Positive Interactions — Mutualism and Commensalism

In a positive interaction, at least one of the two participating species benefits and neither is harmed. The textbook recognises two forms of this: mutualism, in which both species benefit from an obligate (necessary, close) association, and commensalism, in which only one species — the commensal — benefits, while the other species — the host — is neither helped nor hurt. Mutualism shows up throughout the plant world in strikingly different guises: Rhizobium bacteria form root nodules on leguminous plants, drawing food from the plant while fixing atmospheric nitrogen into a usable nitrate form for it in return; the aquatic fern Azolla hosts the free-living nitrogen-fixing cyanobacterium Anabaena; Anabaena also lives inside the coralloid roots of the gymnosperm Cycas, while the cyanobacterium Nostoc is found inside the thalloid body of the bryophyte Anthoceros; fig trees depend on wasps living inside their fruit for pollination; a lichen is itself a permanent mutualistic partnership between an alga and a fungus; and mycorrhiza is the mutualistic association between the roots of terrestrial plants and fungal hyphae. A related but looser arrangement is proto-cooperation, in which both partners benefit but neither is dependent on the relationship for survival — soil bacteria/fungi living alongside plants rooted in the same soil is a typical example. Commensalism, by contrast, is illustrated by epiphytes — orchids and lianas, for instance — that grow perched on a host tree purely for physical support, taking noth …