18.4 Deforestation and Peat Destruction

A free GCSE Biology lesson for AQA (8461) from OBA School: why forests and peat bogs are destroyed, and the effects on biodiversity and carbon dioxide. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Give the reasons for large-scale deforestation in tropical areas
• Explain how deforestation reduces biodiversity and increases carbon dioxide in the atmosphere
• Describe how peat forms and why peat bogs are destroyed
• Explain why burning or decay of peat adds carbon dioxide to the atmosphere
• Calculate rates of change from data on forests and peat

Key words

• Deforestation: The removal of large areas of forest, usually by cutting down or burning trees.
• Biofuel: A fuel made from living or recently living material, e.g. ethanol from sugar cane or oil from palm fruit.
• Peat: Dark, carbon-rich material made from partly decayed plants (mainly moss) that build up in waterlogged bogs.
• Peat bog: A wet, acidic habitat where peat forms. Bogs are home to specialised plants and animals.
• Carbon store: Something that holds carbon for a long time instead of releasing it as carbon dioxide, e.g. wood or peat.

Why forests are cut down

Large areas of tropical forest are cleared each year. The main reasons are to provide land for:

• cattle farming, mainly for beef
• rice fields (paddy fields) to feed growing populations
• growing crops for biofuels, such as sugar cane and oil palm

Timber is also sold, and land is cleared for roads, mines and new towns. All of these demands rise as the human population grows.

Key fact: Cattle and rice fields both release methane, another greenhouse gas, so clearing forest for them adds to global warming in two ways.

Effects of deforestation

• Less biodiversity; Why it happens Tropical forests contain a huge variety of species. Clearing them destroys habitats and food sources, and some species become extinct.
• More CO₂ released; Why it happens Trees are often burned to clear land, and wood left behind is decayed by microorganisms that respire.
• Less CO₂ removed; Why it happens Fewer trees means less photosynthesis, so less CO₂ is taken from the air.
• Less carbon stored; Why it happens Carbon that was locked up in wood for many years is no longer stored.

Worked example: rate of forest loss

In one region the forest area fell from 520 thousand km² in 2000 to 445 thousand km² in 2020.
Loss = 520 − 445 = 75 thousand km² over 20 years
Mean rate = 75 ÷ 20 = 3.75 thousand km² per year
Percentage lost = 75 ÷ 520 × 100 = 14.4%

Answer: 3.75 thousand km² per year; 14.4% of the forest lost

Peat bogs and their destruction

In a bog, dead plants do not fully decay because the soil is waterlogged and acidic, so there is little oxygen for decomposers. The plant remains build up very slowly, about 1 mm a year, as peat. Peat is a huge carbon store.

• Peat is dug up and sold as garden compost, because it helps plants grow. It is also burned as a fuel in some places.
• When a bog is drained, oxygen gets in and microorganisms decay the peat aerobically, releasing CO₂.
• Burning peat releases CO₂ straight away.
• Destroying bogs reduces the area of this habitat, so the plants, insects and birds that live there lose their home and biodiversity falls.

Worked example: how old is a peat bog?

A peat layer is 4.5 m deep. Peat builds up at about 1 mm per year. Estimate how long it took to form.
4.5 m = 4500 mm. 4500 ÷ 1 = 4500 years

Answer: about 4500 years

Exam tip: Peat forms so slowly that it is effectively non-renewable: we use it much faster than it can be replaced. Gardeners can help by choosing peat-free compost.

Common mistakes

• Giving only one way that deforestation increases CO₂. There are several: burning, decay by microorganisms, less photosynthesis, and loss of the carbon store.
• Saying the trees “release CO₂ when cut down”. It is burning or decay by microorganisms that releases it.
• Saying peat forms because there are no decomposers. There are decomposers, but they work slowly because there is little oxygen and the conditions are acidic.
• Forgetting the biodiversity effect: both forests and bogs are habitats, so destroying them removes species.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these is not a main reason for large-scale deforestation in tropical areas? (Choose from: To provide land for cattle, To provide land for rice fields, To grow crops for biofuels, To dig up peat for garden compost.)

Answer: To dig up peat for garden compost

Working: Tropical forests are cleared for cattle, rice fields and biofuel crops. Peat is dug from bogs.

Question 2. True or false? Peat forms because the waterlogged, acidic conditions in a bog have little oxygen, so decomposers work very slowly.

Answer: True

Working: True. Plant remains build up slowly as peat because decay is slow.

Question 3. A peat bog is 2.4 m deep. Peat forms at about 0.8 mm per year. Estimate how many years the bog took to form.

Answer: 3000 years

Working: 2.4 m = 2400 mm. 2400 ÷ 0.8 = 3000 years.

Related lessons

• Previous lesson: 18.3 Air Pollution
• Next lesson: 18.5 Global Warming
• In this chapter: 18.1 The Human Population Explosion
• In this chapter: 18.2 Land and Water Pollution
• In this chapter: 18.6 The Impact of Change

Continue in the full course

This lesson is section 18.4 (AGCBH1804) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.