# Mutualism (biology)

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Hummingbird Hawkmoth drinking from Dianthus. Pollination is a classic example of mutualism.

Mutualism is the way two organisms of different species exist in a relationship in which each individual benefits. Similar interactions within a species are known as co-operation. Mutualism can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, or parasitism, in which one species benefits at the expense of the other. Mutualism is a type of symbiosis. Symbiosis is a broad category, defined to include relationships that are mutualistic, parasitic, or commensal. Mutualism is only one type.

A well-known example of mutualism is the relationship between ungulates (such as Bovines) and bacteria within their intestines. The ungulates benefit from the cellulase produced by the bacteria, which facilitates digestion; the bacteria benefit from having a stable supply of nutrients in the host environment.

Mutualism plays a key part in ecology. For example, mutualistic interactions are vital for terrestrial ecosystem function as more than 48% of land plants rely on mycorrhizal relationships with fungi to provide them with inorganic compounds and trace elements. In addition, mutualism is thought to have 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.[1] However mutualism has historically received less attention than other interactions such as predation and parasitism.[2][3]

Measuring the exact fitness benefit to the individuals in a mutualistic relationship is not always straightforward, particularly when the individuals can receive benefits from a variety 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 and facultative. Defining "closeness," however, is also problematic. 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).[4]

The term "mutualism" was introduced by Pierre-Joseph van Beneden in 1876.[5]

## Types of relationships

Mutualistic transversals can be thought of as a form of "biological barter"[4] in which species trade resources (for example carbohydrates or inorganic compounds) or services such as gamete, 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 primarily phosphate but also nitrogenous compounds. Other examples include rhizobia bacteria that fix nitrogen for leguminous plants (family Fabaceae) in return for energy-containing carbohydrates.[6]

### Service-resource relationships

The Red-billed Oxpecker eats ticks on the impala's coat

Service-resource relationships are also common.

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 defense against predators such as ladybugs.

Phagophiles feed (resource) on ectoparasites, thereby providing anti-pest service, as in cleaning symbiosis. Elacatinus and Gobiosoma, genus of gobies, also feed on ectoparasites of their clients while cleaning them.[7]

Zoochory is an example where animals disperse the seeds of plants. This is similar to pollination in that the plant produces food resources (for example, fleshy fruit, overabundance of seeds) for animals that disperse the seeds (service).

### Service-service relationships

An example of mutual symbiosis is the relationship between Ocellaris clownfish that dwell among the tentacles of Ritteri sea anemones.

Strict service-service interactions are very rare, for reasons that are far from clear.[4] One example is the relationship between sea anemones and anemone fish 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 butterflyfish (family Chaetodontidae), which eat anemones. However, in common with many mutualisms, there is more than one aspect to it: in the anemonefish-anemone mutualism, waste ammonia from the fish feed the symbiotic algae that are found in the anemone's tentacles.[8][9] 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 in the genus Pseudomyrmex 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.

In the Neotropics, the ant, Myrmelachista schumanni makes its nest in special cavities in Duroia hirsute. Plants in the vicinity that belong to other species are killed with formic acid. This selective gardening can be so aggressive that small areas of the rainforest are dominated by Duroia hirsute. These peculiar patches are known by local people as "devil's gardens".[10]

In some of these relationships, the cost of the ant’s protection can be quite expensive. Cordia sp. trees in the Amazonian rainforest have a kind of partnership with Allomerus sp. ants, which make their nests in modified leaves. To increase the amount of living space available, the ants will destroy the tree’s flower buds. The flowers die and leaves develop instead, providing the ants with more dwellings. Another type of Allomerus sp. ant lives with the Hirtella sp. tree in the same forests, but in this relationship the tree has turned the tables on the ants. When the tree is ready to produce flowers, the ant abodes on certain branches begin to wither and shrink, forcing the occupants to flee, leaving the tree’s flowers to develop free from ant attack.[10]

## Humans and mutualism

Dogs and sheep were among the first animals to be domesticated.

