Edexcel GCSE Biology Topic 9: Ecosystems and Material Cycles Exam Questions

About Edexcel GCSE Biology Topic 9: Ecosystems and Material Cycles

Topic 9 of Edexcel GCSE Biology is assessed on Paper 2 and completes the course. It covers the organisation of ecosystems, biotic and abiotic factors, parasitism and mutualism, and the core practical using quadrats and belt transects. It looks at human effects on biodiversity, including fish farming, introduced species and eutrophication, and at the carbon, water and nitrogen cycles. GCSE Biology students also study energy transfer between trophic levels, food security, indicator species, decomposition and how potable water is produced. Examiners often ask for population estimates, efficiency calculations and explanations of eutrophication and the nitrogen cycle.

What you need to know

• Ecosystems and interdependence Levels run from individual to population, community and ecosystem. Organisms are interdependent and compete for resources. Biotic factors include predation, competition and disease; abiotic factors include light intensity, temperature, water, pH and pollutants.
• Parasitism and mutualism In parasitism one organism benefits at the expense of the host, for example fleas, head lice and tapeworms. In mutualism both benefit, for example oxpeckers on large mammals and nitrogen-fixing bacteria in legume root nodules.
• Core practical: fieldwork Use randomly placed quadrats to estimate population size and a belt transect to investigate how the distribution of organisms changes with an abiotic factor.
• Energy transfer (Biology only) Only a small proportion of energy (about 10%) is transferred between trophic levels; the rest is lost in respiration, heat, movement, waste and uneaten parts. Calculate efficiency and interpret pyramids of biomass.
• Biodiversity Fish farming, introduction of non-indigenous species and eutrophication (fertiliser run-off causing algal blooms, plant death, decomposition by bacteria and lowered oxygen) reduce biodiversity. Reforestation and conservation schemes help maintain it.
• Material cycles Carbon is cycled by photosynthesis, respiration, combustion and decomposition. Water cycles by evaporation, transpiration, condensation and precipitation. In the nitrogen cycle, nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds, nitrifying bacteria convert ammonia to nitrates, and denitrifying bacteria return nitrogen to the air. Crop rotation and fertilisers raise soil nitrates.
• Biology only extras Food security is affected by population growth, changing diets, pests, climate change and biofuels. Indicator species such as lichens (air pollution) and freshwater invertebrates (water pollution) show pollution levels. Decomposition depends on temperature, water and oxygen. Potable water can be produced by desalination.

Common mistakes in Topic 9: Ecosystems and Material Cycles

• Describing eutrophication as fertiliser poisoning fish. Fertiliser causes algal growth that blocks light, plants die, and decomposing bacteria use up the oxygen.
• Mixing up nitrifying and nitrogen-fixing bacteria. Nitrogen-fixing bacteria convert nitrogen gas; nitrifying bacteria convert ammonia into nitrates.
• Saying mutualism is when organisms live together. Mutualism means both species benefit.
• Placing quadrats non-randomly for population estimates. Use random coordinates; use a belt transect only for studying distribution.
• Saying energy is lost as 'energy for movement' without explanation. Energy is transferred to the surroundings, mainly by heating, during respiration.

Try three Topic 9: Ecosystems and Material Cycles exam-style questions