Every step leaks: As food is passed along a food chain, energy and organic matter are lost at every step. Only a small part reaches the next trophic level.
The points to remember
- The sun is the main source of energy for almost every food chain; producers capture it.
- At each step only a small part of the energy and organic matter becomes new biomass in the next level.
- The rest is lost: most as heat from respiration, some in waste and in parts not eaten.
- Energy is not destroyed (2.2.2): it leaves as heat, which no organism can use again.
- So there is never 100% transfer of organic matter from one trophic level to the next.
Remember it as: Every step leaks, mostly as heat.
Real example: in an English hay meadow, grasses capture a small part of the sunlight; grasshoppers that eat the grass pass on only a little of its energy to the frogs that eat them.
Where this fits: Why transformations are never 100% efficient (the second law) is on 2.2.8; heat from respiration on 2.2.7.
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Energy and matter are lost at four gates between one trophic level and the next. Each gate is a separate point in an answer.
Where the energy and matter go
- Not all the food available is harvested: prey escapes, some plants are never reached.
- Of what is harvested, not all is consumed: bones, bark, shells, roots and skin are left.
- Of what is consumed, not all is absorbed: undigested food leaves as faeces.
- Of what is absorbed, not all is stored: most is respired for movement and warmth, and lost as heat.
- Some absorbed matter also leaves as urine and other waste (excretion).
Remember it as: Harvested, consumed, absorbed, stored: less at every gate.
Real example: a cabbage white caterpillar egests about half of the leaf it eats and respires about a third; only about 15% becomes caterpillar that a bird can eat.
'Energy is lost' is not enough: Always say how: 'lost as heat from respiration', 'lost in faeces', 'parts not eaten'. The bare phrase 'energy is lost' is not credited.
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Put the four losses together and only a small share passes on each time.
The points to remember
- Typically only about 10% of the energy in one trophic level reaches the next: about 90% is lost.
- The figure varies, roughly 5-20%, between ecosystems, levels and species.
- Aquatic food chains are often a little more efficient than food chains on land.
- Warm-blooded birds and mammals lose more energy as heat than cold-blooded animals such as fish and insects.
- Less energy at each level is why food chains are short (2.2.14).
Remember it as: About a tenth goes up; nine tenths go out.
Real example: a grey heron needs the energy of many frogs, which needed many grasshoppers, which needed a large area of grass.
Numbers, not rules: The 10% figure is a rough guide, not a fixed law. Explain the losses; that is what the marks reward.
How this comes up: Paper 2, Section A: using named examples from a food web, explain the energy transfer between the producer and the primary consumer [2].
In a vegetable garden, cabbage plants are eaten by cabbage white caterpillars, which are eaten by blue tits.
Using named examples, explain the energy transfer between the producer and the primary consumer.
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