A relationship between two organisms in which each of the organisms benefits.♦ In obligate mutualism the interacting species are interdependent and cannot survive without each other. The fungi and algae that combine to form lichen are obligate mutualists.♦ In the more common facultative mutualism the interacting species derive benefit without being fully dependent. Many plants produce fruits that are eaten by birds, and the birds later excrete the seeds of these fruits far from the parent plant. While both species benefit, the birds have other food available to them, and the plants can disperse their seeds when the uneaten fruit drops. Compare amensalism commensalism parasitism.

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