Mutualism

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For the anarchist economic theory, see mutualism (economic theory).

Ant-aphid mutualism: the aphids are protected against predators by the ants who cultivate the aphids for their secretions of honeydew, a food source.

In biology, mutualism is an interaction between two or more species where both species derive benefit. Mutualisms can be lifelong interactions involving close physical and biochemical contact (known as symbiosis) such as those between plants and mycorrhizal fungi; they can also be briefer, non-symbiotic interactions, such as those between flowering plants and pollinators. Mutualisms may also be obligatory or non-obligatory (facultative). For example, bacteria known as rhizobia can reproduce either in the soil or in (usually) mutualistic symbiosis with legume plants (Denison & Kiers 2004). Mycorrhizal fungi, on the other hand, can be totally dependent on their plant hosts. Microbes often band together for mutual benefit in biofilms to break down solid food sources as in rusticles. The relationship between people and their pets is a non-obligatory mutualism for the human, and, depending on the animal, either obligatory or non-obligatory.

The question how and why species might cooperate has received much attention from evolutionary biologists. One way to tackle this question is to look at an interaction between two individuals of these species, and estimate their costs and benefits from each kind of behaviour. Researchers have used the Prisoner's Dilemma known from Game Theory, to model a situation with two possible strategies: 'cooperate' or 'defect' as one way to understand how cooperation might persist. In a one-time encounter, the safest strategy to use for each side would be to defect. However, cooperation might persist when the interacting organisms remember and react to previous behaviour of their partner in strategies such as 'Tit for Tat', where defecting or cooperating follows the behaviour of the partner in the last round. If the partner defected, I should defect and vice versa. This strategy could lead, in some systems to stable cooperation.

Other models trying to explain possible cooperation include 'biological marketing'. Here the decision whether and with whom to cooperate is based on comparison of the different potential partners who attempt to outbid each other with benefits they offer. The price of trade is determined by balance between supply and demand for the benefits exchanged.

In most models trying to explain mutualism, there usually is a place for 'cheaters' or 'exploiters'. These do not 'play along' and try maximizing their own net benefit by reducing the cost of the interaction from their side. That is, by avoiding to deliver their trade 'goods', the other partner is seeking. This could lead, in theory, to repeated exploitation by one species, which some call parasitism.

The question how and why species might cooperate has also been addressed by philosophers. Gilles Deleuze, for example, is interested in the way this questioned the conception of evolutionism and the notion of linear historical progress.

[edit] See also

• Gilles Deleuze's use of the concept of mutualism in the invention of rhizomes (in particular the mutualism between the orchid and the wasp; cf. A Thousand Plateaus)
• Ecological facilitation
• Co-adaptation
• Co-evolution
• A famous land version of symbiosis is the relationship of the Egyptian Plover bird and the crocodile. In this relationship, the bird is well known for preying on parasites that feed on crocodiles which are potentially harmful for the animal. To that end, the crocodile openly invites the bird to hunt on his body, even going so far as to open the jaws to allow the bird enter the mouth safely to hunt. For the bird's part, this relationship not only is a ready source of food, but a safe one considering that few predator species would dare strike at the bird at such proximity to its host.

[edit] References

• Breton, Lorraine M., and John F. Addicott. 1992. Density-Dependent Mutualism in an Aphid-Ant Interaction. Ecology, Vol. 73, No. 6, pp. 2175-2180.
• Bronstein, JL. 1994. OUR CURRENT UNDERSTANDING OF MUTUALISM. QUARTERLY REVIEW OF BIOLOGY 69 (1): 31-51 MAR 1994
• Bronstein JL, 2001. The exploitation of mutualisms. ECOLOGY LETTERS 4 (3): 277-287
• Bronstein JL, 2001. The costs of mutualism. AMERICAN ZOOLOGIST 41 (4): 825-839 S
• Bronstein JL, Alarcon R, Geber M. 2006. The evolution of plant-insect mutualisms. NEW PHYTOLOGIST 172 (3): 412-428
• Denison RF, Kiers ET 2004. Why are most rhizobia beneficial to their plant hosts, rather than parasitic? MICROBES AND INFECTION 6 (13): 1235-1239
• DeVries, PJ; and Baker, I. 1989. Butterfly exploitation of an ant-plant mutualism: Adding insult of herbivory. Journal of the New York Entomological Society [J. N.Y. ENTOMOL. SOC.]. Vol. 97, no. 3, pp. 332-340.
• Hoeksema, J.D. & E.M.Bruna. 2000. Pursuing the big questions about interspecific mutualism: a review of theoretical approaches. Oecologia 125:321-330
• Jahn, G.C. and J.W. Beardsley 2000. Interactions of ants (Hymenoptera: Formicidae) and mealybugs (Homoptera: Pseudococcidae) on pineapple. Proceedings of the Hawaiian Entomological Society 34: 181-185.
• Jahn, Gary C., J. W. Beardsley and H. González-Hernández 2003. A review of the association of ants with mealybug wilt disease of pineapple. Proceedings of the Hawaiian Entomological Society. 36:9-28.
• Noe, R. & P. Hammerstein. 1994. Biological markets: supply and demand determine the effect of partner choice in cooperation, mutualism and mating. Behav. Ecol. Sociobiol. 35:1-11
• Paszkowski, U. 2006. Mutualism and parasitism: the yin and yang of plant symbioses.

Curr. Opin. Plt. Biol. 9 (4): 364-370.