Mutualisms

Mutualism is a type of symbiotic interaction in which two or more species gain a net benefit from their association. Mutualisms can be obligate, in which one or both partners depend on the relationship for survival or reproduction, or facultative, in which the interaction is beneficial but not essential. They differ from parasitism, in which one party benefits at the expense of the other, and from commensalism, in which one partner benefits with little or no effect on the other.

Common examples include plant–pollinator interactions, where flowers provide rewards such as nectar and pollen and pollinators effect reproduction by transfer of pollen; mycorrhizal fungi exchange soil nutrients for plant carbohydrates; and nitrogen-fixing bacteria in legume roots provide bioavailable nitrogen in exchange for carbon and a hospitable environment. Other well-known mutualisms involve animals and microbes: gut microbiota aid digestion and synthesize vitamins in exchange for a habitat and nutrients; corals host symbiotic algae that supply energy through photosynthesis; and reef cleaners such as certain fish receive food while helping keep clients healthy.

Mutualisms can be context-dependent and may shift along a continuum from mutualistic to neutral or even antagonistic as environmental conditions change. They are often maintained by coevolution, partner choice, and ecological feedbacks such as sanctions against cheating or partner-switching. Mutualisms can be diffuse, involving many partners, or tightly coevolved within a specific lineage. They are studied across ecology, evolutionary biology, microbiology, and conservation, and are frequently examined using observational and experimental approaches.