Human Impacts on Ecosystems

This lesson covers the ways human activities affect ecosystems as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the impacts of deforestation, pollution, global warming, land use change and overfishing, and be able to explain eutrophication.

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Why Human Activity Affects Ecosystems

The human population is growing rapidly. As the population increases, so does the demand for:

• Food — more farmland, fishing and intensive agriculture
• Fuel — burning fossil fuels for energy, transport and heating
• Building materials — wood, stone, metals for housing and infrastructure
• Land — for homes, roads, factories and leisure

Meeting these demands impacts ecosystems by reducing habitats, producing pollution and disrupting natural cycles.

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Deforestation

Deforestation is the large-scale cutting down of trees and removal of forests.

Reasons for Deforestation

Reason	Details
Agriculture	Clearing land for crops (e.g. soy, palm oil) or cattle grazing
Timber	Logging trees for wood, paper and building materials
Biofuels	Growing crops (e.g. sugar cane) to produce ethanol as fuel
Mining	Clearing forests to access minerals and metals underground
Urbanisation	Expanding towns and cities into forested areas

Consequences of Deforestation

Consequence	Explanation
Loss of biodiversity	Habitats destroyed; species lose food and shelter; extinction risk increases
Increased CO₂	Fewer trees to absorb CO₂ by photosynthesis; burning trees releases stored carbon
Soil erosion	Tree roots no longer hold soil in place; heavy rain washes topsoil away
Flooding	Trees normally intercept rainfall and absorb water; without them, more surface run-off occurs
Disrupted water cycle	Less transpiration from trees reduces local rainfall

graph TD
    A["Deforestation"] --> B["Loss of habitat"]
    A --> C["Increased atmospheric CO₂"]
    A --> D["Soil erosion"]
    A --> E["Reduced transpiration"]
    B --> F["Biodiversity loss"]
    C --> G["Enhanced greenhouse effect"]
    D --> H["Loss of soil fertility"]
    E --> I["Less rainfall locally"]
    G --> J["Global warming"]

Exam Tip: In an exam question about deforestation, make sure you explain both the direct effects (habitat loss) and the indirect effects (increased CO₂ leading to global warming).

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Pollution

Pollution is the release of harmful substances into the environment. The main types are:

Air Pollution

Pollutant	Source	Effect
Carbon dioxide (CO₂)	Burning fossil fuels	Greenhouse gas; contributes to global warming
Sulfur dioxide (SO₂)	Burning coal and oil	Causes acid rain, which damages trees, lakes and buildings
Nitrogen oxides (NOₓ)	Vehicle engines	Cause acid rain and smog; respiratory problems in humans
Particulates	Diesel engines, industry	Respiratory and cardiovascular diseases; reduced air quality

Water Pollution

Pollutant	Source	Effect
Sewage	Untreated waste water	Increases bacterial growth; reduces oxygen; harms aquatic life
Fertilisers	Agricultural run-off	Causes eutrophication (see below)
Toxic chemicals	Industrial waste, pesticides	Poisons aquatic organisms; bioaccumulates in food chains
Plastics	Litter, industrial waste	Ingested by marine animals; entanglement; habitat damage

Land Pollution

Pollutant	Source	Effect
Pesticides	Agriculture	Kills non-target species; bioaccumulation
Herbicides	Agriculture	Reduces plant diversity
Landfill waste	Household and industrial rubbish	Leachate contaminates groundwater; methane production
Toxic waste	Mining, industry	Contaminates soil; makes land unsuitable for habitation

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Eutrophication

Eutrophication is a process by which excess nutrients (usually nitrates and phosphates from fertilisers) pollute water bodies and cause ecological damage.

The Process of Eutrophication

Step	What happens
1	Fertilisers are washed from farmland into rivers and lakes by rain (run-off)
2	Excess nitrates and phosphates stimulate rapid growth of algae — an algal bloom
3	The dense layer of algae blocks sunlight from reaching underwater plants
4	Underwater plants cannot photosynthesise and die
5	Decomposers (aerobic bacteria) break down the dead plants and algae
6	The bacteria use up large amounts of dissolved oxygen for respiration
7	Oxygen levels in the water drop — this is called deoxygenation
8	Fish and other aerobic aquatic organisms die due to lack of oxygen

graph TD
    A["Excess fertiliser enters water"] --> B["Algal bloom"]
    B --> C["Sunlight blocked"]
    C --> D["Aquatic plants die"]
    D --> E["Decomposers (bacteria) increase"]
    E --> F["Dissolved oxygen used up"]
    F --> G["Fish and aquatic organisms die"]

Exam Tip: Eutrophication is a very common exam question. Learn the full sequence (fertiliser → algal bloom → blocked light → plants die → decomposers use oxygen → fish die). The key link is that it is the decomposers that use up the oxygen, not the algae directly.

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Global Warming

Global warming is the gradual increase in the Earth's average temperature, largely caused by the enhanced greenhouse effect.

The Greenhouse Effect

Step	Description
1	The Sun emits short-wave radiation (light) that passes through the atmosphere and warms the Earth
2	The Earth re-emits longer-wave infrared radiation back towards space
3	Greenhouse gases (CO₂, methane, water vapour) in the atmosphere absorb some of this infrared radiation
4	The absorbed energy is re-radiated in all directions, including back to Earth, warming the surface

The natural greenhouse effect is essential — without it, Earth would be too cold for life. The problem is the enhanced greenhouse effect: human activities are increasing greenhouse gas levels, trapping more heat.

Consequences of Global Warming