Mutualism (biology)
Mutualism describes the ecological interaction between two or more species where each species has a net benefit.[1] Mutualism is a common type of ecological interaction. Prominent examples are:
This article is about the biological term. For the economic theory and other uses, see Mutualism (disambiguation).
Mutualism can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, and with parasitism, 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 term mutualism was introduced by Pierre-Joseph van Beneden in his 1876 book Animal Parasites and Messmates to mean "mutual aid among species".[4][5]
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 (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, 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.[6]
Mutualism plays a key part in ecology and evolution. For example, mutualistic interactions are vital for terrestrial ecosystem function as:
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. [10]
Mutualism has also been linked to major evolutionary events, such as the evolution of the eukaryotic cell (symbiogenesis) and the colonization of land by plants in association with mycorrhizal fungi.
Types[edit]
Resource-resource relationships[edit]
Mutualistic relationships can be thought of as a form of "biological barter"[11] 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 (family Fabaceae) in return for energy-containing carbohydrates.[12] Metabolite exchange between multiple mutualistic species of bacteria has also been observed in a process known as cross-feeding.[13][14]
= 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.
Structure of networks[edit]
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.[28] 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.[29]
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,[30] reduce the spread of indirect effects and thus enhance ecosystem stability[31] and may even lead to strong indirect facilitation between pollinators when conditions are harsh.[32] 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.[33] This simultaneous collapse occurs, because pollinator species depend on each other when surviving under difficult conditions.[32]
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.[32]
Evolution of mutualism[edit]
Evolution by type[edit]
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 undernourished Drosophila are heavily dependent on their fungal symbiont Issatchenkia orientalis for amino acids.[41]
There are many examples of mutualism breakdown. For example, plant lineages inhabiting nutrient-rich environments have evolutionarily abandoned mycorrhizal mutualisms many times independently.[45] 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.[43]
Measuring and defining mutualism[edit]
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).[11]