Cambridge IGCSE Biology 20 Human Influences on Ecosystems Exam Questions

About Cambridge IGCSE Biology 20 Human Influences on Ecosystems

Human influences on ecosystems looks at how people affect the environment in Cambridge IGCSE Biology. Core candidates describe how modern technology increases food production, the effects of monocultures and intensive livestock production, the causes and effects of habitat destruction and deforestation, pollution by fertilisers, sewage and plastics, the greenhouse effect and climate change, and methods of conservation. Extended candidates explain eutrophication step by step, the effects of non-biodegradable plastics, how pollution by female contraceptive hormones affects fish, sustainable resources, and conservation through seed banks, captive breeding and artificial insemination. Long answers on eutrophication and deforestation are frequent and reward accurate sequencing.

What you need to know

• 20.1 Food supply Agricultural machinery, chemical fertilisers, insecticides, herbicides and selective breeding increase food production. Monocultures and intensive livestock production raise output but reduce biodiversity, increase pest spread and pollution, and can raise disease risk.
• 20.2 Habitat destruction Reasons include increased land for food production and housing, extraction of natural resources and freshwater and marine pollution. Deforestation causes extinction, loss of soil, flooding and increased carbon dioxide in the atmosphere.
• 20.3 Pollution Untreated sewage and excess fertiliser cause eutrophication; non-biodegradable plastics harm aquatic and terrestrial ecosystems. Increased carbon dioxide and methane enhance the greenhouse effect, leading to climate change.
• 20.3 Eutrophication (Extended) Fertiliser runs off into water; increased nitrate and other ions cause increased growth of producers; producers die when light is blocked; decomposers break down the dead material and respire aerobically; oxygen concentration falls; fish and other aquatic organisms die.
• 20.4 Sustainable resources A sustainable resource is one that is produced as rapidly as it is removed from the environment, so it does not run out. Forests can be conserved with replanting and quotas; fish stocks with quotas, closed seasons, protected areas, controlled net mesh size and education.
• 20.4 Conservation Endangered species are at risk from habitat loss, hunting, pollution and introduced species. Conservation methods include monitoring and protecting habitats, education, captive breeding and seed banks. Extended: artificial insemination and IVF in captive breeding, and reasons for conservation such as maintaining biodiversity and ecosystem services.

Common mistakes in 20 Human Influences on Ecosystems

• Saying fish die in eutrophication because of the fertiliser. They die from lack of oxygen, used by decomposers respiring aerobically.
• Saying algae use up all the oxygen. The main oxygen loss is from aerobic respiration of decomposers breaking down dead producers.
• Confusing the greenhouse effect with ozone depletion. The enhanced greenhouse effect is due to gases such as carbon dioxide and methane trapping heat.
• Saying deforestation increases carbon dioxide only because trees are burned. Fewer trees also means less photosynthesis to remove carbon dioxide.
• Defining a sustainable resource as one that never runs out. It is produced as rapidly as it is removed.

Try three 20 Human Influences on Ecosystems exam-style questions