How does energy and food pass through an ecosystem, and why are food chains short?
Feeding relationships in ecosystems, food chains and food webs, pyramids of number and biomass, the transfer of energy along a food chain and why energy is lost at each trophic level.
A focused answer to the WJEC GCSE Biology section 1.6 topic on feeding relationships, covering food chains and food webs, pyramids of number and biomass, the transfer of energy along a food chain, why energy is lost at each trophic level, and the implications for food production.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
WJEC wants you to use the terms producer, consumer and decomposer, construct and interpret food chains, food webs and pyramids, explain how energy is transferred and lost along a food chain, and explain why food chains are short.
Feeding relationships
Organisms in an ecosystem depend on each other for food. Three key roles describe them:
- Producers: green plants (and algae) that make their own food by photosynthesis. They are always the start of a food chain.
- Consumers: organisms that eat other organisms. A herbivore eats plants, a carnivore eats animals, and an omnivore eats both.
- Decomposers: bacteria and fungi that break down dead organisms and waste, returning nutrients to the soil.
Food chains and food webs
A food chain shows the feeding relationship between organisms, with arrows pointing in the direction the energy (or food) flows. For example:
grass produces rabbit produces fox
Here grass is the producer, the rabbit is the primary consumer (herbivore), and the fox is the secondary consumer (carnivore). Each stage is a trophic level.
A food web is several food chains linked together, showing that most organisms eat (and are eaten by) more than one type of organism. If one organism in a web changes in number, it can affect many others.
Pyramids of number and biomass
The amounts at each trophic level can be shown as pyramids.
- A pyramid of number shows how many organisms there are at each level. It is usually wider at the bottom, but can be an odd shape (for example one large tree supporting many insects).
- A pyramid of biomass shows the total mass of living material at each level. It is almost always a true pyramid shape, narrowing towards the top, because there is less biomass at higher levels.
Energy transfer and why energy is lost
Energy enters the ecosystem as light, which producers capture by photosynthesis. Energy then passes along the food chain as organisms are eaten. But at each level, much of the energy is lost and does not pass on, because it is:
- used in respiration to release energy for life processes,
- used for movement,
- lost as heat to the surroundings,
- lost in waste materials (faeces and urine), and in uneaten parts.
Because so much energy is lost at each step, only a few trophic levels can be supported. After four or five levels there is too little energy left to support another, so food chains rarely go further.
Implications for food production
The loss of energy along food chains explains why it is more efficient to feed people on crops than on meat: fewer levels means less energy is lost, so more food energy reaches us from a given amount of land. Eating producers (plants) directly wastes less energy than eating animals that ate the plants.
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 style4 marksUsing the food chain grass produces rabbit produces fox, explain what is meant by a producer and why there are fewer foxes than rabbits.Show worked answer →
A 4-mark question.
A producer is an organism, usually a green plant, that makes its own food by photosynthesis; here the grass is the producer. There are fewer foxes than rabbits because energy is lost at each stage of the food chain (in respiration, movement, heat and waste), so less energy is available to the next level. With less energy available, fewer foxes can be supported than rabbits.
Markers reward: producer makes its own food by photosynthesis; energy is lost between levels; so less energy supports fewer organisms higher up. Saying foxes are "bigger" is not the reason.
WJEC style3 marksExplain why food chains rarely have more than four or five trophic levels.Show worked answer →
A 3-mark question on energy loss.
At each trophic level, much of the energy is lost (used in respiration and movement, lost as heat, and lost in waste such as faeces and urine). Only a small fraction of the energy is passed on to the next level. After four or five levels, so little energy remains that it cannot support another level of organisms, so the food chain ends.
Markers reward: energy is lost at each level (respiration, heat, waste); only a little passes on; not enough energy is left to support more levels. Saying "the animals die" is not the reason.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)