This guide covers IGCSE Biology topic 19: organisms and their environment. You will revise ecosystem (habitat, community, population), food chains and food webs, pyramid of numbers and pyramid of biomass, and energy transfer (why only ~10% passes to next level).

Key concepts to revise

• Ecosystem – Habitat + community + non-living factors. Population = same species; community = all species.
• Food chain – Producer → primary consumer → secondary consumer → tertiary consumer. Use arrows (e.g. “is eaten by”).
• Food web – Several interconnected food chains; shows feeding relationships.
• Pyramid of biomass – Mass of organisms at each level; usually pyramid shape; energy lost at each level (respiration, excretion, not all eaten).

Exam tips and command words

• State – What a producer is (makes food by photosynthesis); what a consumer is (eats other organisms).
• Draw – A food chain with at least three levels; arrows in correct direction (energy flow).
• Describe – Why biomass/energy decreases at each level (respiration, waste, not all parts eaten).
• Explain – Why food chains rarely have more than four or five levels (energy lost at each step).

Common mistakes

• Drawing arrows the wrong way (arrow should show energy flow: from eaten to eater, e.g. grass → rabbit).
• Saying “energy is destroyed” (it is lost as heat in respiration; not all biomass is passed on).
• Confusing pyramid of numbers (can be inverted, e.g. one tree, many insects) with pyramid of biomass (usually pyramid shape).

Revision checklist

• Define ecosystem, habitat, population, community, producer, consumer.
• Draw a food chain and a simple food web; use correct arrow direction.
• Draw a pyramid of biomass and explain why it is pyramid-shaped.
• Explain why energy transfer between trophic levels is inefficient (~10%).

Next steps

Book a free trial with an IGCSE Biology tutor to practise ecology and food chain questions, or explore Tutopiya’s learning portal for more revision resources.