Mutualism is most commonly defined in a way that reflects the positive signs characterizing the outcome oftheir interactions, that is, as interactions between individuals of different species that benefit both of them. However, mutualism can be more precisely defined as an interaction between individuals of different species that results in positive (beneficial) effects on per capita reproduction and/or survival of the interacting populations. As in other interspecific interactions, the degree of dependency of each mutualist upon the other ranges from obligate to facultative; hence, they can be obligate-obligate, obligate-facultative, or facultative-facultative interactions. Facultative mutualists are ones whose populations persist in the absence of a mutualist, whereas obligate mutualists are ones whose populations go extinct in the absence of a mutualist. In species-specific mutualisms, only a single partner species confers mutualistic benefits, whereas in generalized mutualisms, an array of species can provide the necessary benefit. For example, a plant that cannot produce seeds in the absence of a single pollinator species is engaged in a species-specific, obligate mutualism, while a plant that can self-pollinate to some extent and that can be pollinated by multiple flower-visitors is involved in a facultative, generalized mutualism.
The term mutualism is not synonymous with symbiosis, cooperation, or facilitation, although ecological and evolutionary parallels do occur among these forms of interaction. The term symbiosis identifies an intimate, close association between species in which the large majority or entire life cycle of one species occurs within or in very close association with another. Often, one species (the symbiont) is not free-living, but inhabits the body of another species (the host). A mutualism can also be a symbiosis, and many symbioses are also mutualistic, but not all symbioses are mutualisms and not all mutualisms are symbioses. Interactions between algae and fungi that comprise lichens and between termites and the protozoa that inhabit their digestive systems are examples of mutualistic symbioses. In contrast, plant-pollinator mutualisms are not symbiotic, as both partner species are free-living. Other symbioses are parasitic rather than mutualistic, including, for example, interactions between humans and protozoa that cause malaria.
While mutualism is an interspecific interaction, the term cooperation is generally used to describe mutually beneficial interactions between individuals of the same species, often involving social interactions. Examples of species in which cooperation is an important feature include naked mole rats and honeybees and other social insects. Finally, facilitation differs from mutualism in that, while it does involve positive feedback, it is not necessarily an interspecific interaction. Facilitation typically refers to the modification of some component of the abiotic or biotic environment by one species that then enhances colonization, recruitment, and establishment of another species, such as occurs during succession.
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