4.4Ecology & the Environment

Human Influences on the Environment — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

10 min read 8 parts Lesson 4 of 7 1 quiz

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Lesson summary

Human Influences on the Environment Edexcel IGCSE Biology 4BI1 notes: Describe the biological consequences of pollution of air by sulfur dioxide and carbon monoxide.The main greenhouse gases are water vapour, CO2, methane, nitrous oxide and CFCs.The Sun's short-wavelength radiati…

Consequences of global warming
Air pollution – sulfur dioxide
Air pollution – carbon monoxide
Greenhouse effect
Water pollution
Deforestation

Part 1 of 8Key ideas

The main greenhouse gases are water vapour, CO₂, methane, nitrous oxide and CFCs.

The Sun's short-wavelength radiation passes through the atmosphere; Earth re-emits longer-wavelength infrared, some absorbed by greenhouse gases.

The greenhouse effect is natural and keeps Earth warm; the enhanced effect causes global warming.

Consequences of global warming

Consequences of global warming melting ice caps, sea level rise, extreme weather, changes in species distribution.

Untreated sewage adds nutrients and pathogens and increases biochemical oxygen demand.

Sulfur dioxide + water forms acidic solutions that damage leaves and release toxic aluminium ions into lakes.

Carbon monoxide combines with haemoglobin more readily than oxygen does.

Part 2 of 8Learning objectives

Describe the biological consequences of pollution of air by sulfur dioxide and carbon monoxide.

Name the greenhouse gases and explain the enhanced greenhouse effect.

Explain eutrophication caused by fertiliser and sewage.

Describe the effects of deforestation.

Part 3 of 8Essential knowledge

1Air pollution – sulfur dioxide

Air pollution – sulfur dioxide produced when fossil fuels containing sulfur, especially coal, are burnt. It dissolves in water droplets forming acid rain, which damages leaves and kills trees, lowers the pH of lakes (killing fish and invertebrates), and leaches minerals from soil. It also irritates the lungs.

2Air pollution – carbon monoxide

Air pollution – carbon monoxide released from incomplete combustion in car engines, faulty boilers and cigarettes. It binds irreversibly to haemoglobin, reducing oxygen transport, causing drowsiness and in high doses death.

3Greenhouse gases

Greenhouse gases water vapour, carbon dioxide (combustion, deforestation), methane (cattle, rice paddies, landfill), nitrous oxide (fertilisers, vehicles) and CFCs (old refrigerants). They trap heat in the atmosphere.

4Greenhouse effect

Greenhouse effect sunlight warms the Earth, which radiates infrared. Greenhouse gases absorb and re-radiate some infrared back to Earth. This natural effect keeps the planet warm enough for life.

5Enhanced greenhouse effect

Enhanced greenhouse effect increased greenhouse gas levels from human activity trap more heat, causing global warming. Consequences include melting polar ice and rising sea levels, flooding, changes in rainfall and more extreme weather, changes in species ranges and migration, and possible extinctions and lower crop yields.

Natural and enhanced greenhouse effects compared

6Water pollution

Water pollution fertiliser leached from fields and untreated sewage cause eutrophication: algal bloom, plant death, bacterial decomposition and oxygen depletion so fish die. Sewage also contains pathogens causing disease.

Eutrophication following fertiliser runoff into water

7Deforestation

Deforestation clearing forests for timber, farming or building reduces photosynthesis so less CO₂ is removed, and burning or decomposing wood adds CO₂.

It destroys habitats and reduces biodiversity, causes soil erosion and leaching of nutrients,
and disrupts the water cycle.
- (less transpiration, less rain locally, more flooding)

8Further effects of global warming

Warmer seas cause coral bleaching; some regions suffer more drought; species shift their ranges, carrying pests and diseases into new areas; disrupted food chains and lost habitats reduce biodiversity.

