Give each partner its own sign
- The idea
- Predict
- See it
- Why
- Check
The idea
“If one species gains and the other is fine, that's mutualism. A small cost still counts as commensalism.”
Two species sharing space without trouble looks like teamwork, and a small cost hardly seems to count.
The short version
The mistake: If one species gains and the other is fine, that's mutualism. A small cost still counts as commensalism.
What's actually true: Mutualism is +/+, commensalism is +/0, parasitism and predation are +/−, and competition is −/−.
- Each partner gets its own sign for how the interaction affects its survival or reproduction. A + is a gain, a − is a cost, 0 is no effect.
- Mutualism is +/+. Commensalism is +/0, where one gains and the other is unaffected.
- Parasitism and predation are both +/−. The predator gets the +.
On the exam: Sign each partner before you name it. Any cost, even a small one, is a −.