A long-term intimate relationship between individuals of different species (symbionts). The term symbiosis is often used to describe relationships in which both species benefit (see cooperation; mutualism), but it may be used for other close associations, such as commensalism, inquilinism, and parasitism. Many symbioses are obligatory (i.e. the participants cannot survive without the interaction); for example, a lichen is an obligatory symbiotic relationship between an alga or a cyanobacterium and a fungus. See also endosymbiont theory.