The food web - what is the role of fungi?

Disclaimer:  Before you read further, this may be the product of a science nerd.  I learned several new terms during the research for this entry, including my new favorite "phyllosphere" - which I will be using in sentences as much as possible from now on!
Someone asked me recently what powdery mildew was and if it harmed the tree.
Before I could answer the question, I had to understand the role of fungus in the food web, since powdery mildew is caused by a fungus.  It turns out that it's all about the disposition of energy and nutrients in the food web.
  • A food web is a graphic concept describing "who eats whom" in an ecosystem.  Every living thing—from single-celled algae to giant blue whales—needs food to survive.  How that living thing gets its food is part of a complex pathway that energy and nutrients follow through the ecosystem.
Organisms within an ecosystem are generally divided into categories called "trophic levels" – referring to whether they produce their own food ("producers"), eat other organic matter that is living or recently killed ("consumers") or eat non-living plant and animal remains ("detritivores, saprotrophs, decomposers").
  • Producers, also known as "autotrophs", make their own food.  Autotrophs are usually plants or single-celled organisms.  Nearly all autotrophs use a process called photosynthesis to create “food” (glucose) from sunlight, carbon dioxide, and water. 
Primary consumers, or herbivores, eat plants.  Secondary consumers eat the herbivores.  Tertiary consumers eat the secondary consumers.  Consumers can be carnivores (animals that eat other animals) or omnivores (animals that eat both plants and animals).  
  • Detritivores feed on any dead or organic biomass including animals, plants, and feces.  They ingest the decomposing biological matter, digest it internally, and shed nutrients in simple forms that plants can easily absorb from the soil.  The earthworm is a classic example of a terrestrial detritivore, but slugs, woodlice, dung flies, millipedes, and most of the worms are some of the other examples. 
  • Saprotrophs feed on decaying or dead plant matter.  Fungi species predominate among saprotrophs due to their ability to digest lignin in the xylem tissues of plants.  Saprotrophs don't have an internal digestive system.  Instead, they secrete digestive enzymes such as proteases, lipases, or amylases onto the substrates.  This type of extracellular digestion transforms lipids into fatty acids and glycerol; proteins into amino acids, and polysaccharides (e.g. lignin, starch) into glucose and fructose.  These simplified nutrients are absorbed into the fungi through an active transport means called endocytosis – this is how they get their nutrition.  
The ecological role of the saprotrophs is vital for nutrient cycling and energy flow since they consume matter that is difficult for others to break down.
  • Detritivores are mostly animals while saprotrophs are mostly fungi.
  • Detritivores consume lumps of dead organic matter separately, while saprotrophs absorb chemically digested food.
  • Saprotrophs digest their food externally, whereas detritivores do it internally in the digestive system.
  • Detritivores shed most of the digested matter unabsorbed, whereas saprotrophs absorb the entire digested matter to use for their own growth, repair, and reproduction.
Decomposers - fungi and bacteria - turn organic wastes into inorganic materials, such as nutrient-rich soil.  
Detritivores, saprotrophs and decomposers function to complete the cycle of life, returning nutrients to the soil or oceans for use by autotrophs.
  • Different habitats and ecosystems provide many possible food chains that make up a food web.  As an example, a grazing food web has plants or other photosynthetic organisms at its base, followed by herbivores and various carnivores.  A detrital food web, mostly bacteria or fungi, recycles organic material back into the biotic part of the ecosystem.  Since all ecosystems require a method to recycle material from dead organisms, most grazing food webs have an associated detrital food web.  For example, in a meadow ecosystem, plants may support a grazing food web of different consumers, while at the same time supporting a detrital food web of bacteria, fungi, and detrivorous invertebrates feeding off dead plants and animals.
  • Producers receive their energy from light by means of photosynthesis.  After this, the energy in organic matter flows from producers to the different levels of consumers.  However, at each trophic level, energy is always lost.  All of the trophic levels lose energy as heat through cell respiration.  Also, as the organic matter passes from one trophic level to the next, not all of it is digested and energy from organic matter is lost through feces.  This energy then passes on to the detritivores and saprotrophs.  Another energy loss occurs through tissue loss and death which can happen at any trophic level.  Once again, this energy would be passed on to detritivores and saprotrophs as they digest these.  Detritivores and saprotrophs, in turn, lose energy as heat through cell respiration. 
