A Crash-Course in Biotic Interactions

Biotic interactions are primary forces driving evolutionary change.  Whether it is organism trying to avoid being eaten, or two individuals competing for the same resource, the many interactions in which an organism partakes over the course of its life can have a huge impact on that organism’s evolutionary success/fitness (i.e. an organism’s ability to reproduce and pass on genetic material).  So how do we as scientists determine the significance of a biotic interaction?

Even as juveniles, predators such as sea stars and crabs compete for food consisting of other organisms (mussels and snails).

Obviously, if an organism is eaten by a predator, that is “game over” for the organism that was eaten.  But what about the predator?  It only ate one meal on one day, which may or may not contribute to its overall fitness.  The significance of the interaction is clearly different for the two organisms participating.  Other, non-lethal interactions can be even more complex.  Plants are an obvious example.  Most of the time, herbivores fail to eat the entire plant, and therefore the plant may still recover and continue to reproduce.  These non-lethal interactions are often of greater interest to biologists, as there are usually individual differences which contribute to the survival of some individuals over others, which can lead to selection of the favourable traits in that interaction over time (e.g. plants and animals with anti-predatory defenses, the bright plumage of many male birds, and even pollinating insects and flowering plants).

The first step in understanding biotic interactions is to understand the nature of the interaction.  Let’s assume that we have an interaction between two individual organisms (which may or may not belong to the same species).  For each individual, there are three possible outcomes of the interaction: 1) positive, 2) negative, or 3) no effect.  Here is a handy chart that is a great teaching tool (from Bronstein et al. 1994):

Fig. 1 Bronstein et al. 1994

From the three types of outcomes, there are several types of interactions that are possible.  Over the next few posts, I’ll dive into some more aspects detail (I am by no means an expert.  It is a huge topic, and each interaction alone could – and has been – the subject of many scientists’ careers).  For now, here is a brief overview of each.

Positive/positive interactions: mutualism
Mutualistic interactions have a positive effect for both organisms involved.  For example, the interaction between a pollinating insect and flowering plant is mutualistic (the insect gets an energy reward in the form of nectar, and the plant receives a reproductive benefit: pollen carried to and from the flower).  Mutualisms can be so extreme that they lead to a symbiotic relationship, such as that of lichen.  Lichen is actually a combination of two organisms: a fungus, and an algae/cyanobacteria.  The fungus provides structural support and protection for the algae/cyanobacteria, and the algae/cyanobacteria provides food (photosynthesis) for the fungus.  Another example would be coral and zooxanthellae in which the coral also provides protection, and the zoxanthellae provide food from photosynthesis.

The drawback of symbiotic relationships is that the two organisms have developed complete dependence on one another, such that if something happens to one organism, the other is also affected.  In the case of coral and zooxanthellae, many coral reefs are experiencing “bleaching” in which adverse water conditions cause the coral to expel the zooxanthellae from their cells, causing the coral to turn white.  The zooxanthellae cannot survive without living within the coral’s tissue, and the coral often dies from lack of food.  As coral reefs are home to major ocean ecosystems, coral bleaching is a huge problem.

Photo credit: http://images.techtimes.com/data/images/full/234535/coral-bleaching.jpg

Positive/negative interactions: Predation, Parasitism, Herbivory
Predation (an animal eating another animal), parasitism (an organism infecting a host), and herbivory (usually an animal eating a plant) are all examples of interactions in which one organism benefits in the form of food (and reproduction in the case of parasites), and the other organism is consumed.  At first glance, such interactions seem simple enough, but are complicated by non-lethal effects.  For example, parasites are more successful when they do not kill their hosts (i.e. the longer the host lives, the longer the parasite benefits from food and/or reproduction).  Additionally, as mentioned earlier, most herbivores fail to kill the entire plant, eating leaves or bark and leaving limbs and roots partially intact.  Even with predation, non-lethal effects are important.  Much of what I and many other palaeontologists study in the fossil record are instances of failed predation where the predator fails to kill the prey (it either escapes, or the predator was interrupted).  In the fossil record, failed predation often leaves traces in the form of scars on hard parts (e.g. bite marks, healing fractures, and repair scars on shells).  See our paper, Molinaro et al. 2014, for an example.

