In biology, mutualism is defined as an ecological relationship between two or more species in which both members benefit. It is a form of symbiosis that organisms develop for any of a number of reasons, including a need for protection, nutrition, shelter, or reproduction.
Types of Mutualism
The two main types of mutualism are obligate mutualism and facultative mutualism.
In obligate mutualism each species depends on the other for its very survival. A lichen is a good example of an obligate mutualistic relationship. A lichen is a symbiotic relationship between fungi and photosynthetic algae or bacteria that lets the participants survive in extreme environments. The fungus needs the photosynthetic species to meet its nutritional needs. The algae or bacteria need the fungus for protection, food, water, and stability.
In facultative mutualism both species benefit from the relationship, but can live without it. For example, humans and cats enjoy a facultative mutualistic relationship. People gain companionship, pest control, and some health benefits from having a cat. But, people can live without cats. House cats, for their part, gain protection, shelter, food, and companionship. Yet, cats (as a species) can live without humans.
Not a Permanent Condition
Symbiotic relationships often change. A mutualistic relationship can transition into a commensal or parasitic relationship. A good example is the relationship between gut bacteria and humans. The bacteria gain a home and food source from the human, while the human gains pathogen protection and nutrients from the bacteria. But, overgrowth of the bacteria can cause disease. Another example is the relationship between certain annelid worms and crayfish. Normally, the worms clean the crayfish gills. This relationship is mutualistic, as the worms get food and the crayfish get health benefits. In times of food scarcity, the worms feed directly on the crayfish gills and the relationship becomes parasitic.
Mutualism is a common form of symbiosis, so there are many examples of these relationships. Here are 10 examples of mutualism:
- Bees and Flowers: The relationship between bees and flowers is an example of obligate mutualism. Many flowering plants can’t reproduce without insect pollinators. Bees are important pollinators that benefit because they feed on nectar and use the pollen mainly to feed their larvae.
- Ants and Aphids: Ants and aphids share a facultative mutualistic relationship. Some ant species herd aphids. The ants farm aphids to eat their honeydew. The aphids gain shelter and protection from predators.
- Woolly Bat and Pitcher Plant: The carnivorous pitcher plant isn’t a great place for most animals, but the woolly bat seeks shelter inside it. The bat gains a safe resting place, while the pitcher plant gets nutrients from bat feces.
- Lichens: A lichen consists of both fungi and photosynthetic algae or bacteria. Photosynthesis helps the fungi get enough nutrients. Meanwhile, the fungi lets the algae or bacteria live in a drier environment than it could otherwise tolerate and offers support and nutrients.
- Humans and Bacteria: Humans have mutualistic relationships with several types of bacteria. Beneficial bacteria live on the skin and in the gut. The bacteria gain food, shelter, and optimal environmental conditions. The human gains protection from pathogens and nutrients.
- Oxpeckers and Grazing Animals: The oxpecker is a bird that eats ticks and other parasites on zebras, rhinos, and other grazing animals. The oxpecker gets food, while the grazing animals get pest control. The oxpecker also screams a warning when a predator is near that protects the herd.
- Shark and Remora: The remora is a fish that attaches to sharks, whales, and other large aquatic fish and mammals. The remora eats parasites that attach to the larger animal and gains a food source and protection. The shark enjoys health benefits.
- Coral and Algae: Coral is a colonial animal that collects a type of algae called zooxanthellae. The coral provides shelter and nutrients for the algae. The photosynthetic algae supplies oxygen and sugars that benefit the coral.
- Legumes and Nitrogen-Fixing Bacteria: Nitrogen-fixing bacteria live in legume plant root hairs. The bacteria convert nitrogen into ammonia, which the plant needs for nutrition. The bacteria get nutrients from the plant and a place to live and grow.
- Clownfish and Anemone: The clownfish and anemone share a facultative mutualistic relationship. Each species can survive without the other. The stinging cells of the anemone protect the clownfish from predators and some parasites. The anemone gains food from clownfish feces.
Commensalism is another common form of symbiosis.
Mutualism vs Symbiosis
All mutualistic relationships are symbiotic; not all symbiotic relationships are mutualistic.
Mutualism and symbiosis are often confused. Mutualism is one form of symbiosis. Symbiosis is when two or more organisms live in close contact with one another. Other types of symbiosis are commensalism, amenalism, and parasitism. In commensalism, one species benefits, while the other neither benefits nor is harmed. In amenalism, one species suffers harm, while the other is unaffected. Parasitism is a form of symbiosis where one species benefits and the other is harmed.
Mutualism vs Cooperation
Mutualism occurs between different species; cooperation occurs between members of one species.
Another pair of terms that are often confused are mutualism and cooperation. In the modern usage of the word, cooperation refers to beneficial intraspecific interactions, while mutualism refers to interspecific interactions. (In the past, facultative mutualism was also called cooperation.) For example, two people working together for mutual benefit is cooperation. A human and a dog are members of two different species, so their jointly beneficial relationship is an example of mutualism.
- Bronstein, Judith (2015). Mutualism. Oxford University Press. ISBN 9780199675654.
- Boucher, D. H. (ed.) (1985). The Biology of Mutualism : Ecology and Evolution. London: Croom Helm. ISBN 0-7099-3238-3.
- Denison, R.F.; Kiers, E.T. (2004). “Why are most rhizobia beneficial to their plant hosts, rather than parasitic”. Microbes and Infection. 6 (13): 1235–1239. doi:10.1016/j.micinf.2004.08.005
- Douglas, Angela E. (2014). The Symbiotic Habit. United States: Princeton University Press. ISBN 9780691113425.
- García-Algarra, Javier (2014). “Rethinking the logistic approach for population dynamics of mutualistic interactions”. Journal of Theoretical Biology. 363: 332–343. doi:10.1016/j.jtbi.2014.08.039