I learned that mushrooms are normally classified as either decomposers or symbiotes. I would like to know the differences between the two types and whether they are detrimental or beneficial to their host, often a tree.

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    $\begingroup$ where have you looked for answers so far? $\endgroup$
    – Ben Bolker
    Aug 27 '19 at 14:53
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    $\begingroup$ I have read an article (sciencing.com/role-fungi-play-food-chains-13253.html) and their separate wikis. $\endgroup$
    – aitía
    Aug 27 '19 at 15:14
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    $\begingroup$ They are saprophytes/ saprotrophs / parasitic / symbiont. Of type. $\endgroup$ Aug 28 '19 at 19:22

It probably makes sense to classify mushrooms (fungi) in two separate ways:

  1. symbiotic vs. free-living: symbiotic organisms are those that live in close association with a host (etymologically sym="with", biosis="life"), free-living organisms don't. So a mycorrhizal fungus that lives within or on the surface of a plant root is a symbiote; a fungus that lives in the soil is free-living.

  2. mutualistic (benefiting from and benefiting a host) vs. parasitic (benefiting from but harming a host) vs. saprotrophic (consuming already-dead material; these are your "decomposers"). (Commensalism [benefiting from, but neither hurting nor harming, a host] is a fourth possibility.) Mycorrhizal fungi are generally mutualistic. Fungi like chestnut blight or the genus of phythophthora, which includes the organism that causes potato blight, are pathogens; so are the myco-heterotrophs. Fungi such as wood-decay fungi are saprophytic.

To answer your question, species of fungi that are associated with trees may be either mutualistic (e.g. mycorrhizal fungi) or parasitic (e.g. chestnut blight).

  • Parasitic fungi might also be called pathogenic (i.e., disease-causing)
  • In popular speech, "symbiotic" is synonymous with "mutualistic" (i.e., organisms that benefit each other); in biology, however, it may refer to either a parasitic or a mutualistic organism. An individual symbiotic organism is often called a "symbiont".
  • In general, saprotrophs would be free-living (since they don't get their resources from a host, it doesn't really make sense for them to live on a host)
  • Individuals of a some fungal species can switch lifestyles/categories depending on their environment: these are called facultatively mutualistic/commensal/parasitic, e.g. see Redman et al. 2001.

Redman, Regina S., David D. Dunigan, and Rusty J. Rodriguez. “Fungal Symbiosis from Mutualism to Parasitism: Who Controls the Outcome, Host or Invader?” New Phytologist 151, no. 3 (2001): 705–16. https://doi.org/10.1046/j.0028-646x.2001.00210.x.


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