Commensalism

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In ecology, commensalism is an interaction between two living organisms, where one organism benefits and the other is neither harmed nor helped. As with all ecological interactions, commensalisms vary in strength and duration from intimate, long-lived symbioses to brief, weak interactions through intermediaries. The term commensalism derives from the Latin com mensa, meaning sharing a table. Originally it was used to describe the use of waste food by second animals, like the carcass eaters who follow hunting animals, but wait until they have finished their meal. Other forms of commensalism include:

Phoresy: Using a second organism for transportation. Examples are the remora on a shark, or mites on dung bugs. Both temporary and permanent phoresy exist.
Inquilinism: Using a second organism for housing. Examples are epiphytic plants (such as many orchids) which grow on trees, or birds that live in holes in trees.
Metabiosis: A more indirect dependency, in which the second organism uses something the first created, however after the death of the first. An example are the hermit crabs who use gastropod shells to protect their bodies.

The question of whether the relationship between humans and some types of our gut flora is commensal or mutualistic is still unanswered.

Some biologists argue that any close interaction between two organisms is unlikely to be completely neutral for either party, and that relationships identified as commensal are likely mutualistic or parasitic in a subtle way that has not been detected. For example, epiphytes are "nutritional pirates" that may intercept substantial amounts of mineral nutrients that would otherwise go to the host plant.Benzing, D.H. 1980. Biology of the Bromeliads. Eureka, California: Mad River Press. Large numbers of epiphytes can also cause tree limbs to break or shade the host plant and reduce its rate of photosynthesis.

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