EDEXCEL IGCSE BIOLOGY
Higher Tier — Complete Revision Guide
Paper 2 Topics: 4–5
Covers: Ecology & the Environment | Use of Biological Resources
Includes worked calculation examples, key definitions, exam-style Q&A, and common mistakes to avoid.
Companion guide to Paper 1 (Topics 1–3) — the complete set covers the full specification.
, Topic 4: Ecology and the Environment
4.1 Key ecological terms
Ecosystem: A community of organisms interacting with each other and with their physical environment
Habitat: The place where an organism lives
Population: All the organisms of one species living in a habitat at the same time
Community: All the populations of different species living and interacting in a habitat
Producer: An organism (usually a green plant or alga) that makes its own food by photosynthesis
Consumer: An organism that gets its energy by eating other organisms
Decomposer: An organism that breaks down dead organisms and waste material, releasing nutrients back into the
ecosystem
4.2 Food chains, food webs and trophic levels
A food chain shows the transfer of energy from one organism to the next, always starting with a producer. Arrows point
in the direction energy flows:
grass → rabbit → fox
Each stage is a trophic level:
• Trophic level 1: Producers (e.g. grass)
• Trophic level 2: Primary consumers / herbivores (e.g. rabbit)
• Trophic level 3: Secondary consumers / carnivores (e.g. fox)
• Trophic level 4: Tertiary consumers (top predators)
A food web shows multiple interconnected food chains — more realistic, since most organisms eat more than one food
source. If one species is removed, it can affect several other populations across the web, not just one chain.
4.3 Energy flow and pyramids of biomass
Energy is lost between each trophic level, mainly through:
• Respiration (released as heat)
• Movement
• Egestion (undigested material lost as waste) and excretion
• Parts of the organism not eaten by the next consumer (e.g. bones, roots)
Because of this loss, only around 10% of energy is transferred from one trophic level to the next. This is why food
chains rarely have more than 4–5 levels, and why pyramids of biomass get narrower at each level going up.
Worked Example: Percentage energy transfer
A population of producers contains 8000 kJ of energy. The primary consumers that eat them contain 800 kJ.
Percentage transferred = (energy in consumer ÷ energy in producer) × 100
= (800 ÷ 8000) × 100 = 10%
This matches the typical ~10% rule for energy transfer between trophic levels.
4.4 Nutrient cycles
The Carbon Cycle — key processes:
• Photosynthesis removes CO₂ from the atmosphere, converting it into glucose in plants
• Respiration (by plants, animals and decomposers) releases CO₂ back into the atmosphere
Higher Tier — Complete Revision Guide
Paper 2 Topics: 4–5
Covers: Ecology & the Environment | Use of Biological Resources
Includes worked calculation examples, key definitions, exam-style Q&A, and common mistakes to avoid.
Companion guide to Paper 1 (Topics 1–3) — the complete set covers the full specification.
, Topic 4: Ecology and the Environment
4.1 Key ecological terms
Ecosystem: A community of organisms interacting with each other and with their physical environment
Habitat: The place where an organism lives
Population: All the organisms of one species living in a habitat at the same time
Community: All the populations of different species living and interacting in a habitat
Producer: An organism (usually a green plant or alga) that makes its own food by photosynthesis
Consumer: An organism that gets its energy by eating other organisms
Decomposer: An organism that breaks down dead organisms and waste material, releasing nutrients back into the
ecosystem
4.2 Food chains, food webs and trophic levels
A food chain shows the transfer of energy from one organism to the next, always starting with a producer. Arrows point
in the direction energy flows:
grass → rabbit → fox
Each stage is a trophic level:
• Trophic level 1: Producers (e.g. grass)
• Trophic level 2: Primary consumers / herbivores (e.g. rabbit)
• Trophic level 3: Secondary consumers / carnivores (e.g. fox)
• Trophic level 4: Tertiary consumers (top predators)
A food web shows multiple interconnected food chains — more realistic, since most organisms eat more than one food
source. If one species is removed, it can affect several other populations across the web, not just one chain.
4.3 Energy flow and pyramids of biomass
Energy is lost between each trophic level, mainly through:
• Respiration (released as heat)
• Movement
• Egestion (undigested material lost as waste) and excretion
• Parts of the organism not eaten by the next consumer (e.g. bones, roots)
Because of this loss, only around 10% of energy is transferred from one trophic level to the next. This is why food
chains rarely have more than 4–5 levels, and why pyramids of biomass get narrower at each level going up.
Worked Example: Percentage energy transfer
A population of producers contains 8000 kJ of energy. The primary consumers that eat them contain 800 kJ.
Percentage transferred = (energy in consumer ÷ energy in producer) × 100
= (800 ÷ 8000) × 100 = 10%
This matches the typical ~10% rule for energy transfer between trophic levels.
4.4 Nutrient cycles
The Carbon Cycle — key processes:
• Photosynthesis removes CO₂ from the atmosphere, converting it into glucose in plants
• Respiration (by plants, animals and decomposers) releases CO₂ back into the atmosphere
