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5.3.6Energy Transfers In and Between Organisms (A Level only)

Trophic Levels — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 18 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

5.3.6 Trophic Levels — AQA A Level Biology 7402 notes: Describe food chains, food webs and pyramids and explain why energy transfer between trophic levels is inefficient.A trophic level is the position of an organism in a food chain, defined by how many energy transfers separate …

  • Pyramid of numbers
  • Pyramid of biomass
  • Pyramid of energy
  • Exam-style question 1
  • Exam-style question 2
  • Primary consumer

Section 1 of 8Food chains, webs and trophic levels

A trophic level is the position of an organism in a food chain, defined by how many energy transfers separate it from the producer.

Producers are level 1, primary consumers (herbivores) level 2, secondary consumers (carnivores or omnivores that eat herbivores) level 3, and so on.
Decomposers and detritivores act at all levels, feeding on dead material and waste.
Food webs
Many animals feed at more than one level, so real communities are better described by food webs, which show interlinked chains.

Section 2 of 8Why energy is lost

Only a small fraction of the energy in one level passes to the next. Typically 10% is transferred, though the true figure varies from about 5% to 20%. The reasons include:

1
Not all of the organism is eaten: roots, bones, shells, fur and so on are left.
2
Some of what is eaten is not digested and is lost as faeces; plant cell walls and animal bones are poorly digested.
3
Much absorbed energy is used in respiration for movement, maintaining body temperature, active transport and synthesis and is lost as heat.
4
Energy is lost in excretory products such as urea.
5
Some organisms die before being eaten and pass to decomposers.

Endotherms use a large share of their food energy for thermoregulation, so they convert less into growth than ectotherms; this is why meat from mammals and birds is more energy-expensive to produce than fish or insects.

Section 3 of 8Ecological pyramids

Ecological pyramids show the relative quantity at each trophic level.

Pyramid of numbers
Pyramid of numbers counts of individuals. It can be irregular if one tree supports thousands of insects.
Pyramid of biomass
Pyramid of biomass dry mass at one time. Usually pyramidal, but can be inverted in aquatic systems when sampled at a time when phytoplankton are scarce.
Pyramid of energy
Pyramid of energy energy transferred per unit area per year. Always pyramid-shaped because energy is lost at each level.

Pyramids of energy are the most reliable representation, but need much more data to produce. Each bar is drawn to scale, with producers at the base.

Section 4 of 8Limits on chain length and applications

Because so much energy is lost at each step, food chains rarely have more than four or five trophic levels: not enough energy remains to support a viable population at higher levels.

Limits on chain length and applications
Top predators tend to be few in number, wide-ranging and sensitive to habitat loss.
Humans can feed more people by eating lower down the food chain, since eating plants directly avoids at least one lossy transfer.
PyramidMeasuredCan be inverted?
NumbersIndividualsYes
BiomassDry mass at one timeYes (aquatic)
EnergykJ m-2 year-1No

Section 5 of 8Worked example: reading a pyramid

Suppose a pyramid of energy has producers 20 000, primary consumers 2 100, secondary consumers 230 and tertiary consumers 20 kJ m-2 year-1.

Worked example: reading a pyramid
The transfer efficiencies are 2 100 / 20 000 = 10.5%, 230 / 2 100 = 11.0% and 20 / 230 = 8.7%.
These are close to the typical 10% figure.
Notice that the total energy decreases at every step and that the amount left at the top is tiny, so a fifth consumer level would have about 2 kJ, too little to support a population.

In questions about food webs, work out what happens if one species is removed.

Worked example: reading a pyramid
If a secondary consumer is lost, populations of its prey may rise, which in turn reduces the plants they eat, a trophic cascade.
Where a consumer feeds at several levels, the effect is buffered.
You should also be able to link the idea to bioaccumulation: persistent toxins such as DDT are not broken down and are stored in fat, so their concentration rises at each level, reaching highest levels in top carnivores.

Finally, remember that decomposers and detritivores release the mineral ions in dead tissue, connecting energy flow to nutrient cycling, which is covered in the next topic.

Remember too that the shape of a pyramid reflects the energy relationships only when the data are collected over a full year and for all organisms at each level.

Worked example: reading a pyramid
Sampling only the large, visible animals underestimates the lower levels, and counting a single tree as one producer distorts a pyramid of numbers.
These limitations are why ecologists prefer energy or dry mass per unit area when comparing communities, and why the data should always be gathered with a standard, repeatable method.

Section 6 of 8Exam-style questions

  1. Suggest why there are rarely more than five trophic levels in a food chain. [3]3 marks

  2. Explain why a pyramid of energy cannot be inverted whereas a pyramid of biomass can. [3]3 marks

Section 7 of 8Key terms

TermMeaning
Trophic levelFeeding position in a food chain.
Primary consumerOrganism that eats producers.
Food webNetwork of interconnected food chains.
Pyramid of energyDiagram showing energy flow through each trophic level per unit area per year.
DetritivoreAnimal that feeds on dead organic matter.
DecomposerMicroorganism that breaks down dead material by extracellular digestion.

Section 8 of 8Exam technique

1
Stating that the missing energy is 'used up'. It is transferred to the surroundings as heat or remains in uneaten or egested material.
2
Saying 90% is lost in every case. It is an approximate average.
3
Thinking the pyramid of numbers is always pyramid-shaped.
4
Placing decomposers at the top of chains only; they feed on all levels.

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