Mutualism

Overview
Mutualism is a biological interaction between individuals of two different species, where both individuals derive a fitness benefit, for example increased survivorship. Similar interactions within a species are known as co-operation.

Mutualism plays a key part in ecology and evolutionary biology. For example, mutualistic interactions are vital for terrestrial ecosystem function as more than 90% of land plants rely on mycorrhizal relationships with fungi to provide them with inorganic compounds and trace elements. In addition, mutualism has driven the evolution of much of the biological diversity we see, such as flower forms (important for pollination mutualisms) and co-evolution between groups of species However mutualism has historically received less attention than other interactions such as predation and parasitism.

Measuring the exact fitness benefit to the individuals is not always straightforward, particularly when the individuals can receive benefits from a range of species, for example most plant-pollinator mutualisms. It is therefore common to categorise mutualisms according to the closeness of the association, using terms such as obligate, facultative and symbiotic. Defining "closeness", however, is also problematical. It can refer to mutual dependency (the species cannot live without one another) or the biological intimacy of the relationship in relation to physical closeness (e.g. one species living within the tissues of the other species). Mutualism and symbiosis are sometimes used to refer to the same thing but this is strictly incorrect: the term symbiosis was originally meant to include relationships which were mutualistic, parasitic or commensal.

Types of relationships
Mutualistic interactions can be thought of as a form of "biological barter" in which species trade resources (for example carbohydrates or inorganic compounds) or services such as gamete or offspring dispersal, or protection from predators.

Resource-resource relationships
Resource-resource interactions, in which one type of resource is traded for a different resource, are probably the most common form of mutualism; for example mycorrhizal associations between plant roots and fungi, with the plant providing carbohydrates to the fungus in return for nitrogenous compounds and water. Other examples include rhizobia bacteria which fix nitrogen for leguminous plants (family Fabaceae) in return for energy-containing carbohydrates.

Service-resource relationships
Service-resource relationships are also common, for example pollination in which nectar or pollen (food resources) are traded for pollen dispersal (a service) or ant protection of aphids, where the aphids trade sugar-rich honeydew (a by-product of their mode of feeding on plant sap) in return for defence against predators such as ladybird beetles.

Service-service relationships
Strict service-service interactions are very rare, for reasons that are far from clear. One example is the relationship between sea anemones and anemonefish in the family Pomacentridae: the anemones provide the fish with protection from predators (which cannot tolerate the stings of the anemone's tentacles) and the fish defend the anemones against butterfly fish (family Chaetodontidae) which eat anemones. However, in common with many mutualisms, there is more than one aspect to the biological barter: in the anemonefish-anemone mutualism, waste ammonia from the fish feed the symbiotic algae that are found in the anemone's tentacles. Therefore what appears to be a service-service mutualism in fact has a service-resource component. A second example is that of the relationship between some ants and trees in the genus Acacia, such as the Whistling Thorn and Bullhorn Acacia. The ants nest inside the plant's thorns. In exchange for shelter, the ants protect acacias from attack by herbivores (which they frequently eat, introducing a resource component to this service-service relationship) and competition from other plants by trimming back vegetation that would shade the acacia. In addition, another service-resource component is present, as the ants regularly feed on lipid-rich food-bodies called Beltian bodies that are on the Acacia plant.

Humans and mutualism
Humans also engage in mutualisms with other species, including our gut flora (without which we would not be able to digest food efficiently) and domesticated animals such as dogs, which provide protection in return for food and shelter. In traditional agriculture, many plants will function mutualistically as companion plants, providing each other with shelter, soil fertility and the repelling of pests. For example, beans may grow up cornstalks as a trellis, while fixing nitrogen in the soil for the corn, as exploited in the Three Sisters gardening technique. The question how and why species might cooperate has been addressed philosophically by a number of writers. Gilles Deleuze, for example, was interested in the way this questioned the conception of evolutionism and the notion of linear historical progress.

General

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 * Ollerton, J. 2006. "Biological Barter": Patterns of Specialization Compared across Different Mutualisms. pp. 411-435 in: Waser, N.M. & Ollerton, J. (Eds) Plant-Pollinator Interactions: From Specialization to Generalization.  University of Chicago Press. ISBN 0226-87400
 * Paszkowski, U. 2006. Mutualism and parasitism: the yin and yang of plant symbioses. Current Opinion on Plant Biology 9 (4): 364-370.
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