Deforestation — Human Impact on the Environment | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

increasing

Medium

Increasing10%

Climate change, deforestation, and biodiversity loss are growing in frequency as contemporary issues.

Deforestation is the large-scale clearing of forests, usually to make room for agriculture (especially cattle ranching, palm oil and soy plantations), logging for timber, road and city building, or mining.

The world has lost about a third of its forests since the start of the agricultural revolution. Tropical rainforests are being lost at a rate of around 10 million hectares per year. Each lost hectare typically contained hundreds of species of plants, insects, and animals.

Five major consequences of deforestation

1. Loss of

Forests, especially tropical rainforests, are home to a huge fraction of Earth's species. Cutting down a rainforest destroys habitats and pushes many species to extinction. Some are lost before they have even been discovered or studied.

2. Soil erosion

Tree roots hold soil together. Without trees, heavy rain washes the topsoil away, leaving infertile subsoil. The lost soil ends up in rivers, where it can clog spawning beds and reduce water quality. Many tropical regions now have impoverished farmland that was tropical forest 50 years ago.

3. Leaching of soil minerals

Trees absorb minerals (especially nitrate) from the soil through their roots, holding them in plant tissue. When the trees are gone, those minerals are washed out of the soil by rainfall (leaching) into waterways. The soil becomes nutrient-poor and unable to support agriculture for long.

4. Disruption of the water cycle

Forests release enormous amounts of water vapour through transpiration. The Amazon rainforest, for example, transpires enough water vapour to generate much of its own rainfall. When the forest is cleared:

Less transpiration → less water vapour in the air → fewer clouds → less rainfall
The area becomes drier
Streams and rivers can dry up
Other vegetation cannot grow back as easily

5. Disruption of the carbon cycle and accelerated

This effect is the biggest at global scale:

Forests are huge carbon sinks: trees hold enormous amounts of carbon in their wood (and in the soil beneath them). When forests are cut down and burned, that stored carbon is released as CO₂ to the atmosphere.
Removing the trees also reduces future photosynthesis, so less CO₂ is being absorbed from the atmosphere in years to come.
Both effects push atmospheric CO₂ higher, accelerating climate change.
Deforestation is responsible for around 10–15% of human-caused CO₂ emissions.

Exam tip

Explain how deforestation increases atmospheric carbon dioxide

What comes up: "explain how deforestation increases carbon dioxide in the atmosphere" — often 2 marks, sometimes combined with a longer question on the effects of deforestation.

Write (two marks): (1) when trees are burned or left to decay, the carbon stored in their wood is released as carbon dioxide (through combustion or decomposition); (2) with fewer trees, less photosynthesis occurs, so less CO₂ is removed from the atmosphere — both effects cause atmospheric CO₂ to rise.

Watch out: both mechanisms are worth a mark; giving only one (e.g. only mentioning burning, or only mentioning less photosynthesis) risks losing half the marks. The mark scheme accepts either mechanism independently, so state both to be safe.

What is being done

Reforestation and afforestation: planting new forests and replanting cleared areas
Protecting old-growth forests through laws and national parks
Sustainable forestry: managed harvesting where trees are replaced as they are taken
Reducing demand: choosing products certified as not linked to deforestation (e.g. FSC-certified timber, RSPO palm oil)
Paying tropical countries to protect their forests, through schemes like REDD+