Human Impact on the Environment
Ecology and the Environment
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.
Worked example
Mean rate of forest loss
A country's forest covered 5 420 km² in 2004 and 5 228 km² in 2024. (a) Calculate the mean rate of decrease in forest area, in km² per year. (b) Calculate the loss in km² per month, giving your answer in standard form.
Solution:
- Decrease in area = 5 420 − 5 228 = 192 km²
- Time = 2024 − 2004 = 20 years
- (a) Mean rate of decrease = 192 ÷ 20 = 9.6 km² per year
- (b) 20 years = 20 × 12 = 240 months, so 192 ÷ 240 = 0.8 km² per month = 8.0 × 10⁻¹ km² per month
Write the subtraction and the division down: in both papers that set this calculation the first mark was for that step (or for the right divisor, such as the number of months), and a correct answer on its own earned both marks.
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.
Common exam question
Explaining the effects of deforestation
Question: Describe the effects of deforestation on an ecosystem, or explain one effect: why it raises atmospheric carbon dioxide, or how it changes a nearby river (2–4 marks).
Asked in 4 of the 23 papers, all on Paper 2. The credited effects: loss of biodiversity and habitat, more carbon dioxide in the atmosphere (less is absorbed; felled trees release it as they burn or decay), less photosynthesis, soil erosion, leaching of minerals so the soil is less fertile, flooding, and a disturbed water cycle with less transpiration and less rainfall. In the 4-mark question the carbon dioxide rise and the fall in photosynthesis are separate marks; in a carbon-sink question either earns the single deforestation mark.
For a river below the cleared land, continue the chain: eroded soil and leached nitrate wash in, algae grow, light is blocked, plants die, bacteria decompose them and respire, oxygen falls, animals die and biodiversity drops. Say "leaching" or "run-off": "nutrients" is ignored.
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+