symbiosis

Symbiosis is a close, long-term interaction between two different biological species. The relationship can affect the fitness of one or both partners and may vary with environmental conditions. Traditionally, symbioses are categorized as mutualism (where both benefit), commensalism (where one benefits and the other is largely unaffected), or parasitism (where one benefits at the expense of the other).

Interactions may be obligate, in which partners depend on each other for survival, or facultative, in which they can live independently but gain from association. Some symbioses are highly specialized and stable over evolutionary time, while others are transient or context-dependent. The boundaries among categories are fluid, and many relationships exhibit varying effects under different circumstances.

Prominent examples include mutualisms such as mycorrhizal fungi and plant roots that enhance nutrient uptake; nitrogen-fixing bacteria in legume nodules; lichens formed by fungi and algae or cyanobacteria; and the microbiota of animal guts that aid digestion. Commensal associations include epiphytic plants on tree surfaces and certain barnacles on whales or other marine animals. Parasitic relationships include tapeworms in vertebrates, leeches, and many parasitic insects.

Symbiosis is a fundamental force in ecology and evolution, influencing species distributions, community structure, and the diversification of life. The study of symbiosis encompasses microbiomes, coevolution, and endosymbiotic events such as the origin of mitochondria and chloroplasts, which illustrate how intimate associations can become indispensable components of organisms.