Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A community is the interacting populations of different species in one area. It changes over time as those interactions change.
- Species composition: which species are present. Species richness: how many. Evenness: how equally individuals are shared among them. Diversity combines richness and evenness.
- Interactions control how each population gets energy and matter, so they drive population dynamics and community structure.
- Predator–prey numbers can cycle, with the predator peak lagging behind the prey peak.
- Similar species can coexist through niche partitioning; without it, one may exclude the other (competitive exclusion).
- A trophic cascade passes effects down a food chain, alternating down, up, down.
Key relationships
| Interaction | Signs | Example of effect |
|---|---|---|
| Competition | − / − | each species gets less of a shared limited resource |
| Predation, herbivory | + / − | consumer gains energy; prey or plant loses |
| Parasitism | + / − | parasite feeds on a living host over time |
| Mutualism | + / + | both gain (pollination, nutrient exchange) |
| Commensalism | + / 0 | one gains; the other is unaffected |
| Equation | Meaning |
|---|---|
| Diversity index = 1 − Σ(n/N)² | n = individuals of one species; N = individuals of all species |
| SE = s ÷ √n; error bars = mean ± 2 SE | non-overlapping bars: the means are probably different |
Assumptions behind the numbers
- Simpson’s index assumes the sample represents the whole community (same method, effort and season at each site).
- Error bars of ± 2 SE give a rough 95% confidence interval; they assume repeated samples were collected independently.
- Comparing indices only makes sense between similar habitats.
Mistakes to avoid
- Calling a community an ecosystem (an ecosystem includes the non-living parts).
- Forgetting to square n/N, or forgetting to subtract the sum from 1.
- Judging diversity by richness alone and ignoring evenness.
- Swapping mutualism (+/+) and commensalism (+/0).
- Saying predators peak at the same time as prey.
- Reading overlapping error bars as proof that there is no difference.
Quick self-check
- Two species with 5 individuals each: index? (1 − (0.25 + 0.25) = 0.50)
- Two species with 9 and 1 individuals: index? (1 − (0.81 + 0.01) = 0.18; same richness, less even, lower index)
- Why does the predator peak come after the prey peak? (Predators need time to turn extra food into extra offspring.)
- Name the +/0 interaction. (Commensalism)
Next: practice questions.