A mutualism is a form of interaction between organisms of two or more species where all the species benefit from the interaction.
- The benefits from the interaction can be protection, pollination, dispersal, or provision of nutrients.
- In most cases, mutualism exists between a pair of species, but some interactions might exist between more than two species.
- Mutualism can be obligate in some species where the species are dependent on the interaction for their survival. In others, however, mutualism provides benefits, but the species are not dependent on the interaction for survival; this is known as facultative mutualism.
- Mutualism can also be species-specific where the interaction is exclusive between two species, but some interactions are diffuse and involve multiple interactions between different species.
- The number of species involved in the interactions depends on how the species benefit from the interaction.
- Mutualistic interactions mostly exist between species that have widely functional and living requirements.
- The term mutualism was used initially by Pierre-Joseph van Beneden in 1876 in his book Animal Parasites and Messmates to indicate the meaning ‘mutual and among species’.
- Mutualism is an important interaction in ecology as well as evolution. Currently, it is assumed that about 80% of land species in a terrestrial ecosystem depend on mutualistic interactions with fungi for nutrients.
- The term of interaction between the species in mutualism can be short-termed or longer depending on the nature of the interaction and the species involved.
- The smaller member of the interaction is termed the mutualist, whereas the larger species is called the host.
- Mutualism involves provisioning a resource by one species and receiving the resource by the other species. During the interaction, providing the resource is costly, whereas receiving the resource is beneficial.
- In order to maintain and evolve mutualism, the benefits must outweigh the cost. If the cost outweighs the benefits, mutualism breaks down and moves to a parasitic association.
What is Symbiosis?
- Symbiosis is a closed and prolonged interaction between two organisms of different species that benefit one or both of the species.
- Symbiosis has been traditionally confused with mutualism, but symbiosis differs in that the species exist in close association. Symbiosis is a broader category that consists of interactions like mutualism, commensalism, and parasitism.
- Symbiosis is of two types depending on the proximity of the involved species. Endosymbiosis is the interaction where one of the species is present within the body of the other. Ectosymbiosis is the relationship where the species are present in close proximity but not within each other.
- Symbiosis, like most ecological interactions, is recognized as one of the essential forces behind evolution.
- In some species, symbiosis is essential for the survival of the species, which is called an obligate symbiotic interaction. In others, it is facultative, and the species can live on their own without the interactions.
Types of Mutualism
Mutualism is of different types depending on various factors. Two important factors resulting in different mutualistic systems are the number of species and the degree of obligateness of the interaction. The following are some of the types of mutualism;
1. Obligate Mutualism
- Obligate mutualism is the interaction between different species where the interaction is essential for their survival, and thus the species are obligated or forced to depend on each other.
- Most of the symbiotic, as well as non-symbiotic interactions, occur through obligate mutualism.
- Obligate mutualism is also termed exclusive mutualism as the interactions are very specific, and the absence of the interaction results in the death of one or both species.
- The benefits in obligate mutualism are usually more than those in facultative mutualism.
- Lichens are the most important example of obligate mutualism consisting of a relationship between a fungus and algae.
- The interaction allows both of the species to survive in conditions where they would not be able to survive otherwise.
- The algae provide nutrients to the fungus by producing organic matter by the process of photosynthesis. The fungus, in turn, protects the algae from the environment with the help of its filaments. The fungus also gathers moisture and nutrients from the surrounding environment.
- Both the fungus and the alga cannot exist in such environmental conditions unless present in the obligate mutualistic interaction.
2. Facultative Mutualism
- Facultative mutualism is the interaction between two or more species where the species benefit from the interaction but can also exist independently of each other.
- The benefits obtained from facultative mutualism are less than that of obligate mutualism.
- Facultative mutualism is not as specific as obligate mutualism and can thus exist between a variety of species.
- Facultative mutualism can be described in one of the three ways; resource-resource mutualism, service-resource mutualism, and service-service mutualism.
- In resource-resource mutualism, both of the species provide a resource to the other species. In service-resource mutualism, one of the species provides a service whereas the other provides a resource. In service-service mutualism, both the species provide service to the other species.
Example- Honeybee and flowering plants
- The mutualism between honeybee and flowering plants represents a service-resource type of facultative mutualism.
- In the interaction, the flowers of the plant provide nectar to the bee, which acts as a source of nutrients for the bee.
- The bees, in turn, provide a service of transferring pollen grains from one flower to another to aid the process of fertilization.
- It is an example of facultative mutualism as the species can exist independently, but the interaction provides benefits to each other.
3. Trophic Mutualism
- Trophic mutualism is a type of ecological interaction that involves the transfer of energy and nutrients between two species.
- Trophic mutualism involves the resource-resource interaction where the species provide resources to one another in the form of nutrients and energy.
- It exists between species of two distinct trophic levels like autotrophs and heterotrophs. Trophic mutualism can be both obligate and facultative.
- Mycorrhiza is the interaction between fungi and the roots of plants where the plants provide carbon to the fungi, and the fungi provide nutrients. Mycorrhizas also improve water uptake and resistance against pathogens.
- The plant is an autotroph that produces sugar by photosynthesis whereas the fungi are heterotrophy that depends on green plants for nutrients.
- The fungi provide phosphorus as well as water to the plant as a part of the interaction.
4. Defensive Mutualism
- Defensive mutualism is a type of service-resource relationship where one of the species provides nutrients whereas the other provides protection against predators or parasites.
- In simple words, defensive mutualism can be explained as a species defending the other for a reward.
- Most of the defensive mutualism exists between insects and plants, especially fast-growing plants with a continuous light source.
- Defensive mutualism is most obligate, but some of the plant-fungi interactions are facultative.
Example- Acacia and Pseudomyrmex ants
- Acacia plants produce newly enlarged thorns (also called bull’s horns) that contain loosely arranged pith.
- The ants can easily chew and remove the pith to create a hollow interior. The small hole in the thorn is used by the ant as a home.
- The plants also provide extra-floral nectar to the ants from the glands at the base of leaves.
- The ants protect the plants against various predators as well as parasites as a part of the interaction.
5. Dispersive Mutualism
- Dispersive mutualism is the interaction between insects or animals and plants where animals acquire nectar from the flower while facilitating the transfer of pollen grains.
- Dispersive mutualism is a service-resource mutualism that enables the dispersal of the plant to new areas.
- Some animals help in the dispersal of seeds to suitable habitats in exchange for nutrients from the fruit.
- The relationship between the plant and pollinator is highly specific and usually exists between the same two species.
Example- Ficus and fig wasps
- The flowers of Ficus are present on the inside of the plant. The female wasps enter the fig flowers and carry the pollen grains to other flowers.
- The wasp lays eggs on the part of the flower which is nourished by the galls present in the flower.
- Thus, the plant helps in the larval development of the wasp while the insect enables pollination of the plant.
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