Human Impact on EcosystemsAQA GCSE Biology: Revision notes
Section 1
What are the main ways humans destroy habitats and reduce biodiversity?
Human activities cause habitat destruction, which is the primary threat to biodiversity worldwide. The main causes include:
- Deforestation: Clearing forests for agriculture, timber, and development removes homes for countless species
- Intensive farming: Monocultures, pesticide use, and removal of hedgerows destroy natural habitats and food sources
- Urban development: Building on natural land converts ecosystems into towns and cities
- Drainage of wetlands: Converting marshes and swamps for agriculture eliminates breeding grounds
Biodiversity refers to the variety of species in an ecosystem. When habitats are destroyed, species cannot survive, leading to extinction or population decline. Invasive species are often introduced accidentally through trade and travel, outcompeting native species for resources and food.
Examiners want you to explain the link between habitat destruction and biodiversity loss. Always state that when habitats are destroyed, organisms lose their food sources, shelter, and breeding grounds, so populations cannot survive.
Think of a forest as an apartment building: if you demolish the building, all the residents lose their homes at once. Species are the same—destroy their habitat and they have nowhere to live.
Section 2
How does pollution from sewage and fertilisers cause eutrophication in aquatic ecosystems?
Eutrophication is the process where excessive nutrients cause algal overgrowth in water bodies, leading to ecosystem collapse. Here is how it occurs:
- Nutrient input: Sewage and agricultural fertiliser (nitrogen and phosphorus) enter waterways
- Algal bloom: Excess nutrients cause rapid algae growth, turning water green and cloudy
- Light blocking: Dense algae blocks sunlight from reaching plants below
- Plant death: Aquatic plants die because they cannot photosynthesise without light
- Decomposition: Dead organic matter is broken down by decomposers (bacteria)
- Oxygen depletion: Decomposition uses up dissolved oxygen, creating anoxic (oxygen-free) conditions
- Animal death: Fish and other aerobic organisms die from lack of oxygen
The water becomes a dead zone where few organisms can survive. This is a major problem in heavily farmed areas and near sewage outlets.
| Stage | What happens | Effect on ecosystem |
|---|---|---|
| Nutrient enrichment | Fertiliser/sewage added to water | Promotes algal growth |
| Algal bloom | Algae multiply rapidly | Blocks light, water becomes murky |
| Plant death | Aquatic plants cannot photosynthesise | Loss of plant-based food sources |
| Decomposition | Bacteria break down dead matter | Uses dissolved oxygen |
| Deoxygenation | Oxygen depleted from water | Fish and aquatic animals die |
Examiners mark this highly. You must describe all stages in sequence: nutrient input → algal bloom → light blocking → plant death → decomposition → oxygen depletion → animal death. Missing any stage loses marks.
A farm near a river uses nitrogen fertiliser. Heavy rain washes the fertiliser into the river. Excess nitrogen causes algae to grow rapidly, blocking sunlight. Underwater plants die and decomposers break them down, using up oxygen. Fish suffocate and die because dissolved oxygen is gone.
Section 3
What is the link between deforestation and increased carbon dioxide levels?
Deforestation contributes to climate change by increasing atmospheric carbon dioxide in two ways:
1. Loss of carbon sinks:
- Trees absorb carbon dioxide from the atmosphere during photosynthesis
- Forests store carbon in their wood and soil
- When forests are cleared, this carbon-storing capacity is removed
- Less CO₂ is removed from the atmosphere each year
2. Release of stored carbon:
- Trees are often burned or left to rot after felling
- Decomposition releases stored carbon as CO₂ back into the atmosphere
- Soil disturbance from clearing releases carbon previously trapped underground
The consequence: Deforestation causes a net increase in atmospheric CO₂ because both the removal of a carbon sink and the release of stored carbon occur together. This accelerates global warming, which causes:
- Rising global temperatures
- Changes in weather patterns
- Rising sea levels
- Increased extreme weather events
Tropical rainforests are particularly important because they store vast amounts of carbon and are being cleared at alarming rates for cattle ranching and agriculture.
Students often say 'deforestation removes trees that make oxygen,' but examiners want the CO₂ explanation. Focus on carbon storage and sinks, not oxygen production.
A forest is like a bank account for carbon. When you cut it down, you lose future deposits (no more CO₂ absorption) and you withdraw all the savings (release stored carbon)—a double loss.
Section 4
How can biological indicators be used to assess pollution levels?
Biological indicators are living organisms used to measure the health and pollution levels of an ecosystem. They work because different species have different pollution tolerance.
