3.4Unit 2 · Ecology & the Environment
Human Influences on the Environment — Unit 2 — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 5 of 5 1 quiz
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 radiation passes through the atmosphere; Earth re-emits longer-wavelength inf…
- Consequences of global warming
- Air pollution – sulfur dioxide
- Air pollution – carbon monoxide
- Greenhouse effect
- Water pollution
- Deforestation
Section 1 of 4Human Influences on the Environment — Unit 2
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.
Air pollution – carbon monoxide:
- released from incomplete combustion in car engines, faulty boilers and cigarettes.
- It binds very strongly (almost irreversibly) to haemoglobin, reducing oxygen transport, causing drowsiness and in high doses death.
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.
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.
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.
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.
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)
Sewage and oxygen:
- Organic matter in sewage is food for aerobic bacteria, which multiply and use up dissolved oxygen.
- Organisms needing high oxygen die, leaving only species tolerant of low oxygen, so biodiversity falls.
More effects of global warming:
- Warmer oceans cause coral bleaching;
- habitats change or are lost;
- species migrate to new areas, which can spread pests and diseases;
- food chains are disrupted and extinction rates may rise.
- Warmer oceans cause coral bleaching;
Section 2 of 4Process & 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.
- Nutrients enter water
- Algal bloom blocks light
- Water plants die
- Decomposers multiply
- Oxygen used up
- Fish suffocate
Section 3 of 4Compare
| 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 |
| 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 |
Section 4 of 4Worked examples
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.
- Rain leaches nitrates from the fertiliser into the river.
Suggest 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.
- 1) Fewer trees are present to remove CO2 by photosynthesis.
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