The other day in class, our AP Biology teacher presented us with the following graph and asked us to determine which of the following interspecific relationships it represents:

enter image description here

(A) commensalism

(B) predation

(C) mutualism

(D) competition

(E) parasitism.

She explained to us that since there is not enough information in the graph to explain why species A suddenly drops off after time "x" and species B suddenly rises after time "x", the best answer out of the choices is (A) commensalism.

But to me, that seems to be a faulty line of thinking. After all, my teacher arrived at that answer only under the assumption that the graph/question itself is good to begin with. I tried finding graphs of commensalism using Google Images, and I actually found the original source of the graph that our teacher presented us.

Now, the question on Regentsprep actually gives us a point of reference and states that we are considering two herbivores in a grassland environment, so in my opinion it's reasonable to conclude that the relationship between species A and B is competition.

The problem is that my teacher never gave us a frame of reference: she just told us to identify the relationship in this graph without any "backstory". So what are your thoughts: is it possible to conclude that "A" is the best answer choice if we are not given any information aside from the graph?

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    $\begingroup$ I agree that the question as your teacher presented it is very unclear, and even the question as it is presented in the original source is still not a great question. I would say that commensalism is tied for the worst answer with mutualism, as with commensalism you would expect one population to go up while the other remains the same. $\endgroup$
    – C_Z_
    Jan 18, 2016 at 1:28

1 Answer 1


Competition makes the most sense, IMO.

For every other type of relationship, A and B should be dependent and roughly proportional: If parasites kill their host, they die in proportion. If predators eat all their prey, they drop in numbers due to food scarcity until food repopulates, the graph would almost look like a double helix. Mutual/commensals also share a dependent and proportional graph. The inverse proportionality of the sample chart, to me, gives away competition: one organism is flat out winning against another through some method.

  • $\begingroup$ Yeah, I agree with why it's competition. That explanation makes so much more sense to me than my teacher's reasoning. EDIT: also, I looked online for competition graphs just now and found a reputable source with competition graphs (bottom of the page) that are very similar in behavior to the graph my teacher showed us: both are steady up until time "x", after which one takes over the other. $\endgroup$ Jan 18, 2016 at 13:07

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