Humans also engage in mutualisms with other species, including their gut flora without which they would not be able to digest food efficiently.[11] Apparently, head lice infestations might have been beneficial for humans by fostering an immune response that helps to reduce the threat of body louse borne lethal diseases.[12]

Some relationships between humans and domesticated animals and plants are to different degrees mutualistic. Agricultural varieties of maize are unable to reproduce without human intervention because the leafy sheath does not fall open, and the seedhead (the "corn on the cob") does not shatter to scatter the seeds naturally.[13]

In traditional agriculture, some plants have mutualist as companion plants, providing each other with shelter, soil fertility and/or natural pest control. For example, beans may grow up cornstalks as a trellis, while fixing nitrogen in the soil for the corn, a phenomenon that is used in Three Sisters farming.[14]

Boran people of Ethiopia and Kenya traditionally use a whistle to call the honey guide bird, though the practice is declining. If the bird is hungry and within earshot, it guides them to a bees' nest. In exchange the Borans leave some food from the nest for the bird.[15]

A population of bottlenose dolphins in Laguna, Brazil coordinates, via body language, with local net-using fishermen in order for both to catch schools of mullet.[16]

## Mathematical modeling

One of the simplest frameworks for modeling species interactions is the Lotka-Volterra equations. In this model, the change in population density of the two mutualists is quantified as:

$\cfrac{dN}{dt}=r_1 N\left(1-\cfrac{N}{K1}+\beta_{12}\cfrac{M}{K_1}\right)$
$\cfrac{dM}{dt}=r_2 M\left(1-\cfrac{M}{K2}+\beta_{21}\cfrac{N}{K_2}\right)$

where,

• N and M = the population density.
• r = intrinsic growth rate of the population.
• K = carrying capacity of its local environmental setting.
• β = coefficient converting encounters with one species to new units of the other.

Mutualism is in essence the logistic growth equation + mutualistic interaction. The mutualistic interaction term represents the increase in population growth of species one as a result of the presence of greater numbers of species two, and vice versa.

In 1989, David Hamilton Wright modified the Lotka-Volterra equations by adding a new term, βM/K, to represent a mutualistic relationship.[17] Wright also considered the concept of saturation, which means that with higher densities, there are decreasing benefits of further increases of the mutualist population. Without saturation, species' densities would increase indefinitely. Because that isn't possible due to environmental constraints and carrying capacity, a model that includes saturation would be more accurate. Wright's mathematical theory is based on the premise of a simple two-species mutualism model in which the benefits of mutualism become saturated due to limits posed by handling time. Wright defines handling time as the time needed to process a food item, from the initial interaction to the start of a search for new food items and assumes that processing of food and searching for food are mutually exclusive. Mutualists that display foraging behavior are exposed to the restrictions on handling time. Mutualism can be associated with symbiosis

### Type II functional response

In 1959, C. S. Holling performed his classic disc experiment that assumed the following: that (1), the number of food items captured is proportional to the allotted searching time; and (2), that there is a variable of handling time that exists separately from the notion of search time. He then developed an equation for the Type II functional response, which showed that the feeding rate is equivalent to

$\cfrac{ax}{1+axT_H}$

where,

• a = the instantaneous discovery rate
• x = food item density
• TH = handling time

The equation that incorporates Type II functional response and mutualism is:

$\cfrac{dN}{dt}=N[r(1-cN)+\beta M(X+M)]$

where,

• N and M = density of the two mutualists
• r = intrinsic rate of increase of N
• c = coefficient measuring negative intraspecific interaction
• X = 1/a TH
• β = b/ TH
• a = instantaneous discovery rate
• b = coefficient converting encounters with M to new units of N

Rearranged:

$\cfrac{dN}{dt}=N\bigg[r(1-cN)+\cfrac{baM}{1+aT_H M}\bigg]$

The model presented above is most effectively applied to free-living species that encounter a number of individuals of the mutualist part in the course of their existences. Of note, as Wright points out, is that models of biological mutualism tend to be similar qualitatively, in that the featured isoclines generally have a positive decreasing slope, and by and large similar isocline diagrams. Mutualistic interactions are best visualized as positively sloped isoclines, which can be explained by the fact that the saturation of benefits accorded to mutualism or restrictions posed by outside factors contribute to a decreasing slope.

