Commensalism, Mutualism and Parasitism

Reviewed by: BD Editors

Symbiosis describes several types of living arrangements between different species of organisms in an ecosystem. These relationships can be beneficial, neutral, or harmful to one or both organisms which are called symbionts. In the complex web of nature, species often have several symbiotic relationship at a time.

Symbiosis can take two forms known as obligatory and facultative. In obligatory symbiosis, one or both organisms are entirely dependent on the relationship and will die without it. Conversely, organisms in facultative relationships can live independently from each other.

Symbiotic relationships are also described by the physical relationship between the symbionts. Conjunctive symbiosis occurs when the symbionts have bodily contact with each other. In contrast, symbionts that do not have physical contact have a disjunctive symbiotic relationship. The term ectosymbiosis is when one organism lives on another, like a flea living in a dog’s fur. Endosymbiosis is a relationship where one symbiont lives in the tissues of another such as bacteria living in the human gut.

Commensalism, mutualism, and parasitism are the three main categories of symbiosis found in nature.


In a commensal relationship, one species benefits and there is a neutral effect on the other—it neither benefits nor is harmed. An example of this relationship is birds building nests in trees. The nests don’t interfere with photosynthesis and are light weight, so they don’t put a strain on the trees. The birds, on the other hand, benefit by having their young protected from predators on the ground and hidden by the leaves and branches of the tree. The tree may also provide an accessible food source for the birds such as berries, grubs, and insects. Other examples of commensalism are spiders spinning webs on plants and hermit crabs that use discarded snail shells to protect themselves.

Commensal relationships are sometimes hard to identify because it can be difficult proving that one symbiont does not benefit in some way from the relationship.


In this type of symbiosis, both organisms benefit from the relationship. A classic example of this is the relationship between termites and the protists that live in their gut. The protists digest the cellulose contained in the wood, releasing nutrients for the benefit of the termite. In turn, the protists receive a steady supply of food and live in a protected environment. The protists themselves also have a symbiotic relationship with the bacteria that live in their gut, without which they could not digest cellulose. This relationship between termites and protists is obligatory—the termites would die of starvation without the protists to digest their food.

Other examples of mutualism are the algae that live in the tissues of coral in reefs, clownfish that live in the tentacles of sea anemones, and the relationship between the Oxpecker bird and zebras and rhinoceroses on the African plains.


Parasitism is a relationship where one symbiont benefits (the parasite) and the other (the host) is harmed in some way and may eventually die. Parasites can damage their hosts or sicken them and make them weak. There is usually a built-in selection process that slows down the rate of damage to the host, giving the parasite time to complete its reproductive cycle and for its offspring to find a new host.

A tapeworm in the digestive tract of a human or other animal is an example of a parasitic relationship. The worm feeds on the food the person eats and grows within the intestines, sometimes reaching 50 feet in length. Other examples are the malaria parasite spread by mosquitoes, fleas and ticks, and aphids that suck the sap from plants.


  • Nelson, D. (2018, February 6). Mutualism, Commensalism, Parasitism: Types of Symbiosis with Examples. Retrieved May 23, 2018, from
  • Symbiosis. (2018, May 9). In Wikipedia. Retrieved from

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MLAAPAChicago Editors. "Commensalism, Mutualism and Parasitism." Biology Dictionary,, 17 Sep. 2018, Editors. (2018, September 17). Commensalism, Mutualism and Parasitism. Retrieved from Editors. "Commensalism, Mutualism and Parasitism." Biology Dictionary., September 17, 2018.

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