AQA GCSE Combined Science (8464), Higher tier · Biology › Ecology › Biodiversity and the effect of human interaction on ecosystems
Practise Deforestation. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Large areas of tropical forest have been cut down to make room for cattle and rice fields and to grow crops for biofuels. Expect to explain why this happens, and how it reduces biodiversity and increases the carbon dioxide in the atmosphere.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define deforestationCutting down large areas of forest so the land can be used for something else.
4
Give reasons for tropical deforestationTo provide land for cattle and rice fields, and to grow crops for biofuels.
5
Explain how deforestation reduces biodiversityForest habitats are destroyed and replaced by grazing land or one crop, so many species are lost.
6
Explain how deforestation increases atmospheric carbon dioxideBurning releases CO2, microorganisms respire as the wood decays, and fewer trees remove CO2 by photosynthesis.
7
Explain how cattle and rice fields add methaneCattle release methane from digestion, and microorganisms in flooded rice fields release methane.
7
Evaluate clearing forest for crops or cattleWeigh food, income and renewable fuel against habitat loss and the carbon dioxide released.
Notes
Why tropical forests are cut down
Large-scale deforestation in tropical areas has happened to:
provide land for cattle and for rice fields, producing food for a growing population;
grow crops for biofuels, such as sugar cane or oil palm, which are used to make fuels.
Effect on biodiversity
Tropical forests have very high biodiversity. Clearing them destroys the habitats of many species.
The forest is replaced by grassland for cattle or by large fields of a single crop, which support far fewer species.
Species that live only in that forest may become extinct.
Effect on the atmosphere
Trees remove carbon dioxide from the air by photosynthesis and store the carbon in their wood. Fewer trees means less carbon dioxide is removed.
The trees are often burned to clear the land. Combustion releases carbon dioxide.
Wood and roots left to rot are broken down by microorganisms, whose respiration releases carbon dioxide.
More carbon dioxide in the atmosphere contributes to global warming.
Cattle and flooded rice fields also release methane, another gas that contributes to global warming. grade 7+
Conflicting needs
People in tropical countries need land, food and income, and biofuels are a renewable alternative to fossil fuels.
But clearing forest releases a large amount of carbon dioxide and destroys habitats that cannot quickly be replaced.
Cheatsheet
Tropical deforestation: land for cattle, rice fields and biofuel crops
Fewer trees → less CO2 removed by photosynthesis
Burning trees → CO2 released by combustion
Rotting wood → microorganisms respire → CO2 released
Habitat loss → lower biodiversity
Cattle and rice fields → methane grade 7+
How to answer each type of question
Give reasons for deforestation
2 marks4
Use the reasons in the specification: land for cattle, land for rice fields, crops for biofuels.
Link them to a growing population needing food and fuel if the question asks you to explain.
Example. Give two reasons why large areas of tropical forest have been cut down.
Show the model answer
Any two from: to provide land for cattle (1); to provide land for rice fields (1); to grow crops for biofuels (1)
Explain how deforestation affects biodiversity
2 to 3 marks5
Say that habitats (food and shelter) are destroyed.
Say that one crop or grazing land replaces the forest, so fewer species can live there.
Conclude that biodiversity falls and species may become extinct.
Example. An area of tropical forest is cleared to grow oil palm. Explain how this affects biodiversity. (3 marks)
Show the model answer
The forest habitat, with food and shelter for many species, is destroyed (1). Only one plant species is grown, so few animal species can find food or shelter there (1). So biodiversity is reduced and some species may become extinct (1).
Explain how deforestation increases carbon dioxide in the atmosphere
3 marks6
Combustion: burning the trees releases carbon dioxide.
Decay: microorganisms respire as they break down the remaining wood, releasing carbon dioxide.
Less photosynthesis: fewer trees take carbon dioxide out of the air.
Example. A forest is cleared by cutting down and burning the trees. Explain three ways in which this increases the concentration of carbon dioxide in the atmosphere.
Show the model answer
Burning the trees releases carbon dioxide by combustion (1). Branches and roots that are left decay, and the microorganisms release carbon dioxide by respiration (1). There are fewer trees to remove carbon dioxide from the air by photosynthesis (1).
6-mark: evaluate clearing forest for a crop
6 marks7
Give benefits: food, income and jobs; biofuels as a renewable fuel.
Give environmental costs: loss of biodiversity, more carbon dioxide, global warming.
Use any data given.
Finish with a justified conclusion.
Example. A government is deciding whether to allow an area of tropical forest to be cleared to grow sugar cane for biofuel. Evaluate this proposal.
