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AP Biology · Unit 8: Ecology

8.5 Community Ecology

Symbiotic relationships

Species interact in a community. Symbiosis describes close relationships: mutualism (+/+, both benefit), commensalism (+/0, one benefits, the other is unaffected), and parasitism (+/−, one benefits at the other's expense). Predation is one organism eating another.

Competition and keystone species

By the competitive exclusion principle, two species can't occupy the exact same niche indefinitely — one outcompetes the other. A keystone species has an outsized effect on its community; removing it can cause dramatic changes (a trophic cascade).

Key terms

Mutualism
A symbiosis where both species benefit (+/+).
Commensalism
A symbiosis where one benefits and the other is unaffected (+/0).
Parasitism
A symbiosis where one benefits at the other's expense (+/−).
Competitive exclusion / niche
Two species can't occupy the same niche indefinitely; one outcompetes the other.
Keystone species
A species with an outsized effect on its community; removing it causes big changes.

Try an AP-style question

Experiment

Two species of barnacles live on rocks. When species A is removed, species B expands into the zone A used to occupy.

Which conclusion is best supported?

  1. A and B have a mutualistic relationship
  2. B is a predator of A
  3. B cannot survive in A's zone
  4. Competition from A was limiting B's range
Show the answer

D. B's realized niche was smaller than its fundamental niche because of competition (Connell's experiment).

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