This article is about the biological term. For the economic theory and other uses, seeMutualism (disambiguation).
Hummingbird hawkmoth drinking fromDianthus, withpollination being a classic example of mutualismArbuscular mycorrhizae forms mutualistic symbiosis with plantsLichens considered to be example of mutualistic symbiosis
Mutualism describes the ecologicalinteraction between two or morespecies where each species has a net benefit.[1] Mutualism is a common type ofecological interaction. Prominent examples are:
the nutrient exchange between vascular plants andmycorrhizal fungi,
the ways plants use fruits andedible seeds to encourage animal aid inseed dispersal, and
the waycorals become photosynthetic with the help of the microorganismzooxanthellae.
Mutualism can be contrasted withinterspecific competition, in which each species experiencesreduced fitness, andexploitation, and withparasitism, in which one species benefits at the expense of the other.[2] However, mutualism may evolve from interactions that began with imbalanced benefits, such as parasitism.[3]
The termmutualism was introduced byPierre-Joseph van Beneden in his 1876 bookAnimal Parasites and Messmates to mean "mutual aid among species".[4][5]
Mutualism is oftenconflated with two other types of ecological phenomena:cooperation andsymbiosis.Cooperation most commonly refers to increases in fitness through within-species (intraspecific) interactions, although it has been used (especially in the past) to refer to mutualistic interactions, and it is sometimes used to refer to mutualistic interactions that are not obligate.[1]Symbiosis involves two species living in close physical contact over a long period of their existence and may be mutualistic, parasitic, orcommensal, so symbiotic relationships are not always mutualistic, and mutualistic interactions are not always symbiotic. Despite a different definition between mutualism and symbiosis, they have been largely used interchangeably in the past, and confusion on their use has persisted.[6]
Mutualism plays a key part inecology andevolution. For example, mutualistic interactions are vital for terrestrialecosystem function as:
about 80% of land plants species rely on mycorrhizal relationships with fungi to provide them with inorganic compounds and trace elements.[7]
estimates of tropical rainforest plants with seed dispersal mutualisms with animals range at least from 70% to 93.5%.[8] In addition, mutualism is thought to have driven the evolution of much of the biological diversity we see, such asflower forms (important forpollination mutualisms) andco-evolution between groups of species.[9]
A prominent example of pollination mutualism is with bees and flowering plants. Bees use these plants as their food source with pollen and nectar. In turn, they transfer pollen to other nearby flowers, inadvertently allowing for cross-pollination. Cross-pollination has become essential in plant reproduction and fruit/seed production. The bees get their nutrients from the plants, and allow for successful fertilization of plants, demonstrating a mutualistic relationship between two seemingly-unlike species.
Mutualism has also been linked to majorevolutionary events, such as the evolution of the eukaryotic cell (symbiogenesis) and the colonization of land by plants in association with mycorrhizal fungi.
Service-resource relationships are common. Three important types arepollination,cleaning symbiosis, andzoochory.
In pollination, a plant trades food resources in the form ofnectar orpollen for the service of pollen dispersal. However, daciniphilousBulbophyllum orchid species tradesex pheromone precursor or booster components via floralsynomones/attractants in a true mutualistic interactions with males ofDacini fruit flies (Diptera: Tephritidae: Dacinae).[14][15]
Zoochory is the dispersal of the seeds of plants by animals. 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). Plants may advertise these resources using colour[17] and a variety of other fruit characteristics, e.g., scent. Fruit of theaardvark cucumber(Cucumis humifructus) is buried so deeply that the plant is solely reliant upon theaardvark's keen sense of smell to detect its ripened fruit, extract, consume and then scatter its seeds;[18]C. humifructus's geographical range is thus restricted to that of the aardvark.
