Fungi's Ecological roles

Typically there are three ecological roles that fungi may play in an ecosystem: parasitic, mycorrhizal and saprophytic.

1. Saprophytes are organisms that consume dead material. These fungi are what eat wood, leaves, manure, corpses. These are the fungi that link death to life, that continue the cycle of nutrient flow on earth. Saprophytes are the typical mushroom cultivated by people. It is much easier to provide a mushroom with dead material and ideal conditions for growth and fruiting than doing the same on a living host. Things like logs and sawdust and wheat bran are very easy to store, handle, and manipulate to create favorable conditions for mushroom growth. Cordyceps militaris is a fungus that can be both parasitic and saprophytic making it possible to grow mushroom fruiting bodies even though a living host is not present.  
2. Mycorrhizal fungi are a type of fungus that creates a symbiotic relationship with plant roots, “myco” meaning fungi and “rhizal” meaning roots. Simply a fungus associated with roots! These fungi are on over 90% of plant species. Most of the trees we look at in our daily lives have fungi attached to their root system. These fungi play multiple roles in assisting plant health, nutrient access, and communication across individuals. Amazing studies have been done to illustrate how connected ecosystems really are. These mycorrhizal networks literally connect individuals of the same and different species to each other. Mycorrhizae allow the exchange of nutrients, flowing from sick and healthy trees as well as old and young trees. Through these mycorrhizal networks, information is passed regarding different stressors like pests that may be invading an ecosystem. On an individual level mycorrhizal fungi also expand the root system of plants and help access nutrients that are locked in the soil like phosphorus and pockets of water. Mycorrhizal filaments are much smaller than roots so can penetrate tiny rocks and cracks that are otherwise inaccessible by the plant. In exchange for these nutrients and water plants trade sugars developed during photosynthesis.
3. Parasitic fungi are fungi that attack a living organism. These fungi give the entire fungal kingdom a bad rap because of their perceived negative impact on human systems. Fungal parasites have a huge impact on crop loss in our agricultural systems. The continued use of monocultures and farming techniques that grow weak plants creates breeding grounds for fungal diseases. These diseases spread rapidly during wet periods and easily travel up the entire country over the course of a growing season. Many of these fungi do not create a mushroom but exist simply in the mycelial and spore stages of the fungal life cycle. Many of these fungi have asexual reproduction cycles where they can rapidly create genetically identical spores for further dissemination. There are some mushroom-forming parasites as well. Honey mushroom, an edible which grows abundantly throughout the U.S., chaga, and cordyceps are all examples of this. One network of honey mushrooms, covering a stretch of forest in Oregon that expands 2000 acres in a continuous mycelial mat has been described as the largest organism in the world. Human perception is typically that parasites are a bad thing, but when the largest organism in the world is a parasite, it gives us something to consider, it must be doing something right. Chaga is currently an extremely popular parasitic mushroom which grows on birch trees. Chaga is thought to have a variety of compounds that positively impact the human immune system. Cordyceps is a parasite not of plants or trees but of insects. Cordyceps attack a living insect larvae or pupae, consume it, and then fruit from the dead body of the insect. Some species of Cordyceps can cross over to being saprophytic allowing humans to produce fruiting bodies without the presence of insects.