Part 4 of 8Process & sequence

- Fertiliser or sewage enters a lake or river.
- Nitrates and phosphates cause rapid algal growth (algal bloom).
- Algae block light, so plants below cannot photosynthesise and die.
- Decomposer bacteria feed on dead plants and algae and multiply.
- Bacteria use up dissolved oxygen in aerobic respiration.
- Fish and other aerobic organisms die from lack of oxygen.
Eutrophicationwrite these steps in this order; each step is a marking point.
1. Nutrients enter water
2. Algal bloom blocks light
3. Water plants die
4. Decomposers multiply
5. Oxygen used up
6. Fish suffocate

Part 5 of 8Worked examples

1Aerobic respiration

Explain how the use of fertiliser on farmland can lead to the death of fish in a nearby river.

Rain leaches nitrates from the fertiliser into the river.
- This causes rapid growth of algae, which blocks light so water plants die.
- Bacteria decompose the dead plants and algae and, as they multiply, use up dissolved oxygen in aerobic respiration.
- The fish die from lack of oxygen.
- This is eutrophication.

2Suggest two ways deforestation increases atmospheric carbon dioxide.

1) Fewer trees are present to remove CO₂ by photosynthesis.
- 2) Trees are often burned, or left to be decomposed by microorganisms, releasing CO₂ through combustion and respiration.

Part 6 of 8Summary

- SO₂ causes acid rain; CO reduces oxygen transport.
- CO₂ and methane are key greenhouse gases.
- The enhanced greenhouse effect leads to global warming.
- Fertilisers and sewage cause eutrophication.
- Deforestation causes erosion, leaching, CO₂ rise and lost biodiversity.

Part 7 of 8Keep going

Human Influences on the Environment Edexcel IGCSE Biology 4BI₁: what to do next
These Human Influences on the Environment Edexcel IGCSE Biology 4BI₁ notes covered the key ideas, worked examples and exam technique for this topic. Check your Human Influences on the Environment Edexcel IGCSE Biology 4BI₁ understanding with the practice questions below, then reread any Human Influences on the Environment Edexcel IGCSE Biology 4BI₁ point you were unsure about before moving on.
Human Influences on the Environment Edexcel IGCSE Biology 4BI₁: revision checklist
- Revise the main Human Influences on the Environment Edexcel IGCSE Biology 4BI₁ definitions until you can write them from memory.
- Practise Human Influences on the Environment Edexcel IGCSE Biology 4BI₁ exam questions under timed conditions.
- Practice: Human Influences on the Environment Edexcel IGCSE Biology 4BI1 practice questions
- Previous lesson: 4.3 Cycles within Ecosystems — Notes
- Next lesson: 5.1 Food Production — Notes
- Course: Edexcel International Gcse Biology 4BI1 — all units
- Official syllabus: Pearson Edexcel International GCSEs

Part 8 of 8Compare: Compare

Air pollutants
Pollutant	Source	Biological consequence
Sulfur dioxide	Burning fossil fuels containing sulfur	Acid rain: kills trees, lowers pH of lakes, killing fish
Carbon monoxide	Incomplete combustion (car engines)	Binds haemoglobin, less O₂ carried; can be fatal
Carbon dioxide	Burning fossil fuels, deforestation	Enhanced greenhouse effect
Methane	Cattle, rice fields, landfill	Strong greenhouse gas
Nitrous oxide / CFCs	Fertilisers, old aerosols/fridges	Greenhouse gases; CFCs damage ozone

Effects of deforestation
Effect	Explanation
Less CO₂ absorbed	Fewer trees photosynthesising; burning releases more CO₂
Soil erosion	No roots to hold soil; rain washes it away
Leaching of nutrients	Minerals washed out of exposed soil
Disturbed water cycle	Less transpiration, less rainfall, more flooding
Loss of biodiversity	Habitats destroyed, species become extinct

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Pollution · Greenhouse gas · Enhanced greenhouse effect · Acid rain · Eutrophication · Deforestation · Carbon monoxide · Leaching · Biochemical oxygen demand

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).

Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated: 24 October 2026