The term symbiosis (Greek for living together) can be used to describe various degrees of close relationship between organisms of different species. Sometimes it is used only for cases where both organisms benefit, sometimes it is used more generally to describe all varieties of relatively tight relationships, i.e. even parasitism, but not predation. Some even go so far as to use it to describe predation. [1] It can be used to describe relationships where one organism lives on or in another, or it can be used to describe cases where organisms are related by mutual stereotypic behaviors.In either case symbiosis is much more common in the living world and much more important than is generally assumed. Almost every organism has many internal parasites. A large percentage of herbivores have mutualistic gut fauna that help them digest plant matter, which is more difficult to digest than animal prey. Coral reefs are the result of mutalisms between coral organisms and various types of algae that live inside them. Most land plants and thus, one might say, the very existence of land ecosystems rely on mutualisms between the plants which fix carbon from the air, and Mycorrhyzal fungi which help in extracting minerals from the ground. In fact the evolution of all eukaryotes (plants, animals, fungi, protists) is believed to have resulted from a symbiosis between various sorts of bacteria: endosymbiotic theory.
Varieties of Symbiosis
Mutualism, a relationship in which members of two different species benefit and neither suffers.[2] (+ +)
Commensalism, a relationship in which 'one party gains some benefit, whilst the other suffers no serious disadvantage.'[2] (+ 0)
Parasitism, in which one member of the association benefits while the other is harmed. (+ -)
Amensalism, in which the association is disadvantageous to one member while the other is not affected. (− 0) Allelopathy can fall into this category.
Competition, in which both members are harmed by the relationship. (- -)
ectosymbiosis, in which the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands.
endosymbiosis, in which the symbiont lives within the tissues of the host; either in the intracellular space or extracellularly.
( the terms in the parentheses refer to whether one or both organisms benefits +, is harmed -, or remains neutral 0)
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