Symbiotic relationships are those in which two (often more) kinds of
organisms exist together in an elaborate, fundamental way.
The word "symbiotic" does not necessary imply a mutualism, where both participating species benefit. This term also includes commensalism, in which one species benefits while the other in not affected, and parasitism, in which one species benefits and the other is harmed.
Many ecosystems depend on the mutualistic association between plant roots and fungi. The plant supplies organic carbon compounds to the fungus, while the fungus supplies important mineral nutrients to the plant. Together in what is often called a mycorrhizal association, each separate species extends their ecological range as well as increasing viability.