mutualism

An intimate relationship between two or more living organisms that is beneficial to all the participants. A *lichen is an example of obligatory mutualism between an alga and a fungus since neither can survive without the other. The relationship between the bacterium Rhizobium and members of the family Leguminosae is an example of facultative mutualism (protocooperation), as the bacterium and the plant can survive independently of one another. See also amensalism , commensalism.