A snail with a huge repair scar caused by shell regrowth after a crab break its shell.

Negative/negative interactions: Competition
At first, competitive interactions might seem like they should be categorized as a positive/negative interaction (there is usually a “winner” and a “loser” in the interaction).  However, even the “winner” experiences a negative effect from the interaction (e.g. two male deer fighting for a mate might injure one another).  Competition may be between different species (interspecific competition), or between individuals of the same species (intraspecific competition).  There are three main types of competition: 1) interference (direct), 2) exploitative (indirect), and 3) apparent competition.  Interference competition usually involves one individual directly trying to gain a resource from another, such a fighting for territory or mates.  Exploitative competition is a bit more subtle, and usually involves a form of resource competition, in which organisms compete for limited space or food (one organism taking food means that there is less food for everyone overall).  Apparent competition is just that: apparent (not real), and is created by an external factor.  For example, if there is a system with one predator and one prey species, and a second prey species is introduced, not only will prey species #1 likely have to compete with prey species #2, but the introduction of a second food source for the predator will increase the predator’s population, resulting in an increased negative effect (predation) on prey species #1, regardless of its interactions with prey species #2 (Holt 1977 is the go-to seminal paper on apparent competition).

Red rock crabs competing for space and food

Neutral/positive interactions: Commensalism
Commensalism is a relationship in which one organism gains a benefit, and the other is unaffected.  This usually occurs when there is a large difference in size between the organisms (the organism receiving the benefit is usually much smaller than the unaffected organism/host).  For example, dust mites receive food and habitats from our bedding and dead skin cells, yet we are basically unaffected.  Other examples are small fish, such as remora, which attach themselves harmlessly to large hosts such as whales and sharks.  The remora then feed off of the food debris from their hosts.

The remora has a modified dorsal fin which it used to attach to sharks like a suction cup. Photo credit: https://kristinabarclay.files.wordpress.com/2016/07/28bfe-remora.jpg

Neutral/negative interactions: Amensalism
Amensalism occurs when one organism is negatively impacted by an interaction with another, unaffected organism.  Like comensalism, amensalism often occurs when the organisms in the interaction are very different in size (e.g. an elephant stepping on an insect or small mammal).  However, amensalism can also occur when one organism secretes a chemical which harms or kills nearby organisms (e.g. organisms which produce chemicals that kill bacteria, such as bread molds that produce penicillin, or the chemicals secreted by some trees which kill nearby grasses or other vegetation).

Black walnuts produce chemicals toxic to nearby plants.  Photo credit: https://seasonsflow.files.wordpress.com/2014/08/0814-1-42.jpg

Neutral/neutral interactions: Neutralism
Neutralism is a recognized term in biology, but it is not often discussed, as it is hard to explain, or even give examples of, an interaction in which both organisms are completely unaffected by an interaction.  It is important to note that neutralism does not mean that there is no interaction.  The best example I can think of would be two organisms bumping into one another, then continuing on their separate ways.  But even this example may indicate that there is potential competition for space.

There are some other ways of classifying biotic interactions based on whether there are any positive or negative outcomes (e.g. antagonism is sometimes used to describe interactions with potentially negative outcomes, such as competition, and predation/parasitism/herbivory, while facilitation is used to describe interactions where one organism is benefited by another, such as mutualism or commensalism).  I personally prefer to use a chart because it is a great teaching tool (put up a blank chart and have students come up with examples for discussion).

As always, such classification systems are merely a human tool for trying to categorize the vast complexity of interactions that exist.  Exceptions and variations always exist.  As is our lab’s mantra, “nature doesn’t like to follow the rules”.


Bronstein, J. L., 1994. Our Current Understanding of Mutualism. The Quarterly Review of Biology. 69;31-51.

Molinaro, D. J., Stafford, E. S., Collins, B. M. J., Barclay, K. M., Tyler, C. L., Leighton, L. R., 2014. Peeling out predation intensity in the fossil record: A test of repair scar frequency as a suitable proxy for predation pressure along a modern predation gradient. Palaeogeography, Palaeoclimatology, Palaeoecology. 412:141-147.

Holt, R. D., 1977. Predation, apparent competition, and the structure of prey communities. Theoretical Population Biology. 12:197-229.


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