Show the model answer
Level-marked. A level 3 answer (5 to 6 marks) covers both sides in detail and gives a justified conclusion, for example: biofuel is renewable, so it can replace some fossil fuels; the crop provides jobs and income; however, burning and decay of the trees release carbon dioxide, and fewer trees remain to remove carbon dioxide by photosynthesis, adding to global warming; the forest habitat is destroyed, so biodiversity falls and species may become extinct. Conclusion: the forest should not be cleared, because the carbon dioxide released and the loss of biodiversity outweigh the benefits; the sugar cane could be grown on land that has already been cleared.
Shortcuts and memory tricks
Reasons, CRB: Cattle, Rice, Biofuels.
Three ways carbon dioxide rises: burning, rotting, and less photosynthesis.
Almost every mark on this subtopic comes from biodiversity or carbon dioxide. Link each point to one of them.
Where marks are lost
Writing that deforestation 'means less oxygen for us to breathe'. This rarely scores; focus on carbon dioxide and biodiversity.
Saying the trees 'release carbon dioxide' without the process: combustion when burned, or respiration by microorganisms when they decay.
Forgetting that fewer trees means less photosynthesis, so less carbon dioxide is removed from the air.
Exam technique
Give three separate ways deforestation increases carbon dioxide: burning, decay and less photosynthesis.
For 6-mark evaluations, include people's needs as well as the environmental costs, and end with a judgement.
Write 'microorganisms respire', not just 'the wood decays'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Deforestation means that less carbon dioxide is removed from the atmosphere. Name the process in trees that removes carbon dioxide from the atmosphere.
Photosynthesis
Complete the sentence. Large-scale deforestation in tropical areas has happened to provide land for cattle and ....................... fields, and to grow crops for ....................... .
rice; biofuels
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Large areas of tropical forest have been cut down.
(a) Which two are reasons for large-scale tropical deforestation? Tick (✓) two boxes.[2]
To provide land for cattle and rice fields
To grow crops for biofuels
To increase biodiversity
To reduce the carbon dioxide in the atmosphere
(b) Deforestation means that less carbon dioxide is removed from the atmosphere. Name the process in trees that removes carbon dioxide from the atmosphere.[1]
(c) Give one effect of deforestation on biodiversity. Give a reason for your answer.[2]
Show the answer and mark scheme
(a)Answer: To provide land for cattle and rice fields; To grow crops for biofuels
(b)Answer: Photosynthesis
photosynthesis
(c)
biodiversity is reduced / there are fewer species
because habitats / food / shelter for many species are destroyed / species may become extinct
Question 2Medium6 marks
In some tropical countries, forests are cleared by cutting down the trees and burning them. The land is then used to grow crops.
(a) Explain how clearing forest in this way increases the concentration of carbon dioxide in the atmosphere.[3]
(c) Some forests are cleared to grow crops for biofuels. Suggest one reason why some people support growing crops for biofuels.[1]
Show the answer and mark scheme
(a)
burning (combustion of) the trees releases carbon dioxide
microorganisms decay the remaining wood and roots, releasing carbon dioxide in respiration
fewer trees means less photosynthesis, so less carbon dioxide is removed from the air
carbon that was stored in the trees for many years is released
(b)
the habitats (food and shelter) of many species are destroyed
tropical forests contain very many species, but the crop field contains only one or a few species / species may become extinct
(c)
biofuels are renewable
they reduce the use of fossil fuels
they provide income / jobs for local people
Question 3Hard6 marks
In one study, clearing 1 hectare of tropical forest to grow a crop for biofuel released about 600 tonnes of carbon dioxide from the burning and decay of the trees and from the soil. Using the biofuel from 1 hectare instead of diesel reduces carbon dioxide emissions by about 7.5 tonnes per year.
(a) Calculate how many years it would take for the biofuel to make up for the carbon dioxide released by clearing the forest.[2]
(b) If the same crop is grown on land that is already farmland, about 60 tonnes of carbon dioxide are released per hectare when the crop is planted. Calculate the number of years needed in this case.[1]
(c) Evaluate growing biofuel crops on land cleared from tropical forest, compared with growing them on existing farmland.[3]
Show the answer and mark scheme
(a)Answer: 80 years
600 ÷ 7.5
80 (years)
(b)Answer: 8 years
60 ÷ 7.5 = 8 (years)
(c)
clearing forest releases much more carbon dioxide, so it takes 80 years (compared with 8 years) before the biofuel gives any benefit
clearing forest destroys habitats and reduces biodiversity
biofuels are renewable / reduce the use of fossil fuels
using existing farmland for biofuels leaves less land to grow food, which could threaten food security
a justified conclusion, e.g. biofuel crops should be grown on existing farmland, not on cleared forest