fungi also help process soil, break down grasses and break down animal dung. Without these decomposers, these materials would sit in the environment for longer before being transformed into materials useful to other organisms.
Other types of fungi help trees grow. These types of fungi grow intertwined with tree roots, receiving water and organic compounds in exchange for making certain essential minerals more accessible to the trees. Individual species of fungi are often specific to a particular species of tree. The fungi usually cannot survive without the trees, while the trees do not grow as well without the fungi.
Parasitic fungi can be very destructive, attacking both plants and animals. These include the most dangerous plant pathogens. One species alone is suspected of destroying billions of chestnut trees. Fungal parasites can also attack humans, causing diseases such as certain types of pneumonia.