AQA GCSE Biology 4.7.2 Organisation of an Ecosystem Exam Questions

About AQA GCSE Biology 4.7.2 Organisation of an Ecosystem

Organisation of an ecosystem is part of AQA Ecology, examined on Paper 2. It covers feeding relationships, fieldwork techniques for estimating population size and studying distribution, and how materials such as carbon and water are cycled. GCSE Biology students also study the factors affecting decomposition, compost and biogas, and (Higher tier) how environmental change affects species distribution. The required practicals are measuring the population size of a common species with quadrats and, for Biology, investigating the effect of temperature on the rate of decay of fresh milk by measuring pH change. Examiners often set population estimate calculations.

What you need to know

• 4.7.2.1 Levels of organisation Food chains start with producers, usually green plants or algae, which make glucose by photosynthesis. Primary consumers eat producers and are eaten by secondary consumers. In a stable community predator and prey populations rise and fall in cycles, with predator peaks lagging behind prey peaks.
• Required practical: sampling Place quadrats randomly to estimate population size: mean number per quadrat x total area / quadrat area. Use a transect with quadrats at intervals to study how distribution changes with an abiotic factor.
• 4.7.2.2 Cycling materials Microorganisms decay dead material, returning carbon to the atmosphere as carbon dioxide and mineral ions to the soil. In the carbon cycle, photosynthesis removes carbon dioxide; respiration and combustion release it. In the water cycle, evaporation, transpiration, condensation and precipitation move water through ecosystems.
• 4.7.2.3 Decomposition (Biology only) The rate of decay depends on temperature, water and oxygen availability. Gardeners and farmers make compost to use as natural fertiliser. Anaerobic decay produces methane, which can be used as biogas fuel.
• Required practical: decay (Biology only) Investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change, as bacteria produce lactic acid. Rate can be calculated as 1000 / time to reach a set pH.
• 4.7.2.4 Impact of environmental change (Biology only, Higher tier only) Changes in temperature, availability of water and composition of atmospheric gases can change the distribution of species. These changes may be seasonal, geographic or caused by human interaction.

Common mistakes in 4.7.2 Organisation of an Ecosystem

• Placing quadrats where organisms are most obvious. Use random coordinates for population estimates to avoid bias.
• Forgetting to scale up correctly. Divide the total area by the quadrat area, not just multiply the mean by the total area.
• Saying decomposers make carbon dioxide by photosynthesis. Decomposers release carbon dioxide by respiration.
• Saying decay is fastest at very high temperatures. Decay slows above the optimum as microbial enzymes denature.
• Saying the prey cycle follows the predator cycle. Predator numbers rise after prey numbers rise.

Try three 4.7.2 Organisation of an Ecosystem exam-style questions