Their are an estimated 1 - 2 million species of fungi with every spadeful of healthy soil home to thousands of different types. Agricultural soils can be almost bare of any type of biological life with regular cultivation and a wide range of pesticides and fungicides killing of all the components of a healthy soil. At the extreme end of this is the use of soil fumigants such as methyl bromide (used in strawberry cultivation amongst other things) which leave the soil sterile, this lack of any life leaves domesticated plants wide open to attack and struggling without their required symbiotic fungi.
Fungi have traditionally played a major role in indigenous cultures with many viewing them as sacred entities, used as food, medicine and religous use. The connection between man and fungi is highlighted by the neolithic man frozen in a glacier 5000 years ago he had on him birch bark fungus used for its antibiotic properties a hat made out of a fungi and a container made out of a fungi used to transport ebers of fire. Fungi are also widely throughout history for alcohol and more recently bread.
What we know of as mushrooms are the reproductive bodies of often fast fungal networks. The mushrooms appear during wet conditions when their spores contained on the underside of the mushroom are able to spread and find ideal conditions to grow. Fungi appear uniquely adapted to cope with natural and unatural disturbances quickly moving into and utilising nutrients after slips fires etc. They then provide a base for other organisms to move in and to colonize the area - more about their unique abilitys in a later post.
Mushroom growing in a mulched onion bed