4.11.8Additional lessons
Ecological Succession — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 99 of 101
Part 1 of 5Types of Succession
Succession is the gradual directional change of a community over time.
Primary
- Colonisation of bare surfaces with no soil, such as new volcanic rock or retreating glacier land.
Secondary
- Recovery on land that still has soil and seeds, for example after a fire or abandoned farmland.
Pioneer species
- The first colonisers, such as lichens and mosses, tolerate harsh conditions and begin building soil.
Climax community
- A relatively stable community in balance with the climate, with high diversity and complex food webs.
| Feature | Early stage | Climax stage |
|---|---|---|
| Soil depth | Thin or absent | Deep, rich in humus |
| Species diversity | Low | High |
| Net primary productivity | High per unit biomass | Close to zero overall |
| Nutrient store | Small, often in soil | Large, mostly in biomass |
Part 2 of 5Stages of Primary Succession
1
Lichens and mosses colonise bare rock, and as they die they add organic matter and help to weather the rock.
2
A thin soil forms, allowing small plants like grasses and herbs to establish and increase the soil's organic content.
3
Shrubs and small trees replace the herbs because they compete better for light, and soil depth and water retention increase.
4
Taller trees form a canopy that shades the ground, and species of the earlier stages decline.
5
A climax community develops, with high biodiversity and balanced energy and nutrient flows.
Part 3 of 5Changes Along Succession
Changes Along Succession
Abiotic change
- Soil depth, nutrient content, water retention and shade increase, while temperature extremes decrease.
Biodiversity
- Species richness rises, then may level off or fall slightly at climax as strong competitors dominate.
Productivity
- Gross production rises, but respiration also rises, so net production falls toward zero at climax.
Food webs
- Simple food chains become complex webs with many niches, which tends to increase stability.
Part 4 of 5Worked Example
Question
- In an early stage GPP is 8 000 and respiration is 2 000 kJ m⁻² year⁻¹;
- at climax GPP is 20 000 and respiration is 19 000 kJ m⁻² year⁻¹.
- Calculate NPP at each stage.
- NPP = GPP − R.
- Early8 000 − 2 000 = 6 000 kJ m⁻² year⁻¹.
- Climax20 000 − 19 000 = 1 000 kJ m⁻² year⁻¹.
Answer
- 6 000 early and 1 000 at climax;
- NPP falls although GPP rises.
Part 5 of 5Exam Checklist & Tips
4.11.8 – Key Points for the Exam
- Succession Gradual change in species composition of a community over time.
- Pioneer species Species that first colonise an area.
- Climax community The stable final stage of succession.
- Primary succession Succession starting on bare ground without soil.
- Secondary succession Succession on land where soil is already present.
Common Mistakes
- Saying that climax communities never change, when they change slowly with climate.
- Treating secondary succession as starting from bare rock.
- Forgetting that net productivity falls as respiration rises in mature communities.
- Confusing the order of colonisation by naming trees as pioneers.
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