Why symbionts matter

Learning Objectives

  1. Name and define the three main types of interspecific interactions: enemy-victim (predation, parasitism, herbivory), mutualism, competition in terms of costs and benefits to the two organisms that interact.
  2. Be able to give a biological example of each type of interaction.
  3. When given a biological example, be able to name the type of interaction.
  4. Identify the benefits and/or costs to each player in the relationship in biological examples.
  5. Be able to define symbiosis, and recognize when an interspecific interaction is a symbiosis. Furthermore, explain why competition is not a good example of a symbiotic relationship.


  1. Define symbiosis and name examples of types of symbioses, explaining why competition is not a good example of a symbiotic relationship.
  2. Name and recognize biological examples of different enemy-victim interactions (predation, herbivory, parasitism), give an example of parasitism, and explain the benefits and costs to each player in the relationship.
  3. Name and recognize biological examples of different mutually beneficial biological interactions (mutualism), give an example of mutualism, and explain the benefits to each player in the relationship.
  4. Name and recognize biological examples of different mutually detrimental biological interactions (competition), give an example of competition in the human microbiota, and explain the costs to each player in the relationship.

A symbiosis is a close and prolonged interaction or relationship between two different species. The three types of symbioses include mutualism (both species benefit), parasitism (one species benefits, but the other is harmed), and commensalism (one species benefits, and the other is unaffected). Thus the interactions in symbiosis can be positive, neutral, or negative for either species, leading to the complexity of species interactions we discussed in the biodiversity unit.

Today’s reading opens with a SciShow video on human parasites. While Hank Green doesn’t usually make mistakes in his videos, this one has at least three. See you you can spot them while you watch.

Did you catch the errors? We noticed:

  • Bacteria are not animals…they are bacteria. Animals are eukaryotes.
  • The “microbiome” actually refers to the DNA of all the critters that live your gut. The critters themselves are called microbiota.
  • Hank says early on that all the residents inside of us are parasites, but then he goes on to describe mutualists and commensalists.
  • Finally, new research revises the estimated number of microbes in a human down to roughly equivalent to the number of human cells (Sender et al 2016).


Parasitism, Predation, and Herbivory are detrimental to one species while beneficial to the other

Enemy-victim interspecific interactions have a negative impact on one species while providing benefits to the other. The three types are predator-prey interactions, parasite-host interactions, and herbivory. These are all win-lose situations.  Parasitism, but not predatory or herbivory relationships, is a type of symbiosis.

In the parasite-host interaction, also called parasitism, one species (the parasite) lives on or in another species (the host) that it harms over time, sometimes even resulting in the death of the host. Parasitism is a win for parasite and a loss for the host.

Another win-lose scenario is predator-prey interactions, or predation. Predation involves one species (the predator) consuming—yes, eating—the other (they prey), which obviously dies in the exchange. A well-studied example is the lynx-hare interaction in the Canadian arctic.

Canada lynx (left) prey upon snowshoe hare in summer (upper right) and winter coloration (lower right) in the Canadian arctic. (Sources: "Canada lynx by Michael Zahra" by Michael Zahra - http://www.flickr.com/photos/mzahra1/4248818181/sizes/l/in/photostream/; "Lepus americanus 5459 cropped" by Lepus_americanus_5459.JPG: Walter Siegmund (talk)derivative work: Wsiegmund - This file was derived from  Lepus americanus 5459.jpg; "Snowshoe Hare, Shirleys Bay" by D. Gordon E. Robertson - Own work.)

Canada lynx (left) prey upon snowshoe hare in summer (upper right) and winter coloration (lower right) in the Canadian arctic. (Sources: “Canada lynx by Michael Zahra” by Michael Zahra; “Lepus americanus 5459 cropped” by Walter Siegmund; “Snowshoe Hare, Shirleys Bay” by D. Gordon E. Robertson – Own work.)

Strong predation on the hare populations reduces hare numbers, which in turn reduces lynx numbers because they die of starvation. So, short term this is a win-lose, but the longer view shows that the dynamics of eating a living resource can ultimately curb the population growth of the predator as well. As a result, predation is a selective force that results in continual co-evolution of traits like agility and speed in both the lynx and the hare, as well as winter cryptic fur color that hides the the hare from the lynx against the snowy arctic background.

