Edexcel International GCSE Biology Section 4: Ecology and the Environment Exam Questions

About Edexcel International GCSE Biology Section 4: Ecology and the Environment

Section 4 of Edexcel International GCSE Biology is examined across Paper 1 and Paper 2. It covers how organisms interact with their environment and each other, how quadrats are used to estimate population size and study distribution, feeding relationships and ecological pyramids, energy transfer, the carbon and nitrogen cycles, and human influences such as air and water pollution, the enhanced greenhouse effect and deforestation. It links to photosynthesis and respiration in Section 2. Examiners frequently ask for population estimates from quadrat data, energy efficiency calculations, the stages of eutrophication and the biological effects of deforestation.

What you need to know

• Key terms and sampling Know population, community, habitat and ecosystem. Use quadrats placed randomly to estimate population size and along a transect to study distribution. Biodiversity is affected by biotic factors (competition, predation, disease) and abiotic factors (light, temperature, water, soil pH).
• Feeding relationships Food chains and food webs show producers, primary, secondary and tertiary consumers and decomposers. Pyramids of number, biomass and energy can be drawn to show each trophic level.
• Energy transfer Only about 10% of energy is transferred between trophic levels; energy is lost through respiration, heat, movement, excretion and uneaten or undigested material. This limits the length of food chains.
• Carbon cycle Carbon dioxide is removed by photosynthesis and released by respiration, combustion of fossil fuels and decomposition. Carbon passes along food chains as carbon compounds.
• Nitrogen cycle Decomposers convert protein and urea into ammonia; nitrifying bacteria convert ammonia to nitrites and nitrates; nitrogen-fixing bacteria in soil and in root nodules of legumes convert nitrogen gas into nitrogen compounds; denitrifying bacteria convert nitrates back to nitrogen gas. Lightning also fixes nitrogen.
• Air pollution and greenhouse gases Sulfur dioxide causes acid rain; carbon monoxide combines with haemoglobin and reduces oxygen transport. Water vapour, carbon dioxide, nitrous oxide, methane and CFCs are greenhouse gases; increased levels cause the enhanced greenhouse effect and climate change.
• Water pollution and deforestation Sewage and fertiliser leaching cause eutrophication, leading to loss of oxygen and death of aquatic organisms. Deforestation causes leaching, soil erosion, disturbance of evapotranspiration and the carbon cycle, and an increase in atmospheric carbon dioxide.

Common mistakes in Section 4: Ecology and the Environment

• Saying energy is lost 'because organisms use it'. Specify how: respiration transfers energy to the surroundings as heat, and some is lost in faeces, urine and uneaten parts.
• Confusing nitrifying and denitrifying bacteria. Nitrifying bacteria make nitrates; denitrifying bacteria remove them, releasing nitrogen gas.
• Linking the greenhouse effect to the ozone layer. The enhanced greenhouse effect is caused by increased greenhouse gases trapping more heat.
• Saying algae use up the oxygen in eutrophication. Bacteria decomposing dead plants and algae use up the oxygen by respiration.
• Putting quadrats only where organisms are found. Random placement avoids bias when estimating population size.

Try three Section 4: Ecology and the Environment exam-style questions