Symbiotic Relationships: Mutualism, Commensalism & Parasitism

Lesson Transcript
Joshua Anderson
Expert Contributor
Amanda Robb

Amanda has taught high school science for over 10 years. They have a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. They also are certified in secondary special education, biology, and physics in Massachusetts.

Understand how different species can exist in symbiotic relationships. Learn about the different types of symbiotic relationships: mutualism, parasitism, commensalism, and amensalism. Updated: 08/18/2021


The word symbiosis literally means 'living together,' but when we use the word symbiosis in biology, what we're really talking about is a close, long-term interaction between two different species. There are many different types of symbiotic relationships that occur in nature.

In many cases, both species benefit from the interaction. This type of symbiosis is called mutualism. An example of mutualism is the relationship between bullhorn acacia trees and certain species of ants. Each bullhorn acacia tree is home to a colony of stinging ants. True to its name, the tree has very large thorns that look like bull's horns. The ants hollow out the thorns and use them as shelter. In addition to providing shelter, the acacia tree also provides the ants with two food sources. One food source is a very sweet nectar that oozes from the tree at specialized structures called nectaries. The second food source is in the form of food nodules called Beltian bodies that grow on the tips of the leaves. Between the nectar and the Beltian bodies, the ants have all of the food they need.

So, the ants get food and shelter, but what does the tree get? Quite a lot actually; you see, the ants are very territorial and aggressive. They will attack anything and everything that touches the tree - from grasshoppers and caterpillars to deer and humans. They will even climb onto neighboring trees that touch their tree and kill the whole branch and clear all vegetation in a perimeter around their tree's trunk, as well. The ants protect the tree from herbivores and remove competing vegetation, so the acacia gains a big advantage from the relationship. In this case, the acacia is considered a host because it is the larger organism in a symbiotic relationship upon or inside of which the smaller organism lives, and the ant is considered to be a symbiont, which is the term for the smaller organism in a symbiotic relationship that lives in or on the host.

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  • 0:11 Symbiosis
  • 2:05 Microorganisms and Mutualism
  • 3:00 Parasitism
  • 4:15 Commensalism
  • 5:28 Amensalism
  • 7:04 Lesson Review
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Microorganisms and Mutualism

Both good and bad bacteria exist in the large intestine.
Bacteria in Large Intestines

An astounding number of mutualistic relationships occur between multicellular organisms and microorganisms. Termites are only able to eat wood because they have mutualistic protozoans and bacteria in their gut that helps them digest cellulose. Inside our own bodies, there are hundreds of different types of bacteria that live just in our large intestine. Most of these are uncharacterized, but we do know a lot about E. coli, which is one of the normal bacteria found in all human large intestines. Humans provide E. coli with food and a place to live. In return, the E. coli produce vitamin K and make it harder for pathogenic bacteria to establish themselves in our large intestine. Whether or not most of the other species of bacteria found in our digestive tract aid in digestion, absorption, or vitamin production isn't completely known, but they all make it harder for invasive pathogens to establish a foothold inside us and cause disease.


Now, let's say by some chance, a pathogenic bacteria does manage to establish itself in a person's large intestine. The host provides a habitat and food for the bacteria, but in return, the bacteria cause disease in the host. This is an example of parasitism or an association between two different species where the symbiont benefits and the host is harmed. Not all parasites have to cause disease. Lice, ticks, fleas, and leeches are all examples of parasites that don't usually cause disease directly, but they do suck blood from their host, and that is causing some harm, not to mention discomfort to their host. Parasites can also act as vectors or organisms that transmit disease-causing pathogens to other species of animals. The bacteria that cause the bubonic plague are carried by rodents, such as rats. The plague bacteria then infect fleas that bite the rats. Infected fleas transmit the bacteria to other animals they bite, including humans. In this case, both the flea and the bacteria are parasites, and the flea is also a vector that transmits the disease-causing bacteria from the rat to the person.


Commensalism is an association between two different species where one species enjoys a benefit, and the other is not significantly affected. Commensalism is sometimes hard to prove because in any symbiotic relationship, the likelihood that a very closely associated organism has no effect whatsoever on the other organism is pretty unlikely. But, there are a few examples where commensalism does appear to exist. For example, the cattle egret follows cattle, water buffalo, and other large herbivores as they graze. The herbivores flush insects from the vegetation as they move, and the egrets catch and eat the insects when they leave the safety of the vegetation. In this relationship the egret benefits greatly, but there is no apparent effect on the herbivore.

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Additional Activities

An Account of a Relationship

Relationships are tricky. Whether it be between two humans or organisms engaged in symbiotic relationships, depending on another living creature is hard. One way for humans to manage their feelings in relationships is to journal about how they are feeling. In this activity, you're going to apply the therapeutic concept of journaling to two organisms engaged in symbiosis. You should research a symbiotic relationship you are interested in, whether it be mutualism, commensalism or parasitism, and find a specific example. Then, write at least three journal entries from each organism in a way that describes some of the benefits and struggles that each organism experiences.

Some examples of experiences you should include in your journal entries:

  • What are some details about the day to day life of these organisms?
  • Where do these organisms live?
  • What is it like to depend on another living thing?
  • What is the organism getting from the relationship?
  • Is one organism being harmed from the relationship?
  • What feelings would come up if these organisms were human and engaged in this type of relationship?

By the end of the activity, you should have six complete journal entries, three from each organism in the relationship,, that reveal scientific details about the relationship and apply human emotions to the experience.

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