WalesGeographySyllabus dot point

How do ecosystems work, and how do energy and nutrients move through them?

Key Idea 5.3: processes and interactions within ecosystems, the components of an ecosystem (biotic and abiotic), the flow of energy through food chains, food webs and trophic levels, the cycling of nutrients, and the global distribution and characteristics of major biomes.

A focused answer on Key Idea 5.3 for WJEC GCSE Geography Unit 2: the biotic and abiotic components of ecosystems, energy flow through food chains, food webs and trophic levels, nutrient cycling, and the global distribution and characteristics of major biomes.

Generated by Claude Opus 4.813 min answerUpdated 2026-06-15

Reviewed by: AI editorial process; not yet individually human-reviewed

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Quick answer

An ecosystem is a community of living things (biotic) interacting with each other and with their non-living surroundings (abiotic). Biotic components are producers, consumers and decomposers; abiotic components are things like climate, water, soil, light and air. Energy flows through the ecosystem in food chains and food webs. Producers (green plants) capture sunlight by photosynthesis; primary consumers (herbivores) eat the plants; secondary and tertiary consumers (carnivores) eat other animals. Each step is a trophic level, and energy is lost as heat at every level, which is why food chains are usually short and there are fewer top predators. A food web links many food chains together, so a change to one species affects others. Nutrients are recycled: when plants and animals die, decomposers (bacteria and fungi) break them down and return nutrients to the soil, where plants take them up again, so nutrients cycle between living things, the soil and the air. At the global scale, ecosystems form large biomes, whose distribution follows climate: tropical rainforests near the Equator (hot and wet, very high biodiversity), hot deserts around 30 degrees north and south (hot and dry), savanna grasslands, temperate forests and grasslands in the mid-latitudes, and cold tundra and polar regions. WJEC studies how energy, nutrients and climate shape these ecosystems.

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What this dot point is asking
The components of an ecosystem
Energy flow: food chains and webs
Nutrient cycling
Global biomes
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What this dot point is asking

This dot point covers Key Idea 5.3 of WJEC Unit 2: processes and interactions within ecosystems. You need the components of an ecosystem (biotic and abiotic), the flow of energy through food chains, food webs and trophic levels, the cycling of nutrients, and the global distribution and characteristics of major biomes.

The components of an ecosystem

Energy flow: food chains and webs

Nutrient cycling

Global biomes

Model answer: "Explain why food chains usually have only a few trophic levels"

A worked structure for a 6-mark explain question.

step Start with energy input

Explain that energy enters the ecosystem when producers capture sunlight by photosynthesis and store it as food.

step Show energy transfer

Show that energy passes up the chain as herbivores eat plants and carnivores eat herbivores, moving from one trophic level to the next.

step Explain the energy loss

Explain that at each level most energy is lost as heat through respiration, movement and waste, so only a small fraction passes to the next level.

step Reach the conclusion

Conclude that because so little energy reaches higher levels, there is not enough to support many top predators, which is why food chains are short and top consumers are few.

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Q1. What is a trophic level? [Knowledge recall]

Cue. A feeding stage in a food chain, such as producers, primary consumers (herbivores) or secondary consumers (carnivores); energy is lost as heat at each level moving up the chain.

Q2. Explain the difference between how energy and nutrients move through an ecosystem. [Short explanation]

Cue. Energy flows in one direction through the food chain from producers to consumers and is lost as heat at each level, so it must be constantly replaced by sunlight; nutrients, by contrast, are recycled round the ecosystem by decomposers and reused by plants again and again.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC Unit 2 (Theme 5)4 marksDescribe how energy flows through a food chain.

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A short data-response describe question. Reward a clear description of energy moving between trophic levels.

The start. Energy enters when producers (green plants) capture sunlight by photosynthesis and store it as food.

The flow. Energy passes to primary consumers (herbivores) that eat the plants, then to secondary and tertiary consumers (carnivores). Energy is lost as heat at each stage, so less is available higher up the chain.

Top marks. Producer to consumers, with energy lost at each trophic level.

WJEC Unit 2 (Theme 5)6 marksExplain how energy and nutrients move through an ecosystem.

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A short explain question (levels marking). Reward developed links between energy flow and nutrient cycling.

Energy. Producers capture solar energy by photosynthesis; it then flows along the food chain to consumers, with energy lost as heat at each trophic level, which is why food chains are usually short.

Nutrients. When plants and animals die, decomposers break them down, returning nutrients to the soil; plants take these up again, so nutrients are recycled between living things, the soil and the air.

Top band. Show energy flows through (and is lost) while nutrients cycle round and are reused.

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Sources & how we know this

WJEC GCSE Geography (Wales) specification (3110) — WJEC (2019)