Mutualism is a type of ecological interaction between two or more species in which each species benefits from the relationship, such as with regard to food, shelter or protection.
It is one of three distinct types of symbiotic relationships that describe how species interact with one another, along with parasitism (in which one species benefits while the other is harmed) and commensalism (in which one species benefits while the other is neither helped nor harmed). As with these other types, mutualism is fundamental to many ecological processes and contributes greatly to biodiversity and ecosystem health.
Mutualistic relationships can be classified by the type of benefits exchanged and the degree of dependency between the partners. However, many overlap in type and can simultaneously exhibit characteristics of multiple types. Moreover, many are not static and can evolve or break down over time due to, for example, changes in environmental conditions.
In obligate mutualism, both species are completely dependent on each other for survival. This can enable life in extreme and otherwise uninhabitable environments, as exemplified by lichens, in which fungi protect the algae from harsh environmental conditions and collect moisture and nutrients, while the algae produce organic matter via photosynthesis. Likewise, fig plants depend entirely on the fig wasp for pollination, while the fig wasp requires the fig plant for reproduction.
In facultative mutualism, each partner can survive independently but both benefit from the association. An example is honeybees visiting various flowers for nectar.
Trophic mutualism involves the exchange of nutrients or energy, which occurs when, for example, cows and the bacteria in their stomachs work together to digest cellulose.
In defensive mutualism, one partner provides protection against predators in exchange for food or shelter. An example is ants protecting aphids in return for the honeydew (a sweet, nutrient-rich food source) that the aphids secrete.
Dispersive mutualism involves the transport of pollen or seeds, such as when bees, birds and bats pollinate flowers while feeding on their nectar. It is fundamental for the reproduction of some plants and the maintenance of their genetic diversity, especially in terrestrial ecosystems.
Multiple mutualism relationships exist within most ecosystems, often creating complex networks. The weakening or loss of even a minor species can trigger large and unpredictable effects on biodiversity and ecosystem health. Some types of mutualism can be particularly vulnerable to environmental changes, often depending on the degree of dependency between partner species, the specificity of the interaction, and the ability of the partners to adapt or shift to new species.
Because mutualistic networks are complex and ever-changing, it is often difficult to fully understand their role in ecosystems. Adding to the difficulty is that many are subtle and difficult to observe and that many occur at a microscopic scale involving bacteria and other microbes.
Humans are highly dependent on mutualistic interactions, including some taking place within their bodies. For example, the human intestine provides a safe and nutrient-rich environment for trillions of individual bacteria, while the bacteria have essential roles in the digestion of food, the production of vitamins and supporting the immune system.