Energy is not recycled.  Since the energy in organic matter is continually being lost as it flows through the ecosystem, energy in the form of sunlight must be constantly re-supplied.  Nutrients on the other hand have to be recycled.  There is only a finite supply of them - they are absorbed from the environment, used by living organisms and then returned to the environment.
  • Fungi that act as decomposers are essential recyclers of nutrients in an ecosystem.  Without these fungi, forest floors would be covered in plant debris and animal carcasses; similarly other ecosystems would have a vast amount of waste piled up.  Without fungal decomposition, nutrients in the soil would be used up, and plants would not have food and couldn’t survive.  If plants don’t survive, the animals that depend on plants for food would also suffer, and the whole food chain would collapse. 
Since transferring nutrients from fungi to the soil is such an integral part of the food chain, some organisms team up with fungi to form symbiotic relationships.  Mycorrhizal fungi, for example, form a symbiotic relationship with plant roots; the plant provides the fungi with carbohydrates, and the fungi in return transfer nutrients like phosphorus to the plant.
  • Endophytic fungus lives within a plant for at least part of its life without causing apparent disease.‪   Endophytes are ubiquitous and have been found in all the species of plants studied to date, but most of these endophyte/plant relationships are not well understood.  Many economically important grasses (e.g., Festuca spp. [Fescue], Lolium spp. [Ryegrass], Zea [Maize]) carry fungal endophytes that are believed to enhance host growth‪ and improve the plant's ability to tolerate abiotic stresses, such as drought, and resistance to insects and mammalian herbivores.‬‬‬‬   For example, endophyte-containing tall fescue is now being planted in areas where people want to deter geese from eating the grass, since it seems to be unpalatable to them.‬‬
While most fungi aid the function of the ecosystem and contribute positively to the food chain, some fungi are harmful to and can even destroy plant life.  An epiphytic fungus is a fungus that grows upon, or attached to, a living plant.  Ephiphytic fungi are part of the mycobiota infesting the plant's phyllosphere, or leaf surface, along with other species of fungus and other organisms.  If cultural conditions (temperature, humidity, soil moisture etc ) result in a disturbance of the equilibrium between "good" and "pathogenic" organisms in the phyllospere, a pathogenic epiphyte like the fungus causing powdery mildew can precipitate plant disease.
  • Powdery mildew fungi are obligate, biotrophic parasites.  Infection by the fungus is favored by high humidity but not by free water.  During the growing season, hyphae are produced on leaf surfaces and specialized absorption cells, termed "haustoria", extend into the plant epidermal cells to obtain nutrition - this can eventually kill a heavily infected leaf.  "Conidia" (asexual spores) are also produced on plant surfaces during the growing season.  They develop on specialized hyphae called conidiophores that are frost-resistant and can overwinter in leaf litter. 
Certain fungal species that cause wood rot are also epiphytic and gain access to wood at a wound site where the bark is breached.  Although trees have mechanisms to compartmentalize the spread of fungi, if large areas of dead tissue spread through the tree, its nutrient supply is cut off and its structural integrity is compromised.  
  • But even as pathogens, the role of fungi can be seen as beneficial to the ecosystem as a whole.  Tree death is a natural and necessary aspect of the forest ecosystem.  The death of large old trees is necessary for the regeneration of new trees and the continuation of the forest.  Death is necessary for life.  Tree diseases that attack especially weak trees can improve the overall vigor of a stand.  In addition, the death of a large tree in the forest creates a gap in the canopy, letting sunlight reach the forest floor and allowing the regeneration of species that would not otherwise grow in the shade of the tree.  Therefore, tree death can improve the diversity of the forest.  This is important because diversity confers resilience to a system.  Pathogens often have some degree of host specificity, meaning they only attack certain species of trees.  A forest that contains only one type of tree is in danger of being completely obliterated by a single pathogen, whereas the damage would be more confined in a forest with greater species diversity.