Common biological indicators:
| Indicator | Tolerance level | What it shows |
|---|---|---|
| Lichens | Very sensitive to air pollution | Low lichen presence = high air pollution (SO₂, nitrogen oxides) |
| Mayfly nymphs | Sensitive to water pollution | Presence indicates clean water; absence suggests pollution |
| Bloodworms (midge larvae) | Tolerant of pollution | Presence in water indicates poor oxygen levels, high pollution |
| Tubifex worms | Very tolerant of pollution | Found only in highly polluted, anoxic water |
| Freshwater shrimp | Sensitive | Indicates good water quality |
How they work:
- Sensitive species are only found in clean environments; their absence indicates pollution
- Tolerant species survive in polluted conditions; their presence indicates pollution
- Biodiversity index: The number and variety of species present reflects ecosystem health—high diversity = low pollution
Advantages of biological indicators:
- Show long-term pollution effects (not just current levels)
- Reflect cumulative impacts on the ecosystem
- Cheaper than chemical testing
- Easy to identify and collect
Limitations:
- Require specialist knowledge to identify species
- Results take time to assess
- Cannot pinpoint exact pollution sources
Examiners test whether you understand the principle: sensitive species = clean water (absence means pollution), and tolerant species = polluted water (presence means pollution). Don't confuse these.
A river downstream of a factory contains only bloodworms and tubifex worms but no mayfly nymphs. This indicates high pollution because tolerant species are present and sensitive species are absent. Oxygen levels are likely very low.
Section 5
How is global warming affecting species distribution and breeding seasons?
Global warming causes changes to ecosystems by altering temperature and climate conditions. Species are responding in two main ways:
1. Changes in species distribution:
- Species are moving towards cooler habitats to find suitable temperatures
- Poleward migration: Species moving towards the poles (north in Northern Hemisphere, south in Southern Hemisphere)
- Altitude migration: Species moving up mountains to find cooler temperatures
- Range expansion: Species spreading into areas that were previously too cold
- Some species cannot migrate fast enough and face local extinction
Examples:
- Polar bears losing sea ice habitat as Arctic temperatures rise
- Insects and plants moving further north in Europe
- Alpine species moving higher up mountains, reaching summit peaks with nowhere left to go
2. Changes in breeding seasons:
- Earlier breeding: Many species breed earlier in spring due to warmer temperatures
- Longer growing seasons: Plants flower and leaf out weeks earlier than historical records
- Phenological mismatch: When species breed at different times than their food sources (e.g., chicks hatch after insects have emerged and died)
- Desynchronisation: Predators and prey breeding cycles no longer align, disrupting food chains
Examples:
- Birds laying eggs 2–3 weeks earlier than 50 years ago
- Spring arriving earlier, but some species not adapting quickly enough
- Caterpillars pupating before birds have eggs to feed chicks
Ecosystem consequences:
- Food chain disruption and population decline
- Loss of biodiversity in regions unable to support shifting species
- Increased competition when multiple species invade the same habitat
- Stress on specialist species dependent on specific climatic conditions
Examiners expect you to explain both migration and breeding season changes, and then link these to ecosystem impacts. Always mention food chain disruption or population decline as a consequence.
A warbler bird species arrives in the UK earlier in spring due to global warming. However, the caterpillars it feeds to its chicks have already emerged and pupated. The birds cannot find enough food to feed their young, so breeding success declines and populations fall.
Must Know
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Habitat destruction (deforestation, intensive farming, development) is the primary cause of biodiversity loss. Invasive species introduced by human activity outcompete native species.
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Eutrophication occurs in sequence: excess nutrients → algal bloom → light blocked → plants die → decomposition uses oxygen → anoxic conditions → aquatic animals die. Sewage and fertiliser are the main sources.
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Deforestation increases atmospheric CO₂ because forests are carbon sinks that absorb CO₂, and felled trees release stored carbon. This accelerates global warming and climate change.
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Biological indicators assess pollution: sensitive species (mayflies, freshwater shrimp) indicate clean water, tolerant species (bloodworms, tubifex) indicate pollution. High biodiversity = low pollution.
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Global warming causes poleward and altitude migration of species seeking cooler habitats. Breeding seasons shift earlier, causing phenological mismatch when food sources do not align with predator breeding times, disrupting food chains.
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Climate change disrupts ecosystems through species range shifts, local extinctions, altered food chains, and loss of biodiversity in regions that cannot support migrating species.
That's the notes covered.
Carry on to the next subtopic.