Deforestation and habitat lossIB MYP Biology: Revision notes
Section 1
What is deforestation?
Deforestation is the large-scale removal of trees from an area without replacing them. It is happening fastest in tropical rainforests such as the Amazon, Central Africa and South-East Asia. The land is cleared by cutting the trees down or by burning them.
Section 2
Why are forests cleared?
People clear forests for several reasons:
- Farming: land is needed for crops (such as soya and palm oil) and for cattle ranching. A growing population needs more food.
- Timber: wood is sold for building, furniture and paper.
- Building: land is cleared for roads, houses, towns and mines.
Many of these bring money and jobs to the people and countries involved, which is why deforestation is hard to stop.
In an exam, give a specific reason such as 'to grow palm oil' or 'to graze cattle', not just 'for land'.
Section 3
Consequences for the climate
Trees take in carbon dioxide during photosynthesis and store the carbon in their wood. When forests are cleared:
- Burning and decay of the trees release carbon dioxide into the atmosphere.
- There are fewer trees to take carbon dioxide out of the air.
- Carbon dioxide is a greenhouse gas, so more of it traps more heat and adds to global warming.
Forests also release water vapour, so losing them can reduce local rainfall.
Do not say that burning trees 'uses up the ozone layer' or 'makes holes in the sky'. The point is that more carbon dioxide stays in the air, which increases the greenhouse effect.
Section 4
Consequences for the soil
Tree roots bind the soil together and the leaves and branches (the canopy) shelter it from heavy rain. Without trees:
- Rain hits bare soil and washes the fertile topsoil away. This is soil erosion.
- The eroded soil is washed into rivers, where it causes silting and can increase flooding.
- The land becomes less fertile, so crops and new trees grow poorly.
Section 5
Consequences for biodiversity
Biodiversity is the variety of living things in an area. Forests, especially tropical rainforests, have very high biodiversity.
- Clearing trees destroys the habitats of animals and plants, so they lose their food and shelter.
- Populations fall, and species that live nowhere else may become extinct.
- Food webs are broken, because losing one species affects the species that eat it or are eaten by it.
- Replacing forest with one crop (monoculture) supports far fewer species.
Section 6
Peat bog destruction
Peat forms in waterlogged bogs, where the water slows decay so that dead plant material builds up as a carbon-rich store.
When peat bogs are drained and cleared (for farming, plantations or fuel):
- The peat dries and decomposes, or is burned, releasing carbon dioxide.
- The specialised plants and animals that live only in the bogs lose their habitat.
Peat takes thousands of years to form, so it cannot be replaced quickly. For this reason it is described as a non-renewable resource.
Link peat to climate change in three steps: peat stores carbon, draining releases carbon dioxide, carbon dioxide is a greenhouse gas.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Deforestation and habitat loss
- A research team in the Brazilian Amazon has compared satellite records of one region taken 20 years apart. The earlier records show unbroken rainforest. The recent records show large rectangular areas of bare ground, new roads and industrial-scale farms where forest once stood.Outline two reasons why rainforest is cleared in regions such as this.2 marks
- Students at a school in the Kenyan highlands wanted to find out how plant cover affects soil erosion. They filled two identical trays with the same mass of soil and placed both on the same slope. One tray was left as bare soil and the other was covered with grass turf. They poured 2 dm³ of water over each tray at the same rate and collected the water that ran off. The runoff was filtered and the soil in it was dried and weighed. The bare tray lost 48 g of soil and the grass tray lost 6 g.Explain what the results show about the effect of plant cover on soil erosion.2 marks
- In Sumatra, Indonesia, large areas of peat bog forest have been drained and cleared to make way for oil palm plantations. Peat forms where waterlogged ground slows the decay of dead plant material. The bogs were home to specialised species such as orangutans, which live only in these swampy forests.Explain why destroying peat bogs adds to climate change.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).