Mutualism

Mutualism is an ecological interaction between two or more species in which every participant gains a net benefit, a term the naturalist Pierre-Joseph van Beneden introduced in 1876 to mean mutual aid among species. In resource-resource mutualism, species trade tangible materials, as when plants supply carbohydrates to mycorrhizal fungi in exchange for phosphate and nitrogen, or legumes feed carbohydrates to nitrogen-fixing rhizobia bacteria in their roots. In service-resource mutualism, one partner provides a service for a resource, as in pollination, where plants trade nectar or pollen for a pollinator's help spreading their seeds, or in cleaning symbiosis, where a small fish removes parasites from a larger animal in exchange for food. Service-service mutualism, in which both partners provide services rather than resources, is rarer, as in the relationship between sea anemones, which shelter clownfish, and the clownfish, which defend their host from predators; a fourth, looser category, protocooperation, describes species that benefit from cooperating without depending on each other to survive.