Edexcel GCSE Biology Topic 9 Ecosystems and material cycles: a complete overview of ecosystems, energy transfer, biodiversity and the material cycles

What Topic 9 actually demands

Ecosystems and material cycles is the final topic and a strong source of data and calculation questions. The examiners test the levels of organisation, abiotic and biotic factors, sampling with quadrats and transects, energy transfer with biomass efficiency, human impact on biodiversity, and the carbon, water and nitrogen cycles. This guide ties together the three dot-point pages, each with its own practice questions.

Ecosystems and sampling

Ecosystems run from the organism, to the population (one species), to the community (all species), to the whole ecosystem (community plus environment). What lives where depends on abiotic factors (light, temperature, water, pH) and biotic factors (food, predators, disease, competition). Organisms are interdependent: parasitism harms one and benefits the other; mutualism benefits both. Population size is estimated with quadrats (random sampling) and belt transects (along a gradient), the latter being the core practical.

Energy transfer, biodiversity and human impact

Energy enters through producers and passes up trophic levels, but only about 10% transfers each step, lost in respiration (heat), waste and uneaten parts, which limits food-chain length. Humans reduce biodiversity through deforestation, pollution and overhunting, but can protect it through reforestation and conservation. Food security depends on population, pests and climate, and indicator species reveal pollution levels.

The carbon, water and nitrogen cycles

Materials are recycled. In the carbon cycle, photosynthesis removes carbon dioxide, and respiration, decomposition and combustion return it. In the water cycle, water evaporates, condenses and falls as rain, and potable water is made by treatment or desalination. In the nitrogen cycle, bacteria turn nitrogen into nitrates plants can use. Decomposition by microorganisms is fastest when warm, moist and oxygen-rich, the basis of food preservation and composting.

How Topic 9 is examined

• Calculations. Estimating population size from quadrats and energy-transfer efficiency between trophic levels.
• Data interpretation. Reading sampling data and food-chain or pyramid information.
• Cycles. Explaining the carbon, water and nitrogen cycles and the role of microorganisms.
• Evaluation. Weighing human impacts on biodiversity and the factors affecting food security.

Check your knowledge

A mix of recall and calculation questions covering Topic 9. Attempt them under timed conditions, then check against the solutions.

1. Define a community in an ecosystem. (1 mark)
2. State the difference between an abiotic and a biotic factor, with an example of each. (2 marks)
3. Explain the difference between parasitism and mutualism, with an example of each. (3 marks)
4. A mean of $5$ daisies is found in $1\ m^{2}$ quadrats in a $400\ m^{2}$ field. Estimate the total number of daisies. (2 marks)
5. State two reasons why energy is lost between trophic levels. (2 marks)
6. Producers contain $60\,000\ kJ$; the primary consumers contain $4800\ kJ$. Calculate the percentage of energy transferred. (2 marks)
7. Name the two processes that return carbon dioxide to the air in the carbon cycle. (2 marks)
8. Name the type of bacteria that convert nitrogen gas into nitrogen compounds. (1 mark)