Humans and their effects on the environmentOxford AQA IGCSE Biology: Revision notes
Section 1
What is biodiversity and why does it matter?
Biodiversity is the variety of different species of living organisms in an area, including the different varieties within species. It encompasses genetic diversity (variation within a species), species diversity (number of different species), and ecosystem diversity (range of different habitats).
Importance of biodiversity:
- Provides ecosystem stability by ensuring multiple species can perform similar functions; if one species declines, others can compensate
- Maintains food webs and nutrient cycles that support all life
- Provides food, medicine, and raw materials for humans
- Offers genetic resources for breeding programmes and scientific research
- Supports cultural and recreational value (tourism, education)
- Increases resistance to disease and environmental change
Examiners expect you to explain why biodiversity matters in terms of ecosystem function and human needs. Link stability to multiple species performing similar roles — this shows deeper understanding.
Section 2
What causes biodiversity loss?
Causes of reduction in biodiversity:
| Cause | Mechanism | Example |
|---|---|---|
| Habitat destruction | Direct removal or fragmentation of ecosystems | Deforestation, urbanisation, agricultural land conversion |
| Climate change | Rising temperatures and altered precipitation patterns disrupt species' habitats | Polar species losing ice, coral bleaching |
| Pollution | Introduction of harmful substances damages organisms and ecosystems | Pesticides, heavy metals, plastic |
| Overexploitation | Taking organisms from the wild faster than they can reproduce | Overfishing, illegal hunting |
| Invasive species | Non-native species outcompete native species for resources | Rabbits in Australia, grey squirrels in Britain |
Human activities causing these problems:
- Deforestation for agriculture, timber, and development destroys habitat for millions of species
- Land use change converts natural ecosystems to farmland or urban areas
- Water pollution from fertiliser runoff, sewage, and industrial discharge
- Air pollution from fossil fuel combustion damages respiratory systems and contributes to climate change
Students often list causes without explaining the mechanism. Remember: examiners want to see HOW each cause reduces biodiversity, not just WHAT it is.
Think of habitat destruction like knocking down houses in a town — the fewer homes available, the fewer families can live there. Similarly, fewer habitats mean fewer species can survive.
Section 3
How can we conserve biodiversity?
Conservation methods:
| Method | Description | Effectiveness |
|---|---|---|
| National parks and protected areas | Legal designation of land to prevent exploitation and allow ecosystem recovery | Moderate to high; but requires enforcement and may displace local communities |
| Seed banks | Storage of plant seeds in controlled conditions for future replanting | High for preservation; supports future restoration but doesn't address current habitat loss |
| Captive breeding programmes | Breeding endangered species in controlled environments for eventual reintroduction | Moderate; can rescue species from extinction but costly and may reduce genetic diversity |
| Reforestation and habitat restoration | Active replanting and ecosystem recovery to increase biodiversity | Moderate to high; slow process but directly addresses habitat loss |
| International agreements | Treaties like CITES (Convention on International Trade in Endangered Species) control exploitation | Moderate; effectiveness depends on enforcement and compliance |
Additional conservation strategies:
- Reducing pollution and addressing climate change
- Sustainable land use and agriculture
- Education and awareness programmes
- Regulation of fishing and hunting quotas
When evaluating conservation methods, consider both advantages (what they achieve) and limitations (cost, time, effectiveness, unintended consequences). Examiners reward balanced evaluation.
Captive breeding of Arabian oryx saved the species from extinction, but birds bred in captivity showed reduced genetic diversity. This illustrates a key trade-off: rescue from extinction versus long-term genetic health.
Section 4
What are the specific impacts of pollution and climate change?
Water pollution and eutrophication:
Water pollution occurs when harmful substances (fertilisers, sewage, industrial waste) enter water bodies. This causes eutrophication — a chain reaction:
- Excess nutrients (nitrogen and phosphorus) from fertilisers and sewage enter water
- Algae and aquatic plants grow rapidly (algal bloom)
- Plants and algae die; decomposers increase, using oxygen
- Oxygen depletion (hypoxia) kills fish and other aerobic organisms
- Dead zone develops where few organisms can survive
Greenhouse effect and climate change:
Greenhouse gases trap heat in the atmosphere:
- Carbon dioxide (CO₂) from fossil fuel combustion and deforestation
- Methane (CH₄) from livestock farming and wetlands
These gases increase global temperature, causing:
- Rising sea levels (melting ice caps, thermal expansion)
- Habitat loss for polar and coastal species
- Altered growing seasons affecting food chains
- Extreme weather events (droughts, floods, storms)
- Range shifts where species migrate to cooler areas
- Coral bleaching and ocean acidification
- Disruption of migration and breeding cycles
Eutrophication questions often ask you to explain the sequence of events. Always structure your answer as a chain: nutrients → algal bloom → decomposition → oxygen depletion → death of organisms.
A farm uses nitrogen fertiliser; heavy rain washes it into a river. Algae grows rapidly and dies; decomposers respire, consuming oxygen. Fish suffocate because oxygen levels fall below what they need to survive.
Section 5
How do we measure biodiversity and assess ecosystem health?
Methods of measuring species diversity:
Quadrats (fixed square areas):
- Place a square frame (e.g. 1 m²) randomly in the habitat
- Count and identify all species within the quadrat
- Repeat multiple times to calculate average species numbers
- Useful for sessile organisms (plants, fungi) in uniform habitats
Transects (line or belt surveys):
- Stretch a line across a habitat
- Record species at regular intervals along the line
- Useful for showing how species change across gradients (e.g. up a hillside, from water to land)
- Can be line transects (point) or belt transects (area around line)
Calculating species richness:
Species richness = the total number of different species in an area
Example: If quadrat 1 has oak, ash, birch (3 species), quadrat 2 has oak, elm (2 species), the combined species richness is 4 (oak, ash, birch, elm).
Indicator species (Higher Tier):
Some organisms are sensitive to specific environmental conditions and indicate pollution or habitat quality:
| Indicator | Indicates | Reason |
|---|---|---|
| Mayfly nymphs | Clean, high-oxygen water | Cannot survive in polluted water |
| Sludge worms | Polluted, low-oxygen water | Tolerant of poor conditions |
| Lichens | Clean air | Sensitive to air pollution and acid rain |
| Bloodworms (Chironomus) | Moderate pollution | Tolerant of some organic pollution |
Presence or absence of indicator species tells us about water or air quality without needing to measure all species.
Quadrats are for sampling small areas with organisms you can easily see and identify; transects are for showing how communities change across space. Choose the right method based on the question.
A stream has sludge worms and bloodworms but no mayfly nymphs. This indicates moderate to high pollution. If the same stream had mayflies, you would conclude the water is clean.
Must Know
- Biodiversity is the variety of species in an area; it is important for ecosystem stability, human food/medicine, and resilience to environmental change
- Five major causes of biodiversity loss: habitat destruction, climate change, pollution, overexploitation, and invasive species — all driven by human activities
- Conservation methods include national parks, seed banks, captive breeding, reforestation, and international agreements like CITES; each has benefits and limitations
- Eutrophication is a specific water pollution problem: excess nutrients → algal bloom → decomposition → oxygen depletion → death of organisms
- Greenhouse gases (CO₂ and methane) trap heat and cause climate change, leading to habitat loss, species range shifts, and ecosystem disruption
- Quadrats and transects are field methods to measure biodiversity; indicator species (like mayfly nymphs for clean water, lichens for clean air) assess ecosystem health without measuring all species
- Species richness = total number of different species; higher richness generally indicates better ecosystem health
That's the notes covered.
Carry on to the next subtopic.