Fungi play a critical role in nearly every ecosystem. They are key in recycling dead vegetation and making the nutrients available for the next generation of plant life. They provide a source of evolutionary pressure as plant pathogens, and help keep rampant monoculture plant populations in check. They form symbioses with the vast majority of herbaceous and woody plants, allowing them to colonize poor soils and pull otherwise unavailable nutrients from the soil. Without fungi, the dominance of the plant kingdom would be in serious jeopardy, and all other life that depends on it.

The following are the primary roles that fungi play in the world's ecosystems:

Saprotrophs

Saprotroph is Latin for 'lives on rotten stuff', or more concisely, 'dead eater'. Saprotrophs are fungi that assimilate dead plant tissues and decompose them. Many, many fungi fall into this category, especially the molds. Saprotrophs play a key role in recycling nutrients in most ecosystems, and are many times more effective than alternative organisms such as bacteria. Imagine what the world would look like if there were no saprobic fungi to decompose dead vegetation...

Most fungi are capable of digesting the large quantities of cellulose contained in plant tissue. A select group of fungi are also capable of digesting the tough, supportive lignin contained in woody plants and trees. Without these wood rotting fungi, fallen trees would take forever to fully decompose. Even with the aid of burrowing and cutting insects, their lignin would never be reintroduced into the carbon cycle. These wood rotters are absolutely critical to forest ecology. Wood rotters are often classified as brown rot and white rot fungi. These labels are regarding the appearance of the wood after the fungus has acted on it, and are used to aid identification.

Brown rot: Brown rot is caused by a wood rotter using enzymes that digest cellulose but leave the brown lignin. Often you will encounter wood that has been attacked by a brown rot fungus that looks as though it is crumbling into cubes. This is called brown cubic rot, and is the result of major connective tissues being selectively digested.

White rot: In contrast to brown rot, white rot is the result of lignin being digested, with some or all of the cellulose remaining behind. In South America, ranchers know a secret about a particular type of white rot in certain woods: cattle can digest the cellulose-rich remains after the white rotter has removed all the lignin. This white-rot wood is called palo podrido. Another use for white-rot fungi is as an alternative to using chemicals to prepare wood for paper manufacture.

Wood is just one of many sources of nutrients for saprotrophs. Fungi that can't digest or can't compete with wood rotters find their niche in the leaf litter and forest humus. There are saprobic fungi species for every possible niche in an ecosystem's decomposition cycle. An important aspect of saprotrophic activity is that different fungi act on dead vegetation in different stages of decomposition. A pine needle can take up to 10 years to decompose into soil in a healthy conifer forest, and hundreds of different saprotrophs specialize in digesting the compounds that become available in that needle throughout the process. As in many other food webs, there are primary and secondary consumers. On the far end of the saprotroph spectrum, coprophageous fungi are adapted to act on the dung of herbivores, especially ruminants, which contains plenty of partially digested vegetation. Many fungi are adapted to include ruminant dung in their lifecycle, including the commercial white button mushroom.

Pathogenic Fungi

Many specially adapted fungi are able to overcome a plant's natural defenses and aquire their nutrients from a living host. Often, these fungi have co-evolved with their host and are able to attack only a narrow range of host species. With some exceptions, the fungus does not kill its host, but the attack(s) can seriously damage the host's health. The list of microscopic pathogenic fungi is especially long, and most every plant species has as least one pathogenic fungi that is adapted to attack it. The majority of pathogenic fungi are microscopic or nearly so, and include the rusts and smuts.

A good share of pathogenic fungi require a specific, living host. Others are capable of attacking both live and dead vegetation. Some microscopic fungi have adapted to infect living plant tissue, especially leaves, and wait until the tissue dies before actively attacking it (begging the question, are they parasites or saprobes, or both?). In some cases, such as heart rot of trees, the fungus invades the trunk but only attacks the dead heartwood. But because heart-rot is still harmful to the tree in that it weakens the trunk and increases the chance of windfall, these fungi are classified as pathogenic.

The forms and methods of pathogenic fungi are as widely varied as the plants they attack. A major branch of mycology is in plant pathology, which involves more than just kingdom fungi, but thoroughly involves it. Some well known fungal diseases in the past and present are the Irish potato blight, Dutch elm disease, ergot, wheat rust, and powdery mildew.

