Commensalism

Commensalisms  is a term employed in ecology to describe a relationship between two living organisms where one benefits and the other is not significantly harmed or helped (like a bird living in a tree). It is derived from the English word commensal, meaning the sharing of food, and used of human social interaction. The word derives from the Latin com mensa, meaning sharing a table.

As with all ecological interactions, commensalisms vary in strength and duration from intimate, long-lived symbioses to brief, weak interactions through intermediaries. Originally it was used to describe the use of waste food by second animals, like the carcass eaters that follow hunting animals, but wait until they have finished their meal. Other forms of commensalism include:
 * Phoresy: One animal attaching to another animal for transportation only. This concerns mainly arthropods, examples of which are mites on insects (such as beetles, flies, or bees), pseudoscorpions on mammals and millipedes on birds. Phoresy can be either obligate or facultative (induced by environmental conditions).
 * 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 is the hermit crabs that 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. Large numbers of epiphytes can also cause tree limbs to break or shade the host plant and reduce its rate of photosynthesis. Similarly, the phoretic mites in the image above may hinder their host by making flight more difficult, which may affect its aerial hunting ability or cause it to expend extra energy while carrying these passengers.