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Cambridge IGCSE Biology · 0610

Chapter 20: Human influences on ecosystems (Part 2)

Pollution of land, water, and air

Aquatic ecosystems
Polluted by untreated sewage and excess fertilisers, which reduce biodiversity.
Plastics
Non-biodegradable plastics accumulate in both terrestrial and aquatic habitats. Animals can become entangled or ingest plastic, leading to injury, death, and disruption of food chains.
Air pollution
Human activities, particularly the burning of fossil fuels and intensive farming, increase levels of methane and carbon dioxide. This leads to the enhanced greenhouse effect and climate change.

Exam Traps

  • Do not say plastics biodegrade quickly — they accumulate in the environment.
  • Avoid confusing the natural greenhouse effect (essential for life) with the enhanced effect from excess gases.

Eutrophication

This process occurs when excess nutrients (like nitrates from fertilisers) enter waterways:

  1. Fertilisers are washed into water bodies by rain.
  2. The nutrient increase causes an algae bloom across the water surface.
  3. The algae block sunlight, preventing submerged plants from photosynthesising, causing them to die.
  4. Decomposers break down the dead plants, using up the remaining dissolved oxygen through aerobic respiration.
  5. Organisms requiring oxygen (like fish) die, leading to a massive reduction in biodiversity.
Six stages of eutrophication from nitrate leaching through algae bloom, plant death, decomposition, oxygen depletion, to fish death
Diagram 1: The process of eutrophication. (1) Nitrates leaching into a pond, (2) algae covering the surface, (3) submerged plants dying from lack of light, (4) bacteria decomposing dead plants, (5) dissolved oxygen levels dropping, and (6) fish dying at the bottom.

Examiner Report Insights

  • State the steps in order — examiners expect the full chain from fertiliser runoff to oxygen depletion.

Exam Traps

  • Do not say fish die because of the algae bloom directly — they die from low dissolved oxygen after decomposition.
  • Avoid claiming algae increase oxygen for submerged plants — they block light and cause plant death.

Conservation of species and resources

Sustainable resource
A resource produced as rapidly as it is removed from the environment, ensuring it does not run out. Forests and fish stocks can be managed this way.
Conservation methods
Endangered species are protected through monitoring, education, captive breeding programmes, and seed banks.
Risks of small populations
A decrease in population size reduces genetic variation (the gene pool), making a species less able to adapt to changes or survive diseases.
Advanced breeding
Artificial insemination (AI) and in vitro fertilisation (IVF) are used to increase genetic diversity by allowing breeding between geographically separated individuals.

Exam Traps

  • Do not say small populations increase genetic variation — the gene pool shrinks.
  • Avoid confusing seed banks (plant genetic material) with captive breeding (animal programmes).

Management of forests and fish stocks

Forests
Conserved through replanting, quotas on logging, protected areas, and educating companies on sustainable practices.
Fish stocks
Managed using quotas, closed seasons to allow breeding, controlled net/mesh sizes (to let young fish escape), and creating protected marine areas.

Exam Traps

  • Do not say smaller mesh sizes protect young fish — larger mesh lets undersized fish escape.
  • Avoid claiming unlimited fishing is sustainable — quotas and closed seasons prevent stock collapse.

Reasons for conservation programmes

Programmes are vital for:

  1. Maintaining and increasing biodiversity.
  2. Reducing the risk of extinction.
  3. Protecting vulnerable ecosystems.
  4. Maintaining ecosystem functions, such as nutrient cycling and the provision of food, fuel, genes, and medicinal drugs.

Exam Traps

  • Do not give only one reason when asked to explain why conservation matters — link biodiversity, extinction, and ecosystem function.
  • Avoid saying conservation has no economic value — genes and medicines from wild species are important resources.

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