Pathogenic fungi that you are likely to encounter on a foray or a hike in the woods mostly involve macrofungi that are capable of attacking trees. Some of the wood rotting fungi that form shelves (also called conks) are pathogenic on trees. It is fairly easy to determine if a shelf forming fungus is pathogenic by observing the health of the tree from which it protrudes. Obviously, if the tree looks alive, it is probably being parasitized. A dead tree with shelves may have died from parasitism, or it may just be colonized by saprobic wood rotters. It is also possible for damaged / abandoned sections of a tree to be saprobically attacked while the rest remains healthy and unmolested.

Below are a few examples of pathogenic fungi:

	Phellinus, a white heartwood rot fungus. This genus has many species, each specialized to a particular host. This particular species, Phellinus tremulae attacks aspen. The fungus will penetrate the bark through wounds (sometimes created by bark beetles) and begin assimilating the heartwood. Once it reaches a critical mass, it will begin to produce hoof-shaped conks that increase in size annually.
	Armillaria mella (ssp?), the honey mushroom. A. mella is complex of subspecies and is a major pathogen of trees and some herbaceous plants; attacking and girdling the root collar. It is also great for the table! A. mella also acts as a saprotroph, so is quite versatile and very successful in old growth forests. It uses large, reinforced hyphae called rhizomorphs to transport nutrients over far distances, which allows it to occupy massive areas. In the Michigan upper peninsula, genetically identical samples of an A. mella colony were taken over a 37 acre area. With further research, mycologists concluded that a single honey mushroom colony that was over 1500 years old and had an estimated weight of over 100 tons was living in the forest!
	Hypomyces lactifluorum, the lobster mushroom. Hypomyces is actually a microscopic mold-like fungus that attacks other mushrooms! H. lactifluorum assimilates mushrooms in the Russula and Lactarius genera, many of which are edible. It also seems to make less edible and inedible hosts edible when fully "lobsterized"! Other members of Hypomyces attack other mushroom species, but are of a different color and are in no way edible.

Mycorrhiza

As stated in the previous section, mycorrhiza are fungi which have adapted to live in symbiosis with the roots of plants. Nearly all land plants on earth have mycorrhizal relationships. These relationships range from merely being helpful, to being absolutely essential to the plant's survival. When addressing the relationship between a mycorrhizal fungus and its plant partner, the fungus is called the mycombiont, and the plant is called the photobiont. These terms are also used when referring to the partner relationship of lichens.

The primary role of all mycorrhiza involves the uptake of phosphorous from the soil. Many plants have difficulty processing the form that phosphorous takes in the soil, but mycorrhizas have an easy time at it. There are plenty of other advantages, such as increased surface area and defense against root pathogens.

There are two major types of mycorrhizal fungi:

Photo: mycorrhizas.info

Glomerales: The Glomerales have an entire phylum to themselves (the next step down from kingdom, which means they are a MAJOR division of fungi). They are mycombionts with herbaceous plants, and produce microscopic fruiting bodies underground. The glomerales are truly integrated with their photobionts: they penetrate the root cells and form interfaces called arbuscles (as shown at left). These fungi are also called endomycorrhiza, because part of them exist within the root cells. Although many plants do not require these mycombionts to survive, they provide a major advantage, especially in poor soils. Well known members of the Glomerales are Gigaspora and Glomus. Premium potting soil is often inoculated with members of the Glomerales, and you can also buy mycorrhizal additives to enhance normal potting soil, mulch, compost, etc. prior to planting. An interesting fact about the Glomerales, is that the phylum has a limited number of species (about 150), but are compatible with a huge range of plant species (around 90% of the entire plant kingdom).

Ectomycorrhiza: Most of the ectomycorrhiza are macrofungi, which produce mushrooms, truffles, corals, and earthballs / earthstars. Were you expecting that? Mycombionts in this category mostly form relationships with trees. They form fungal sheaths around root hairs, which are called hartig nets. Many trees absolutely require mycombionts to survive. All pines, oaks, alders, willows, poplars, and most eucalyptus have mycombionts, and many other tree species as well. The great thing about EM fungi, on top of their production of great edible mushrooms, is their specificity to particular tree species. The photo to the left is of Leccinium insigne, the aspen bolete. If you want to find this great edible, look near aspens! They are the only tree served by this species. Quite the opposite of the endomycorrhiza, EM fungi have a large number of species, but specifically serve a relatively small number of plant species. Below are some well known ectomycorrhizal fungi: Ramaria, a coral fungus Amanita muscaria, the fly agaric Boletus edulis, the highly sought after king bolete; cep; porchini; beliy gribe.