An interaction between two species in which both species benefit. (The term symbiosis is often used synonymously with mutualism.) There are many examples in nature spanning all types of organisms. In some cases—facultative mutualisms—the partners can survive on their own, whereas in others—obligate mutualisms—one or both partners have evolved to depend on the symbiotic association for their survival. A well-known example of mutualism is the association between termites and the specialized protists that inhabit their guts. The protists, unlike the termites, are able to digest the cellulose of the wood that the termites eat and release sugars that the termites absorb. The termites therefore depend on the association to use wood as a foodstuff, while the protists are supplied with food and a suitable environment. See also mycorrhiza.