Symbiotic Relationships: Mutualism, Commensalism & Parasitism

  • 0:11 Symbiosis
  • 2:05 Microorganisms and Mutualism
  • 3:00 Parasitism
  • 4:15 Commensalism
  • 5:28 Amensalism
  • 7:04 Lesson Review
Create An Account
To Start This Course Today
Used by over 10 million students worldwide
Create An Account
Try it free for 5 days
Lesson Transcript
Instructor: Joshua Anderson
If your cat or dog has ever had fleas, you've witnessed symbiosis in action. In this lesson, learn the many types of symbiosis in biology, and how these relationships can have a positive, negative, or neutral effect on the individual species.

Symbiosis

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 ahost 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.

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.

Parasitism

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

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.

Some biologists maintain that algae and barnacles growing on turtles and whales have a commesalistic relationship with their hosts. Others maintain that the presence of hitchhikers causes drag on the host as it moves through the water and therefore the host is being harmed, albeit slightly. In either case, it is unlikely that the fitness of the host is really affected by the hitchhikers, so commensalism is probably the best way to describe these relationships as well.

Amensalism

Microscopic image of penicillium mold
Amensalism Penicillium Slide

To unlock this lesson you must be a Study.com Member.
Create your account

Unlock Your Education

See for yourself why 10 million people use Study.com

Become a Study.com member and start learning now.
Become a Member

Already a member? Log In

Earning College Credit

Did you know… We have over 100 college courses that prepare you to earn credit by exam that is accepted by over 2,900 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Welcome to a video lesson! You now have full access to our video lessons, watch this video now if you are ready or keep exploring the other features you have available to you.
You've watched a video! Now you are officially smarter, check out the next video or take the quiz to keep learning.
You took a quiz! Getting a perfect score on a quiz is how you gain course progress. If you aced it, great job! If not, don't worry, you can try again.