Edexcel IGCSE Biology · Spec 4.6-4.9
Feeding Relationships
Food chains and food webs, energy transfer between levels, and pyramids of number and biomass.
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Feeding Relationships
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Explained
Food chains, and why they are short
An arrow in a food chain means is eaten by, and it points in the direction the energy travels: from the organism being eaten to the one eating it.
Drawing an arrow the wrong way round is the commonest error in this topic, and it reverses the meaning of the whole chain.
The trophic levels
Every chain starts with a producer, which is a plant or an alga, because only producers can make their own food by photosynthesis. Everything else in the chain ultimately depends on them.
Then comes the primary consumer, which eats the producer, then the secondary consumer, then the tertiary consumer, and occasionally a quaternary consumer.
Count positions from the producer to name a level. The third organism along is the secondary consumer, because the producer is not a consumer at all.
Decomposers, mainly bacteria and fungi, break down dead material at every level and return the nutrients to the soil.
Food webs
A food web is many chains joined together, which is closer to what really happens, because few animals eat only one thing.
To read one, follow the arrows. To answer a what if question, follow the arrows outwards in both directions: if one species is removed, what ate it has less food, and what it ate becomes more numerous, and both of those changes then spread further along.
Why energy runs out
Only about ten per cent of the energy at one level becomes biomass at the next. The rest is lost, and a question asking where wants the specific routes rather than the total.
Energy is lost through respiration, and as thermal energy to the surroundings. It is lost in movement. It is lost in waste: faeces, which is egested, and urine and urea, which are excreted. And some is never taken in at all, because parts of the organism are not eaten or cannot be digested.
That loss is why chains are rarely longer than four or five levels. After a few steps there is not enough energy left to support another population.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for a fill in the blanks question on energy flow awards eight separate marks, and the list is effectively this whole topic in eight words: direction, producer, trophic, heat or thermal, respiration, faeces or manure, urine or urea or sweat, and bacteria.
Notice that faeces and urine are listed separately. They are different losses by different routes, and both are credited, so naming only waste covers one of two available marks.
On pyramids, another mark scheme awards one mark for the correct shape and one for the correct order, and adds that having the producer at the bottom alone earns one of the two.
Asked why a pyramid of numbers is not always pyramid shaped, it credits that the mass or size of organisms varies, that the number of producers may be small, giving the example of a single oak tree, and that the base may therefore be narrow so the pyramid can be inverted. It then contrasts this with biomass, which is always greater for producers and always decreases up the trophic levels.
That contrast is the point of the question. A pyramid of numbers counts organisms regardless of size, so one large tree supporting many insects gives a narrow base. A pyramid of biomass measures mass, so it is always the right way up.
Pyramids of number and biomass
A pyramid of numbers shows how many organisms are at each level. It can be inverted or an odd shape when the organisms differ greatly in size.
A pyramid of biomass shows the total mass at each level, so it is always widest at the bottom, because the mass available cannot increase as energy is lost.
When a question gives you an unusual pyramid and asks you to explain it, the answer is almost always the size of the producers.
Spec 4.6-4.9
What you need to know
- Name the trophic levels in a food chain
- Read food webs
- Explain energy loss and ecological pyramids
Active recall
Quick check
Answer each question before opening the answer.
What do the arrows in a food chain represent?
The direction of energy/biomass transfer — they point from the food to the organism that eats it.
Why are food chains usually short?
Energy is lost at each level (in respiration, movement and waste), so there is less to support organisms higher up.
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