## References

1. ^ Thompson, J. N. 2005 The geographic mosaic of coevolution. Chicago, IL: University of Chicago Press.
2. ^ Bronstein, JL. 1994. Our current understand of mutualism. Quarterly Review of Biology 69 (1): 31-51 March 1994
3. ^ Begon, M., J.L. Harper, and C.R. Townsend. 1996. Ecology: individuals, populations, and communities, Third Edition. Blackwell Science Ltd., Cambridge, Massachusetts, USA.
4. ^ a b c 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.
5. ^ van Beneden, Pierre-Joseph (1876). Animal parasites and messmates. London, Henry S. King.
6. ^ Denison RF, Kiers ET 2004. Why are most rhizobia beneficial to their plant hosts, rather than parasitic. Microbes and Infection 6 (13): 1235-1239
7. ^ M.C. Soares, I.M. Côté, S.C. Cardoso & R.Bshary (August 2008). "The cleaning goby mutualism: a system without punishment, partner switching or tactile stimulation". Journal of zoology 276 (3): 306–312. doi:10.1111/j.1469-7998.2008.00489.x. 10.1111/j.1469-7998.2008.00489.x.
8. ^ Porat, D. & Chadwick-Furman, N. E. 2004 Effects of anemonefish on giant sea anemones: expansion behavior, growth, and survival. Hydrobiologia 530, 513–520. doi:10.1007/s10750-004-2688-y
9. ^ Porat, D. & Chadwick-Furman, N. E. 2005 Effects of anemonefish on giant sea anemones: ammonium uptake, zooxanthella content and tissue regeneration. Mar. Freshw.Behav. Phys. 38, 43–51. doi:10.1080/102362405000_57929
10. ^ a b Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
11. ^ Sears CL (October 2005). "A dynamic partnership: celebrating our gut flora". Anaerobe 11 (5): 247–51. doi:10.1016/j.anaerobe.2005.05.001. PMID 16701579.
12. ^ Rozsa, L; Apari, P. (2012). "Why infest the loved ones – inherent human behaviour indicates former mutualism with head lice". Parasitology 139: 696–700. doi:10.1017/s0031182012000017.
13. ^ "Symbiosis - Symbioses Between Humans And Other Species". Net Industries. Retrieved December 9, 2012.
14. ^ Mt. Pleasant, Jane (2006). "The science behind the Three Sisters mound system: An agronomic assessment of an indigenous agricultural system in the northeast". In John E. Staller, Robert H. Tykot, and Bruce F. Benz. Histories of maize: Multidisciplinary approaches to the prehistory, linguistics, biogeography, domestication, and evolution of maize. Amsterdam. pp. 529–537.
15. ^ Gibbon, J Whitfield; foreword by Odum, Eugene P. (2010). Keeping All the Pieces: Perspectives on Natural History and the Environment. Athens, Georgia: University of Georgia Press. pp. 41–42.
16. ^ http://news.discovery.com/animals/whales-dolphins/helpful-dolphins-120502.htm
17. ^ Wright, David Hamilton. 1989. A Simple, Stable Model of Mutualism Incorporating Handling Time. The American Naturalist, Vol. 134, No. 4, pp. 664-667.

## Further 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 March 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 ISSN 1286-4579
• 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. ISSN 0028-7199
• Hoeksema, J.D. & E.M. Bruna. 2000. Pursuing the big questions about interspecific mutualism: a review of theoretical approaches. Oecologia 125:321-330 ISSN 0029-8549
• 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. ISSN 0073-134X
• 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.ISSN 0073-134X
• Noe, R. & P. Hammerstein. 1994. Biological markets: supply and demand determine the effect of partner choice in cooperation, mutualism and mating. Behavioral Ecology and Sociobiology 35:1-11 ISSN 0340-5443
• 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 978-0-226-87400-5
• Paszkowski, U. 2006. Mutualism and parasitism: the yin and yang of plant symbioses. Current Opinion in Plant Biology 9 (4): 364-370. doi:10.1016/j.pbi.2006.05.008. PMID 16713732
• Porat, D. & Chadwick-Furman, N. E. 2004. Effects of anemonefish on giant sea anemones:expansion behavior, growth, and survival. Hydrobiologia 530, 513–520. doi:10.1007/s10750-004-2688-y
• Porat, D. & Chadwick-Furman, N. E. 2005. Effects of anemonefish on giant sea anemones: ammonium uptake, zooxanthella content and tissue regeneration. Mar. Freshw. Behav. Phys. 38, 43–51. doi:10.1080/102362405000_57929
• Thompson, J. N. 2005. The Geographic Mosaic of Coevolution. University of Chicago Press. ISBN 978-0-226-79762-5
• Wright, David Hamilton. 1989. A Simple, Stable Model of Mutualism Incorporating Handling Time. The American Naturalist, Vol. 134, No. 4, pp. 664–667.