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named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key point
Interpret food webs and predict effects of organism removal. Understand energy loss at each trophic level. Calculate population estimates from quadrat data. Extended: carbon and nitrogen cycles.
Topic 19 covers ecosystems, food chains and webs, energy flow through trophic levels, nutrient cycling (carbon and nitrogen cycles for Extended), and population sampling with quadrats. Food web interpretation is one of the most common question types.
Priority 1: Food chains and webs
Arrows show direction of energy transfer (prey → predator). Trophic levels: producer → primary consumer → secondary consumer → tertiary consumer. Be able to predict effects of removing one organism from a food web.
Priority 2: Energy transfer
Energy is lost at each trophic level through: respiration (heat), egestion (undigested material), and excretion. This is why food chains rarely have more than 4–5 trophic levels. Pyramid of energy always has a pyramid shape (unlike pyramids of numbers or biomass, which can be inverted).
Priority 3: Sampling and population estimates
Random sampling with quadrats: use random coordinates, count organisms, calculate mean per quadrat, scale up using the formula: population estimate = mean per quadrat × (total area ÷ quadrat area).
Frequently asked questions
Are nutrient cycles Core or Extended?
The carbon cycle and nitrogen cycle in detail are Extended. Core students need to know the basic concept that nutrients are recycled in ecosystems.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.