<P> Symbiosis can vary between mutualism, commensalism, and parasitism, though these grade into each other, . In mutualism, both organisms benefit . In commensalism, one benefits and the other is unaffected; in parasitism, one benefits and the other is harmed . </P> <P> Some of these relationships are so close that we speak of the composite of two species as one unit; for example, we speak of the composite of algae and fungi as lichens . </P>

The three main types of symbiotic relationships in nature are and