Food chains, food webs and energy flow
How energy moves through an ecosystem, and why it runs out.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 19.1.1Describe food chains and webs and the terms producer, consumer, herbivore, carnivore and decomposer.
- 19.1.2Explain energy loss between trophic levels and the shape of pyramids of biomass.
6 minute read
Food chains, food webs and energy flow
Almost all energy in an ecosystem enters as sunlight and is captured by producers through photosynthesis.
The vocabulary
- Producer — an organism that makes its own organic nutrients, usually by photosynthesis. Always the start of a food chain.
- Consumer — an organism that gets energy by feeding on other organisms. Primary consumers eat producers, secondary consumers eat primary consumers, and so on.
- Herbivore eats plants; carnivore eats other animals.
- Decomposer — an organism that gets energy from dead or waste organic matter, returning nutrients to the soil.
- Trophic level — the position an organism occupies in a food chain.
A food chain shows one pathway; a food web shows many interconnected chains, which is far more realistic.
Why energy runs out
Only about 10% of the energy at one trophic level passes to the next. The rest is lost through:
- Respiration — energy released as heat to the surroundings.
- Excretion and egestion — energy remaining in urine and faeces.
- Uneaten parts — bones, roots and other material not consumed.
This is why food chains rarely have more than four or five links: after four transfers, too little energy remains to support another level.
Consequence for food supply
Because energy is lost at each transfer, feeding grain to humans directly supports far more people than feeding the grain to cattle and eating the beef. Shortening the food chain wastes less energy.
Think of it like this
Each trophic level is a leaky bucket poured into the next. Roughly nine-tenths spills as heat and waste on the way, so by the fourth bucket there is barely a splash left.
Worked examples
Method, step by step
Explain why a food chain rarely contains more than five trophic levels.
- 1Only about 10% of energy is transferred from one trophic level to the next.
- 2The rest is lost as heat from respiration, and in egestion and excretion.
- 3After several transfers, so little energy remains that it cannot support the biomass of another level.
Because roughly 90% of energy is lost at each transfer — mostly as heat from respiration and in waste — too little energy remains after four or five levels to support another consumer.
Common misconceptions
- Drawing arrows the wrong way. Arrows in a food chain show the direction of **energy flow**, so they point from the eaten to the eater.
- Thinking energy is "recycled" like nutrients. Nutrients cycle; energy flows one way and is ultimately lost as heat.
- Believing a pyramid of numbers is always pyramid-shaped. One oak tree supporting thousands of insects inverts it — which is why pyramids of **biomass** are more reliable.
In the exam
- When explaining energy loss, name at least two specific processes — respiration/heat, and egestion or excretion — rather than saying energy is "lost".
- For "why are food chains short", link it explicitly: only ~10% transfers, so after several levels there is insufficient energy to support another consumer.