Every habitat supports a web of species that depend on each other. When people clear land for houses and farms, dig out natural resources or pollute rivers and seas, those habitats shrink. Deforestation is the best-studied example: it removes far more than the trees.
Biodiversity is the number of different species that live in an area. Habitats are destroyed to provide land for housing, crop plants and livestock, to extract natural resources and by freshwater and marine pollution. By altering food chains and food webs, humans can have a negative impact on habitats. Deforestation reduces biodiversity, can cause extinction, and leads to loss of soil, flooding and an increase of carbon dioxide in the atmosphere.
Fig. 1: the undesirable effects of deforestation
Section 01
Biodiversity and why habitats disappear.
Drag the slider to clear the forest, 5% at a time. Watch the number of species and the share of the original carbon dioxide uptake that is left, then press Heavy rain to see what happens to the soil and the river.
Biodiversity: the number of different species that live in an area. In the sim the intact forest holds 28 species; clearing it all leaves only 2.
Land for people: more area for housing, crop plants and livestock as the population grows.
Extraction of natural resources: timber, and mining for minerals, metals and fossil fuels.
Pollution: freshwater and marine pollution destroys aquatic habitats (see the Pollution lesson).
Food chains and webs: removing producers, hunting predators or introducing new species changes food chains and food webs, so humans can have a negative impact on habitats even without clearing them.
Examiner trap
Biodiversity is the number of different species, not the number of individual organisms. A field of a million wheat plants has very low biodiversity.
Stage 1 · Learn
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In the deforestation sim, what happens to the number of species as more of the forest is cleared?
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With 50% of the forest cleared, what does the CO2 TAKEN UP reading show?
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What happens when heavy rain falls on a largely cleared forest in the sim?
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Which best describes biodiversity?
Section 02
Five effects of deforestation.
Deforestation is the clearing of forest, often by cutting and burning. Each effect follows from something the trees were doing.
Effect
Explanation
Reduced biodiversity
habitats and food sources are lost, so many species cannot survive in the area
Extinction
species found only in that forest may die out completely
Loss of soil
no roots hold the soil and no canopy shields it, so rain washes it away (soil erosion); the remaining soil loses its mineral ions
Flooding
trees no longer absorb water or slow its flow, so rain runs straight off into rivers, which flood; eroded soil also silts up the river
More carbon dioxide in the atmosphere
fewer trees take in carbon dioxide for photosynthesis, and burning or decomposing wood releases carbon dioxide
Knock-on effects: the extra carbon dioxide adds to the enhanced greenhouse effect; less transpiration can mean less rainfall in the region.
Examiner trap
Do not write that deforestation "reduces oxygen so animals suffocate". The expected effects are lower biodiversity, extinction, soil loss, flooding and more carbon dioxide.
Stage 2 · Exam
Exam-style questions
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Which is NOT one of the main reasons for habitat destruction?
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Why does deforestation increase the risk of flooding downstream?
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Clearing and burning forest increases carbon dioxide in the atmosphere because:
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Farmers kill all the foxes in an area. Rabbit numbers then rise and crops are damaged. This shows that humans:
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Habitat destruction, mastered.
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