Strict service-service interactions are very rare, for reasons that are far from clear.[10] One example is the relationship betweensea anemones andanemone fish in the familyPomacentridae: the anemones provide the fish with protection frompredators (which cannot tolerate the stings of the anemone's tentacles) and the fish defend the anemones againstbutterflyfish (familyChaetodontidae), which eat anemones. However, in common with many mutualisms, there is more than one aspect to it: in the anemonefish-anemone mutualism, wasteammonia from the fish feeds thesymbioticalgae that are found in the anemone's tentacles.[19][20] 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 someants in the genusPseudomyrmex and trees in thegenusAcacia, such as thewhistling thorn andbullhorn acacia. Theants nest inside the plant's thorns. In exchange for shelter, the ants protect acacias from attack byherbivores (which they frequently eat when those are small enough, 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 onlipid-rich food-bodies calledBeltian bodies that are on theAcacia plant.[21]
In theneotropics, the antMyrmelachista schumanni makes its nest in special cavities inDuroia hirsuta. Plants in the vicinity that belong to other species are killed withformic acid. This selective gardening can be so aggressive that small areas of the rainforest are dominated byDuroia hirsute. These peculiar patches are known by local people as "devil's gardens".[22]
In some of these relationships, the cost of the ant's protection can be quite expensive.Cordia sp. trees in theAmazon rainforest have a kind of partnership withAllomerus 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 ofAllomerus sp. ant lives with theHirtella 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.[22]
The term "species group" can be used to describe the manner in which individual organisms group together. In this non-taxonomic context one can refer to "same-species groups" and "mixed-species groups." While same-species groups are the norm, examples of mixed-species groups abound. For example, zebra (Equus burchelli) and wildebeest (Connochaetes taurinus) can remain in association during periods of long distancemigration across theSerengeti as a strategy for thwarting predators.Cercopithecus mitis andCercopithecus ascanius, species of monkey in theKakamega Forest ofKenya, can stay in close proximity and travel along exactly the same routes through the forest for periods of up to 12 hours. These mixed-species groups cannot be explained by the coincidence of sharing the same habitat. Rather, they are created by the active behavioural choice of at least one of the species in question.[23]
Protocooperation is a form of mutualism, but the cooperating species do not depend on each other for survival. The term, initially used for intraspecific interactions, was popularized byEugene Odum (1953), although it is now rarely used.[24]
Mutualistic symbiosis can sometimes evolve fromparasitism orcommensalism.Symbiogenesis, a leading theory on the evolution ofEukaryotes states the origin of themitochondria andcell nucleus emerged from a parasitic relationship of ancientArchaea andBacteria. Fungi's relationship to plants in the form ofmycelium evolved from parasitism and commensalism. Under certain conditions species of fungi previously in a state of mutualism can turn parasitic on weak or dying plants.[25] Likewise the symbiotic relationship ofclown fish andsea anemones emerged from a commensalist relationship.[26][27][28] Once a mutualistic relationship emerges both symbionts are pushed towardsco-evolution with each other.[29][30]
Mathematical treatments of mutualisms, like the study of mutualisms in general, have lagged behind those forpredation, or predator-prey, consumer-resource, interactions. In models of mutualisms, the terms "type I" and "type II"functional responses refer to the linear and saturating relationships, respectively, between thebenefit provided to an individual of species 1 (dependent variable) and thedensity of species 2 (independent variable).[citation needed]
One of the simplest frameworks for modeling species interactions is theLotka–Volterra equations.[31] In this model, the changes in population densities of the two mutualists are quantified as:
where
= the population density of speciesi.
= the intrinsic growth rate of the population of speciesi.
= the negative effect of within-species crowding on speciesi.
= the beneficial effect of the density of speciesj on speciesi.
Mutualism is in essence thelogistic growth equation modified for mutualistic interaction. The mutualistic interaction term represents the increase in population growth of one species as a result of the presence of greater numbers of another species. As the mutualistic interactive term β is always positive, this simple model may lead to unrealistic unbounded growth.[32] So it may be more realistic to include a further term in the formula, representing a saturation mechanism, to avoid this occurring.
In 1989, David Hamilton Wright modified the above Lotka–Volterra equations by adding a new term,βM/K, to represent a mutualistic relationship.[33] Wright also considered the concept of saturation, which means that with higher densities, there is a decrease in the benefits of further increases of the mutualist population. Without saturation, depending on the size of parameter α, species densities would increase indefinitely. Because that is not 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.[citation needed]
Handling time interactions
In 1959,C. S. Holling performed his classic disc experiment that assumed that
the number of food items captured is proportional to the allottedsearching time; and
that there is ahandling time variable that exists separately from the notion of search time. He then developed an equation for the Type IIfunctional response, which showed that the feeding rate is equivalent to
where
a = the instantaneous discovery rate
x = food item density
TH = handling time
The equation that incorporates Type II functional response and mutualism is:
where
N andM = densities of the two mutualists
r = intrinsic rate of increase ofN
c = coefficient measuring negative intraspecific interaction. This is equivalent to inverse of thecarrying capacity, 1/K, ofN, in thelogistic equation.
a = instantaneous discovery rate
b = coefficient converting encounters withM to new units ofN
or, equivalently,
where
X = 1/aTH
β =b/TH
This model is most effectively applied to free-living species that encounter a number of individuals of the mutualist part in the course of their existences. Wright notes that models of biological mutualism tend to be similar qualitatively, in that the featuredisoclines 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.
The type II functional response is visualized as the graph ofvs.M.
Mutualistic networks made up out of the interaction between plants and pollinators were found to have a similar structure in very different ecosystems on different continents, consisting of entirely different species.[34] The structure of these mutualistic networks may have large consequences for the way in which pollinator communities respond to increasingly harsh conditions and on the community carrying capacity.[35]
Mathematical models that examine the consequences of this network structure for the stability of pollinator communities suggest that the specific way in which plant-pollinator networks are organized minimizes competition between pollinators,[36] reduce the spread of indirect effects and thus enhance ecosystem stability[37] and may even lead to strong indirect facilitation between pollinators when conditions are harsh.[38] This means that pollinator species together can survive under harsh conditions. But it also means that pollinator species collapse simultaneously when conditions pass a critical point.[39] This simultaneous collapse occurs, because pollinator species depend on each other when surviving under difficult conditions.[38]
Such a community-wide collapse, involving many pollinator species, can occur suddenly when increasingly harsh conditions pass a critical point and recovery from such a collapse might not be easy. The improvement in conditions needed for pollinators to recover could be substantially larger than the improvement needed to return to conditions at which the pollinator community collapsed.[38]
Humans are involved in mutualisms with other species: theirgut flora is essential for efficientdigestion.[40] Infestations ofhead licemight have been beneficial for humans by fostering animmune response that helps to reduce the threat ofbody louse borne lethal diseases.[41]
Some relationships between humans anddomesticated animals and plants are to different degrees mutualistic.[citation needed] For example, domesticatedcereals that provide food for humans have lost the ability to spread seeds byshattering, a strategy that wild grains use to spread their seeds.[42]
One researcher has proposed that the key advantageHomo sapiens had overNeanderthals in competing over similar habitats was the former's mutualism with dogs.[44]
Themicrobiota in the human intestine coevolved with the human species, and this relationship is considered to be a mutualism that is beneficial both to the human host and the bacteria in the gut population.[45] Themucous layer of the intestine contains commensal bacteria that producebacteriocins, modify the pH of the intestinal contents, and compete for nutrition to inhibit colonization by pathogens.[46] The gut microbiota, containing trillions ofmicroorganisms, possesses the metabolic capacity to produce and regulate multiple compounds that reach the circulation and act to influence the function of distal organs and systems.[47] Breakdown of the protective mucosal barrier of the gut can contribute to the development ofcolon cancer.[46]
Every generation of every organism needs nutrients – and similar nutrients – more than they need particular defensive characteristics, as the fitness benefit of these vary heavily especially by environment. This may be the reason that hosts are more likely to evolve to become dependent on vertically transmitted bacterial mutualists which provide nutrients than those providing defensive benefits. This pattern is generalized beyond bacteria by Yamada et al. 2015's demonstration that undernourishedDrosophila are heavily dependent on their fungal symbiontIssatchenkiaorientalis for amino acids.[48]
There are many examples of mutualism breakdown. For example, plant lineages inhabiting nutrient-rich environments have evolutionarily abandoned mycorrhizal mutualisms many times independently.[52] Evolutionarily, headlice may have been mutualistic as they allow for early immunity to various body-louse borne disease; however, as these diseases became eradicated, the relationship has become less mutualistic and more parasitic.[50]
Measuring the exactfitness 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 asobligate andfacultative. 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).[10]
^Jordano, P. (2000). "Fruits and frugivory". In Fenner, M. (ed.).Seeds: the ecology of regeneration in plant communities (2nd ed.). CABI. pp. 125–166.ISBN978-0-85199-947-0.OCLC228171397.
^abcOllerton, J. 2006. "Biological Barter": Interactions 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.
^See alsoAttractant related to synomone; and references therein
^Tan, K.H.; Nishida, R. (2000). "Mutual reproductive benefits between a wild orchid,Bulbophyllum patens, and Bactrocera fruit flies via a floral synomone".Journal of Chemical Ecology.26 (2):533–546.Bibcode:2000JCEco..26..533T.doi:10.1023/A:1005477926244.
^Bastolla, U.; Fortuna, M. A.; Pascual-García, A.; Ferrera, A.; Luque, B.; Bascompte, J. (2009). "The architecture of mutualistic networks minimizes competition and increases biodiversity".Nature.458 (7241):1018–20.Bibcode:2009Natur.458.1018B.doi:10.1038/nature07950.PMID19396144.S2CID4395634.
^Mount Pleasant, Jane (2006). "The science behind the Three Sisters mound system: An agronomic assessment of an indigenous agricultural system in the northeast". In Staller, John E.; Tykot, Robert H.; Benz, Bruce F. (eds.).Histories of Maize: Multidisciplinary Approaches to the Prehistory, Linguistics, Biogeography, Domestication, and Evolution of Maize. Left Coast Press. pp. 529–537.ISBN978-1-59874-782-9.OCLC785782398.
Bastolla, U.; Fortuna, M. A.; Pascual-García, A.; Ferrera, A.; Luque, B.; Bascompte, J. (2009). "The architecture of mutualistic networks minimizes competition and increases biodiversity".Nature.458 (7241):1018–20.Bibcode:2009Natur.458.1018B.doi:10.1038/nature07950.PMID19396144.S2CID4395634.
DeVries, PJ; Baker, I (1989). "Butterfly exploitation of an ant-plant mutualism: Adding insult of herbivory".Journal of the New York Entomological Society.97 (3):332–340.
Jahn, G.C.; Beardsley, J.W. (2000). "Interactions of ants (Hymenoptera: Formicidae) and mealybugs (Homoptera: Pseudococcidae) on pineapple".Proceedings of the Hawaiian Entomological Society.34:181–5.hdl:10125/8370.
Ollerton, J. (2006). ""Biological Barter": Patterns of Specialization Compared across Different Mutualisms". In Waser, N.M.; Ollerton, J. (eds.).Plant-Pollinator Interactions: From Specialization to Generalization. University of Chicago Press. pp. 411–435.ISBN978-0-226-87400-5.OCLC1109234356.
