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Symbiotic relationships are a special type of interaction between species.

Sometimes beneficial, sometimes harmful, these relationships are essential to many organisms and ecosystems, and they provide a balance that can only be achieved by working together.

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Have you ever heard the phrase, ‘I’ll scratch your back if you scratch mine’? This idea of helping someone to get some help in return is the essence of a symbiotic relationship.Symbiosis describes close interactions between two or more different species. It is different from regular interactions between species, because in a symbiotic relationship, the two species in the relationship live together.

Many organisms are involved in symbiotic relationships because this interaction provides benefits to both species. However, there are types of symbiosis that are not beneficial and may in fact harm one or both of the species.Symbiotic relationships can be obligate or facultative.

Obligate symbiosis is when two organisms are in a symbiotic relationship because they can’t survive without each other. Facultative symbiosis is when the species live together by choice. There are four main types of symbiotic relationships: mutualism, commensalism, parasitism and competition.


Mutualism occurs when both species benefit from the interaction. Because mutualism is beneficial to both species involved, there are a wide variety of mutualistic interactions, and these are most common in nature.For example, there may be a nutritional benefit to be gained from the symbiosis, such as with lichen.

Lichen is made up of both algae and fungi, and together they provide each other with food and structure. This type of symbiosis is both obligate and mutualistic.Pollination symbiosis is another example of an obligate, mutualistic symbiosis. Pollinators, such as bees and birds, receive nectar from plants while transporting pollen that the plants need for fertilization.Cleaning symbiosis is a facultative mutualistic symbiosis. In this case, one organism cleans parasites off another organism’s body, which in turn provides a source of food. This can sometimes lead to transport symbiosis, since the first organism provides not only food but transportation for the second organism.

Defense symbiosis is another mutualistic symbiosis. A good example of this is the relationship between clownfish and sea anemones. The anemones have stinging tentacles, but the clownfish are not affected by them. Because of this, clownfish can find safety from predators in the anemone, and in return they protect the anemone from its predators.


The second main type of symbiosis is commensalism. This is when one species benefits and the other does not gain or lose anything.

A good example of this is cattle and cattle egrets. Cattle egrets are birds that are often seen in cattle pastures. They live with the cattle because as the cattle walk around they stir up insects, which the birds can eat. The cattle do not benefit from the egrets, but they are not harmed either.


The third main type of symbiosis is parasitism. This is when one species benefits from the interaction and the other species is harmed. A parasite is an organism that lives on or within a host species. The host provides food and shelter to the parasite, but at a cost to the host itself.

Examples of parasites include tapeworms, ticks, leeches and lice.


The fourth main type of symbiosis is competition, which is when one or both individuals are harmed. Sometimes competition is between members of the same species, such as two males competing for a mate.

Competition can also be between two different species if they are competing for the same type of food and shelter.

Lesson Summary

Symbiosis describes close interactions between two or more different species. There are four main types of symbiotic relationships: mutualism, commensalism, parasitism and competition. Symbiotic relationships are important because they are a major driving force of evolution. This networking and cooperation among species allows them to survive better than they would as individuals. In fact, these relationships are so important that many species have evolved to be entirely dependent on their symbiotic partner.

Without them, life couldn’t go on!

Learning Outcomes

When you are done with this lesson, you should be able to:

  • Characterize a symbiotic relationship
  • State the two categories of symbiotic relationships
  • Discuss the four main types of symbiotic relationships

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