A community is all the populations living in one place. In middle school you named the interactions between them. Here we add the idea of a niche, and what happens when two species need the same one.
Key idea
A species' niche is its role and needs: what it eats, when it is active, where it shelters, what conditions it tolerates. Its habitat is only where it lives. Two species cannot share exactly the same niche for long: one outcompetes the other (competitive exclusion).
Why it works
If two species need exactly the same limited resource in the same way, any small advantage lets one take a bit more each generation, and the other shrinks. In the 1930s Georgii Gause grew two kinds of Paramecium on the same food. Alone, each grew well. Together, one always drove the other out. Species that do coexist usually split the resource: they eat at different times, in different spots or different sizes of food (resource partitioning).
| Interaction | Effect on species 1 | Effect on species 2 | Example |
|---|---|---|---|
| Mutualism | + | + | bees and flowers; legumes and nitrogen-fixing bacteria |
| Commensalism | + | 0 | barnacles riding on a whale |
| Parasitism | + | − | a tick feeding on a deer |
| Predation | + | − | a lynx eating a hare |
| Competition | − | − | hawks and owls hunting the same mice |
Worked example: Naming an interaction
Clownfish shelter among the stinging tentacles of a sea anemone, which do not sting them, and the clownfish chase away fish that eat anemones. What kind of interaction is this?
- The clownfish gains shelter: +.
- The anemone gains protection: +.
Answer: Mutualism.
Worked example: Coexisting competitors
Two warbler species feed on insects in the same spruce trees. One feeds near the treetops, the other on the lower branches. Why can both survive there?
- They live in the same habitat.
- But they feed in different parts of the trees, so they do not compete fully.
Answer: They partition the resource, so their niches differ.
Common mistake
Habitat and niche are easy to confuse. The habitat is the address: where the species lives. The niche is the job: how it lives there, what it eats, when it is active and what it needs. Many species share one habitat, but each has its own niche.