symbiotic

Symbiotic relationships are close, long-term interactions between two different organisms. The term derives from Greek symbiosis, meaning living together. Symbiosis encompasses several interaction types that may be mutualistic, commensal, or parasitic. In mutualism, both partners gain fitness benefits; examples include mycorrhizal fungi with plants, pollinators and flowering plants, and gut microbiota providing digestion assistance and vitamins, while receiving carbohydrates and a habitat. Lichens arise from a mutualistic association between a fungus and a photosynthetic partner.

Commensalism is where one partner benefits without noticeable effect on the other, as with barnacles on whales or epiphytic plants on trees. Parasitism involves one organism benefiting at the expense of the other; parasites can be endoparasites or ectoparasites and may be obligate or facultative.

Symbioses have shaped evolution. Endosymbiotic theory posits that mitochondria and chloroplasts originated as free-living bacteria that became integrated into early eukaryotes. Such interactions influence nutrient cycles, ecosystem structure, and species distributions. In practical terms, managing symbiotic relationships can improve agriculture, livestock, and human health, as gut microbiota influence digestion and immunity. Relationships are dynamic and can shift along a continuum from mutualism to parasitism depending on environmental context and partner fitness.