4.11.9Additional lessons
Human Influence on Succession — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 100 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Ways Humans Intervene
Human activity can keep a community at an earlier stage than the natural climax.
Grazing
- Grazing Livestock feed on tree seedlings and tall plants, so grasses are favoured and woodland cannot establish.
Burning
- Burning Regular fires kill young trees and shrubs but allow fire-adapted grasses and heathers to regrow.
Mowing
- Mowing Cutting vegetation at intervals stops woody plants growing and maintains a short grassland.
Clearing and farming
- Clearing and farming Removing the community completely sends the area back to an early stage of succession.
| Activity | Effect on succession | Resulting community |
|---|---|---|
| Sheep grazing | Arrested | Upland grassland |
| Heather burning | Arrested | Moorland |
| Abandoning farmland | Secondary succession starts | Scrub then woodland |
| Mowing a meadow | Arrested | Species-rich grassland |
Part 2 of 5Plagioclimax Formation
1
A natural woodland is cleared by people to create farmland or pasture.
2
Regular grazing or cutting prevents tree seedlings from growing, so the land remains a grassland.
3
The grassland community becomes stable under this management, forming a plagioclimax, which differs from the natural climax.
4
If the management stops, secondary succession resumes and shrubs and trees return over several decades.
5
Conservation managers may then choose between allowing natural succession to continue or maintaining the managed stage, depending on which species they wish to protect.
Part 3 of 5Consequences
Consequences
Biodiversity gain
- Biodiversity gain Managed meadows and heathland support rich plant and insect communities that would be lost under woodland.
Biodiversity loss
- Biodiversity loss Clearing large areas of forest removes species that need mature habitats.
Conservation
- Conservation Some reserves use grazing or cutting deliberately to keep valued habitats.
Land use change
- Land use change Urban expansion and intensive farming often prevent succession completely.
Climate and planning
- Climate and planning Planners balance food production, housing and conservation,
- so protected areas often include management that maintains valued habitats.
Part 4 of 5Worked Example
Question
- On a grazed moor 0 of 40 sample plots contain tree seedlings, whereas 18 of 40 plots in a fenced, ungrazed area do.
- Calculate the percentage of plots with seedlings in each area.
- Grazed0 ÷ 40 × 100 = 0%.
- Ungrazed18 ÷ 40 × 100 = 45%.
- The difference suggests grazing prevents woody seedlings establishing.
Answer 0% grazed and 45% ungrazed
Part 5 of 5Exam Checklist & Tips
4.11.9 – Key Points for the Exam
- Plagioclimax A community held at a stage different from the natural climax by human activity.
- Arrested succession Succession that is stopped at an early stage.
- Grazing Feeding by animals on vegetation.
- Land management Planned actions to maintain or alter vegetation.
- Secondary succession Recovery that follows disturbance of an existing community.
Common Mistakes
- Saying that human activity always reduces biodiversity.
- Confusing plagioclimax with true climax.
- Forgetting that stopping management lets succession continue.
- Describing grazing as a cause of primary succession.
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