Biodiversity and Human Impact (B7.3)
Biodiversity
Biodiversity is the variety of all living organisms in an area — includes the number of different species (species richness) and the genetic variation within species.
High biodiversity = stable, resilient ecosystems. Low biodiversity = fragile — vulnerable to environmental change.
Human threats to biodiversity
| Threat | Mechanism | Example |
|---|---|---|
| Deforestation | Destroys habitat, releases CO₂ | Amazon rainforest clearance |
| Land use (farming, urbanisation) | Reduces wild habitat, increases runoff | UK farmland, urban sprawl |
| Water pollution | Eutrophication, toxic run-off | Fertiliser leaching to rivers |
| Air pollution | Acid rain, global warming | SO₂, CO₂, CH₄ emissions |
| Global warming | Climate change, sea level rise, habitat loss | Coral bleaching, Arctic ice melt |
Deforestation
Cutting down forests (especially tropical):
- Loss of habitat → biodiversity reduction → species extinction
- Reduced photosynthesis → less CO₂ absorbed
- Release of CO₂ if burned
- Disrupts water cycle (less transpiration → less rainfall)
- Soil erosion (no roots to hold soil)
Eutrophication
Excess fertilisers (nitrates) leach from farms into rivers:
- Nitrates → rapid algal bloom on water surface
- Algae block light → aquatic plants die
- Decomposers break down dead plants → use up O₂
- Low O₂ (deoxygenation) → fish and invertebrates die
Global warming
Greenhouse gases (CO₂, CH₄, N₂O) absorb infrared radiation → warming Earth's surface. Human activities (burning fossil fuels, farming, deforestation) increase concentrations.
Consequences: sea level rise (ice melt, thermal expansion), more extreme weather, habitat loss, species distribution shifts, crop failure.
Conservation
Reasons to maintain biodiversity:
- Ethical: all species have a right to exist
- Ecological: ecosystem stability and services
- Medical: potential future drugs
- Agricultural: genetic diversity for crop breeding
Methods:
- Marine and terrestrial nature reserves/national parks
- Seed banks (Svalbard) — preserve genetic diversity
- Captive breeding programmes / wildlife corridors
- International agreements (Paris Agreement, CITES)
- Sustainable fishing (quotas, size limits, no-catch zones)
Common exam errors
- Confusing eutrophication mechanism — fertilisers cause algal bloom which causes O₂ depletion, not directly.
- Forgetting that CO₂ is released both by burning forests AND by reduced photosynthesis after deforestation.
- Saying global warming causes the greenhouse effect — the greenhouse effect is natural; ENHANCED greenhouse effect is the human problem.
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