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:
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.
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.
A few examples of saprotrophs include:
Agaricus, including the commercial white button mushroom
Pleurotis, the oyster mushroom
Ganoderma, a shelf forming white rot fungus
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:
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: