Mutualism describes the ecological interaction
between two or more species
where each species has a net benefit. Mutualism is a common type of ecological interaction. Prominent examples include most vascular plants engaged in mutualistic interactions with mycorrhizae
, flowering plants being pollinated by animals, vascular plants being dispersed by animals, and coral
s with zooxanthellae
, among many others. 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.
The term ''mutualism'' was introduced by Pierre-Joseph van Beneden
in his 1876 book ''Animal Parasites and Messmates'' to mean "mutual aid among species".
Mutualism is often conflated with two other types of ecological phenomena: 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. Symbiosis
involves two species living in close physical contact over a long period of their existence and may be mutualistic, parasitic, or commensal
, so symbiotic relationships are not always mutualistic, and mutualistic interactions are not always symbiotic. Despite a different definition between mutualistic interactions and symbiosis, mutualistic and symbiosis have been largely used interchangeably in the past, and confusion on their use has persisted.
Mutualism plays a key part in ecology
. For example, mutualistic interactions are vital for terrestrial ecosystem
function as about 80% of land plants species rely on mycorrhizal
relationships with fungi
to provide them with inorganic compounds and trace elements. As another example, the estimate of tropical forest trees with seed dispersal mutualisms with animals ranges from 70–90%.. 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.
Mutualism has also been linked to major evolutionary events, such as the evolution of the eukaryotic cell (Symbiogenesis
) or the colonization of land by plants in association with mycorrhizal fungi.
Despite the importance of mutualism in both evolution and ecology, some authors have argued that mutualism has been less studied than other interspecific interactions such as predation or parasitism, with the main argument being a smaller consacration of ecology and biology textbooks towards mutualism. Other authors have argued that this is not the case when considering the primary research literature.
Mutualistic relationships can be thought of as a form of "biological barter"
associations between plant root
s 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
Service-resource relationships are common. Three important types are pollination, cleaning symbiosis, and zoochory.
, a plant trades food resources in the form of nectar
for the service of pollen dispersal.
s feed (resource) on ectoparasite
s, thereby providing anti-pest service, as in cleaning symbiosis
'' and ''Gobiosoma
'', genera of gobies
, feed on ectoparasites of their clients while cleaning them.
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).
Another type is ant
protection of aphids
, where the aphids trade sugar
(a by-product of their mode of feeding on plant sap
) in return for defense against predator
s such as ladybug
and Ritter's sea anemone
s live in a mutual service-service symbiosis, the fish driving off butterflyfish
and the anemone's tentacles protecting the fish from predators.]]
Strict service-service interactions are very rare, for reasons that are far from clear.
One example is the relationship between sea anemone
s and anemone fish
in the family Pomacentridae
: the anemones provide the fish with protection from predator
s (which cannot tolerate the stings of the anemone's tentacles) and the fish defend the anemones against butterflyfish
), 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 feeds 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
in the genus ''Pseudomyrmex
'' and trees in the genus
'', 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 garden
[Piper, Ross (2007), ''Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals'', Greenwood Press.]
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.
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 distance migration
across the Serengeti
as a strategy for thwarting predators. ''Cercopithecus mitis
'' and ''Cercopithecus ascanius
'', species of monkey in the Kakamega Forest
, 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.
Mathematical treatments of mutualisms, like the study of mutualisms in general, has lagged behind those of predation
, or predator-prey, consumer-resource, interactions. In models of mutualisms, the terms "type I" and "type II" functional response
s refer to the linear and saturating relationships, respectively, between ''benefit'' provided to an individual of species 1 (''y''-axis) on the ''density'' of species 2 (''x''-axis).
Type I functional response
One of the simplest frameworks for modeling species interactions is the Lotka–Volterra equation
[May, R., 1981. Models for Two Interacting Populations. In: May, R.M., Theoretical Ecology. Principles and Applications, 2nd ed. pp. 78–104.]
In this model, the change in population density of the two mutualists is quantified as:
= the population densities.
= the intrinsic growth rate of the population.
= the negative effect of within-species crowding.
= the beneficial effect of a mutualistic partner's density.
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. As the mutualistic term is always positive, it may lead to unrealistic unbounded growth as it happens with the simple model.
So, it is important to include a saturation mechanism to avoid the problem.
Type II functional response
In 1989, David Hamilton Wright modified the Lotka–Volterra equation
s by adding a new term, ''βM''/''K'', to represent a mutualistic relationship.
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.
Handling time interactions
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
* a=the instantaneous discovery rate
* x=food item density
The equation that incorporates Type II functional response and mutualism is:
* ''N'' and ''M''=densities of the two mutualists
* ''r''=intrinsic rate of increase of ''N''
* ''c''=coefficient measuring negative intraspecific interaction. This is equivalent to inverse of the carrying capacity
, 1/''K'', of ''N'', in the logistic equation
* ''a''=instantaneous discovery rate
* ''b''=coefficient converting encounters with ''M'' to new units of ''N''
* ''X''=1/''a'' ''T''H
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 featured isocline
s 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 of
Structure of networks
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. 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.
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, reduce the spread of indirect effects and thus enhance ecosystem stability and may even lead to strong indirect facilitation between pollinators when conditions are harsh.
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.
This simultaneous collapse occurs, because pollinator species depend on each other when surviving under difficult conditions.
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.
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Humans are involved in mutualisms with other species: their [[gut flora]] is essential for efficient [[digestion]].
Infestations of [[head lice]] ''might'' have been beneficial for humans by fostering an [[immune]] response that helps to reduce the threat of body louse
borne lethal diseases.
Some relationships between humans and domesticated
animals and plants are to different degrees mutualistic. For example, agricultural varieties of maize
provide food for humans and 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.
In traditional agriculture
, some plants have mutualist as companion plants
, providing each other with shelter, soil fertility and/or natural pest control
. For example, bean
s may grow up corn
stalks as a trellis, while fixing nitrogen in the soil for the corn, a phenomenon that is used in Three Sisters farming
One researcher has proposed that the key advantage ''Homo sapiens
'' had over Neanderthals
in competing over similar habitats was the former's mutualism with dogs.
Evolution of mutualism
Mutualisms are not static, and can be lost by evolution.
Sachs and Simms (2006) suggest that this can occur via 4 main pathways:
# One mutualist shifts to parasitism, and no longer benefits its partner,
such as headlice
# One partner abandons the mutualism and lives autonomously
# One partner may go extinct
# A partner may be switched to another species
There are many examples of mutualism breakdown. For example, plant lineages inhabiting nutrient-rich environments have evolutionarily abandoned mycorrhizal mutualisms many times independently.
[Wang, B. and Qiu, Y.-L. (2006) 'Phylogenetic distribution and evolution of mycorrhizas in land plants.', Mycorrhiza, 16(5), pp. 299–363.]
Measuring and defining mutualism
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
. 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).
[Ollerton, 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.]
* Arbuscular mycorrhiza
* Ecological facilitation
* Greater honeyguide
– has a mutualism with humans
* Interspecies communication
* Müllerian mimicry
* Mutualisms and conservation
* ''Mutual Aid: A Factor of Evolution
* Bronstein JL. 2001. The costs of mutualism. ''American Zoologist
'' 41 (4): 825-839 S
* 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.
* Thompson, J. N. 2005. ''The Geographic Mosaic of Coevolution''. University of Chicago Press.
* Boucher, D. H. (editor) (1985) ''The Biology of Mutualism : Ecology and Evolution'' London : Croom Helm