symbiosis sĭmbēō´sĭs [key], the habitual living together of organisms of different species. The term is usually restricted to a dependent relationship that is beneficial to both participants (also called mutualism) but may be extended to include parasitism, in which the parasite depends upon and is injurious to its host; commensalism, an independent and mutually beneficial relationship; and helotism, a master-slave relationship found among social animals (e.g., the ant and the aphid). True symbiosis is illustrated by the relationship of herbivorous animals (e.g., cockroaches, termites, cows, and rabbits) to the cellulose-digesting protozoans or bacteria that live in their intestines; neither organism could survive without the other. Other symbiotic relationships include the interdependence of the alga and the fungus (and sometimes a basidiomycete yeast) that form a lichen and the relationship between leguminous plants and the nitrogen-fixing bacteria, which is important in agriculture (see nitrogen cycle). Two obvious examples of a plant-to-animal relationship are yucca and yucca moth, fig and fig wasp; in both cases the insect fertilizes the plant, and the plant supplies food for the larvae of the insect.
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