So here we have an example problem that says, suppose you remove species a from a community, which you notice reduces the fitness of species b. After introducing species a back into the community, you then remove species b and find that it has no significant impact on the fitness of species a. What is the relationship between species a and species b? And we've got these 4 potential answer options down below. Now to solve this problem, I've gone ahead and drawn a little sketch, which you can see right here, to help us better visualize what's happening in this problem.
Notice on the far left, we've got the community with both species a and species b present. But then we remove species a and find that it has a negative impact on the fitness of species b, indicated by the minus sign here. Then we reintroduce species a back into the community and remove species b, and find that that has no impact on the fitness of species a, indicated by the 0 here. And so what this seems to indicate is that species b has no effect on the fitness of species a, so we can put a 0 there. And because species b's fitness goes down in the absence of species a, that suggests that species a helps to improve the fitness of species b.
So we can put a plus sign for that. And so this is a plus-zero interaction, which is commensalism. Notice that we can eliminate answer option a, which says mutualism. That's a plus-plus interaction where both organisms benefit. That's not the case here.
We can eliminate option b, which says competition. That's a minus-minus interaction where both organisms are negatively impacted. And again, that's not the case here. So now we're between either option c or option d, which both say commensalism. But notice that it's species b that benefits, and species a that is unaffected.
So that matches with answer option c, which says commensalism where species b benefits while species a is unaffected. So we can go ahead and indicate that c here is the correct answer to this example problem, and d is not correct because it says species a benefits while species b is unaffected, but it's the other way around. Species b is benefiting, species a is unaffected. So c here is the correct answer. That concludes this problem, and I'll see you all in our next video.