Finally, herbivory is similar to predation in the sense that a grazer eats a plant. However, some grazing interactions can be argued to provide a benefit because grazing can promote the growth of new plant material. To determine if an herbivore-plant interaction is a win-lose, the researcher needs to measure the cost and benefit to the plant.

Competition, where two species use the same limited resource and thus are in conflict with each other over that resource, can be confused as an enemy-victim relationship; however, it doesn’t fit the definition because both species are harmed rather than one benefiting.


Interspecific (between-species) interactions are not static but can evolve

Think about predator-prey or parasite-host interactions, and you’ll recognize that these biological interactions are dynamic, or constantly changing. Sometimes the changes are so dramatic that the interaction shifts from win-lose parasitism to win-win mutualism! For instance, the parasitic bacterium Wolbachia can infect the fruit fly Drosophila. Infected female Drosophila have impaired reproduction. Weeks and colleagues (2007) observed this parasite-host interaction evolve rapidly into a mutualism.

Mutualism is a type of symbiosis where both species benefit in the interspecific interaction. For example, plants have fungi that live on or in their roots, called mycorrhizae. The fungus gains access to sugars like glucose and sucrose from the root system, and the plants use the phosphorus and nitrogen that the fungi “fix,” or process into a biologically usable form. Without this fairly universal mutualism in almost all plants, we would not have crops to eat or as much oxygen to breathe.

Mutualisms are vulnerable to “cheating,” where one pair in the interaction saves time or energy and circumvents the benefit to the other species. For example, pollinating insects transfer pollen (analogous to sperm in plants) from one plant to another as they forage for nectar in the plant’s flowers. Exact matches between the flower structure and the pollinator’s mouth and tongue structure show evidence of strong co-evolution. However, some insects cheat on this interaction, such as the bumblebee in the images below:

Bumblebee biting open the stem of a flower to gain direct access to the nectar…and nectar robbing from the damaged flower. The bee obtains nectar without providing pollinator services. (Source: “Bumble bee feeding” by Pahazzard – Own work. Licensed under CC BY-SA 3.0.

Persistent cheating in a mutualism can select for resistance to cheating, shifting the mutualistic interaction to…parasitism.

One key set of mutualisms are the mammalian gut microbiota, usually referred to as the microbiome. Watch Hank Green at SciShow as he provides a general background on the human microbiome:

Interestingly, while the human hosts the microbiota in a (usually) mutualistic interaction, the different microbes themselves are pitted against each other for the limited resources inside the human. As different microbial species vie against each other for space and for food, they experience competition, a lose-lose interaction that is not a symbiosis because its not usually a close, prolonged interaction. Competitive interactions between two bacterial species in your gut sounds like it should give you indigestion, and sometimes it does. Usually, though, the battles for space and food happen without your knowledge.

Why live inside another organism?

It seems like a risky strategy to live your life inside another organism that will eventually die. Yet all organisms play host to other organisms. Humans carry around about one single-celled symbiont for every one of our own cells. What do those symbionts see in us? Food resources and shelter from a variable temperature and variable humidity environment. Our bodies are 50-75% water, which sounds pretty amazing if you are a single-celled organism who moves by rotating a flagella around, or just an organism that requires a liquid environment to metabolize food and to reproduce.

E. coli bacteria reproduce optimally at 37°C, which is also human body temperature, but not all bacteria are adapted to 37°C. In fact, Thermus aquaticus is a heat-loving bacterium discovered in a Yellowstone geyser basin, and prefers 70°C and cannot even survive in temperatures as low as the 37°C of your average human body. Every organism has a range of conditions in which it can survive and thrive; outside that range it will hibernate or die. For symbionts, at least part of their life cycle depends upon another living organism:

  • Mycorrhizal fungi require the nutrients from plant roots, and the plants require the nitrogen the mycorrhizae make available.
  • Plasmodium is a eukaryotic single-celled parasite that lives part of its life cycle inside human red blood cells, and another part of its life cycle is spent inside mosquitoes. Without mosquitoes or red blood cells, Plasmodium would not survive.

We’ll learn about more examples in class and dive into the co-evolutionary implications of close and prolonged associations between species.



Sender R, Fuchs S, Milo R (2016) Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLOS Biology 14(8): e1002533.https://doi.org/10.1371/journal.pbio.1002533

Weeks et al. 2007. From Parasite to Mutualist: Rapid Evolution of Wolbachia in Natural Populations of Drosophila. PLoS Biol 5(5): e114. doi:10.1371/journal.pbio.0050114