Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Biodiversity works at three levels: genetic, species and ecosystem.
- Resistance: how little an ecosystem changes when disturbed. Resilience: how fast and how fully it recovers.
- Natural and artificial ecosystems with few parts and little variety among the parts are often less resilient (a one-variety crop field is the classic artificial case).
- Diversity gives back-up species (functional redundancy), alternative food-web pathways and a variety of responses to stress.
- Producers, keystone species and essential abiotic and biotic factors maintain an ecosystem’s diversity.
- A keystone species has a disproportionate effect relative to its abundance, usually by controlling a dominant competitor or grazer. Examples: the ochre sea star on rocky shores; sea otters in kelp forests.
- Adding or removing any component changes structure in the short term (direct partners) and the long term (whole community).
Key relationships
| Change | Short-term effect | Long-term effect |
|---|---|---|
| Keystone predator removed | its prey (often a dominant competitor) increases | dominant species spreads; many others lost; community simplifies |
| Producer lost | herbivores and species that shelter in it decline | habitat and food web collapse; species that depended on them leave or die |
| Essential abiotic factor changed (e.g. water, temperature) | sensitive species decline | community composition shifts to tolerant species |
| Species added to a low-diversity system | depends on its interactions | may fill a gap and add resilience, or out-compete natives (see Topic 8.7) |
| Equation | Use |
|---|---|
| Percentage change = (new − original) ÷ original × 100 | size of a fall (resistance) |
| Recovery = after ÷ before × 100 | how completely an ecosystem returns (resilience) |
Assumptions behind the numbers
- Comparisons of resilience assume the ecosystems faced the same disturbance and were measured the same way.
- A single disturbance with no replicates can support a pattern; it cannot prove it.
Mistakes to avoid
- Calling the most abundant species the keystone species.
- Confusing resilience (recovery) with resistance (how little it changes).
- Giving only the short-term effect when a question asks for the long term.
- Forgetting abiotic factors when asked what maintains diversity.
- Saying diverse ecosystems cannot be damaged.
Quick self-check
- Biomass falls from 300 to 240 g m⁻² in a drought. Percentage fall? (60 ÷ 300 × 100 = 20%)
- Why does removing a keystone predator reduce diversity? (Its prey, often a dominant competitor, is released and crowds out other species.)
- Why is a single-variety crop field fragile? (Every plant responds the same way, so one pest or disease can affect them all.)
Next: practice questions.