Key Idea: Most marks in topic 2.5 come from how an ecosystem changes during succession. At HL, also learn productivity in succession, r and K strategies, and plagioclimax.
1. Changes during succession (most marks)
- Soil gets deeper and holds more water and nutrients.
- Species diversity and biomass increase.
- Food webs become more complex, and nutrient cycling speeds up.
- The community becomes more stable and resilient.
2. Succession
- Succession: one community replaces another over time.
- Each sere changes conditions so the next can move in.
- Primary: new ground with no soil, such as lava. Secondary: soil left, such as after a fire.
- Pioneers (lichens, mosses) start it; the climax community ends it.
3. HL: productivity, r/K, people
- Early succession: NP is high, so biomass builds up fast.
- At climax, GP is high but respiration uses almost all of it, so NP is near zero.
- Pioneers are r-strategists; climax species are K-strategists.
- Grazing, burning or mowing stops succession: a plagioclimax.
How it is examined:
| Question | How to score |
|---|---|
| [1] Identify a zone or vegetation type from a figure | Read the figure; copy the name exactly. |
| [2] Outline why vegetation changes with altitude | Name the abiotic factor (temperature) and its effect. |
| [2] to [3] Describe the stages of a succession | Pioneers, then soil builds, then taller plants, then climax. |
| [4] Outline how a climax differs from a pioneer community | Diversity, biomass, soil depth, stability: one each. |
| [7] Explain how succession changes an ecosystem | Soil, diversity, productivity, nutrient cycling, resilience, with a named example. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Succession | the replacement of one community by another over time |
| Transect | a line across a gradient along which samples are taken |
| r-strategist | many small offspring, little care, grows fast: good pioneers |
| K-strategist | few large offspring, much care, competes well: climax species |
| Plagioclimax | a community held back from its climax by human activity |
| Polyclimax | the idea that an area can have several possible climax communities |
Last-minute check: answer, then tap to see.
What happens to NP at the climax? It falls to near zero: respiration uses almost all of GP.
Are pioneer species r- or K-strategists? r-strategists: many offspring, fast growth.
Name a plagioclimax. Heather moorland kept by burning, or a hay meadow kept by mowing.
How is a transect shown? On a kite diagram: width shows how much of each species.
Why may a succession not reach the same climax? Climate, soil, species and chance differ.
On the North York Moors, heather moorland is burnt in small patches every 10 to 15 years. Explain why the moorland does not develop into woodland.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- Say what changes AND in which direction: soil gets deeper, diversity increases.
- Zonation is space, succession is time: never mix them up.
- Name a real example: Surtsey, Mount St Helens, a glacier foreland.
- For productivity, compare GP, respiration and NP at the start and at the climax.