Mutualism (biology)
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 are:
- the nutrient exchange between vascular plants and mycorrhizal fungi,
- the fertilization of flowering plants by pollinators,
- the ways plants use fruits and edible seeds to encourage animal aid in seed dispersal, and
- the way corals become photosynthetic with the help of the microorganism zooxanthellae.
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 and symbiosis. Cooperation most commonly refers to increases in fitness through within-species interactions, although it has been used 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 mutualism and symbiosis, they have been largely used interchangeably in the past, and confusion on their use has persisted.
Mutualism plays a key part in ecology and evolution. 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.
- estimates of tropical rainforest plants with seed dispersal mutualisms with animals range at least from 70% to 93.5%. In addition, mutualism is thought to have driven the evolution of much of the biological diversity we see, such as flower forms and co-evolution between groups of species.
Mutualism has also been linked to major evolutionary events, such as the evolution of the eukaryotic cell and the colonization of land by plants in association with mycorrhizal fungi.
Types
Resource-resource relationships
Mutualistic relationships can be thought of as a form of "biological barter" in 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 in return for energy-containing carbohydrates. Metabolite exchange between multiple mutualistic species of bacteria has also been observed in a process known as cross-feeding.Service-resource relationships
Service-resource relationships are common. Three important types are pollination, cleaning symbiosis, and zoochory.In pollination, a plant trades food resources in the form of nectar or pollen for the service of pollen dispersal. However, daciniphilous Bulbophyllum orchid species trade sex pheromone precursor or booster components via floral synomones/attractants in a true mutualistic interactions with males of Dacini fruit flies.
Phagophiles feed on ectoparasites, thereby providing anti-pest service, as in cleaning symbiosis.
Elacatinus and Gobiosoma, genera of gobies, feed on ectoparasites of their clients while cleaning them.
Zoochory is the dispersal of the seeds of plants by animals. This is similar to pollination in that the plant produces food resources for animals that disperse the seeds. Plants may advertise these resources using colour and a variety of other fruit characteristics, e.g., scent. Fruit of the aardvark cucumber is buried so deeply that the plant is solely reliant upon the aardvark's keen sense of smell to detect its ripened fruit, extract, consume and then scatter its seeds; C. humifructuss geographical range is thus restricted to that of the aardvark.
Another type is ant protection of aphids, where the aphids trade sugar-rich honeydew in return for defense against predators such as ladybugs.
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 anemone fish in the family Pomacentridae: the anemones provide the fish with protection from predators 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 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 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 hirsuta. 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".
In some of these relationships, the cost of the ant's protection can be quite expensive. Cordia sp. trees in the Amazon 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 and wildebeest 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 of Kenya, 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.
Protocooperation
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 by Eugene Odum, although it is now rarely used.Evolution
Mutualistic symbiosis can sometimes evolve from parasitism or commensalism. Symbiogenesis, a leading theory on the evolution of Eukaryotes states the origin of the mitochondria and cell nucleus emerged from a parasitic relationship of ancient Archaea and Bacteria. Fungi's relationship to plants in the form of mycelium 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. Likewise the symbiotic relationship of clown fish and sea anemones emerged from a commensalist relationship. Once a mutualistic relationship emerges both symbionts are pushed towards co-evolution with each other.Mathematical modeling
Mathematical treatments of mutualisms, like the study of mutualisms in general, have lagged behind those for predation, 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 the benefit provided to an individual of species 1 and the density of species 2.Type I functional response
One of the simplest frameworks for modeling species interactions is the Lotka–Volterra equations. In this model, the changes in population densities of the two mutualists are quantified as:where
- = the population density of species i.
- = the intrinsic growth rate of the population of species i.
- = the negative effect of within-species crowding on species i.
- = the beneficial effect of the density of species j on species i.