Score each partner +, −, or 0 from the data. On predator-prey graphs, prey peak first.
A community is all the species in one place, pushing on each other. Every interaction helps (+), harms (−), or doesn't affect (0) each partner. Those effects shape who persists, who spreads, and who gets squeezed out.
Key terms
- Niche — a species' whole way of life: what it eats, where it lives, when it's active, what conditions it tolerates.
- Fundamental niche — the full range a species could use without competitors. Its realized niche is the narrower range it uses with them.
- Symbiosis — a close, long-term relationship between two species. Mutualism is +/+, commensalism +/0, parasitism +/−.
- Keystone species — one whose effect on the community is far bigger than its abundance. Remove it and the community often loses species.
- Species diversity — richness (how many species) plus evenness (how equal their numbers are). Simpson's index on your formula sheet combines both.
How interactions shape a community
- Competition (−/−): species that need the same limiting resource each end up with less of it.
- If their needs overlap completely, the stronger competitor eventually eliminates the other from that habitat. That's competitive exclusion.
- Similar species that coexist usually split the resource by height, depth, time of day, or food size. That resource partitioning eases competition without ending it.
- Predation (+/−) couples two populations: prey boom → predators multiply → prey crash → predators decline → prey recover.
Why it matters
Expect this topic as data. A table of species grown alone and together is a competition experiment. The alone rows are the control: they show each species can live there, so a drop when they're together is the other species' doing. Interactions explain invasions too. An introduced species often spreads fast because the predators and competitors that held it back at home didn't come along.
Exam traps
| Trap | Actually |
|---|
| On a graph, predator and prey peak together, or the predator peaks first. | Prey peak first. Predators need that surplus food to multiply, so their peak lags behind. |
| One partner probably helps the other somehow, so call it mutualism. | Classify from measured effects. If one partner gains and the other shows no measurable change, it's commensalism (+/0). |
| Competing species must fight or poison each other. | Competition can run through the resource alone. One species draws a shared food supply down faster, and the other starves out. |
| Removing a predator helps the community, since fewer animals get eaten. | Remove a keystone predator and the dominant competitor takes over. Weaker species get crowded out, and diversity falls. |
Worked example
Two invented lizard species share a chain of small islands. Where both live, species R perches only below 1 m on tree trunks, and species S perches above 1 m. On islands with no S, R perches from the ground up to 4 m. Researchers removed S from one mixed island, and within two years R was perching up to 4 m there too. What best explains R's low perches on mixed islands?
- Find R's fundamental niche, its range with no S around: ground to 4 m.
- Find R's realized niche with S present: below 1 m only. That's a narrower slice.
- Use the removal. Same island, one change: S leaves and R moves up. So S was holding R down.
Competition with S restricts R to low perches, so R's realized niche is narrower than its fundamental niche.
The removal turns a pattern into a cause: same island, one change, and R's niche expands. The near-miss is 'R prefers low perches.' If low perches were R's own preference, removing S wouldn't send it upward. The two coexist by splitting the trees by height, which is resource partitioning.