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4.2Ecology and the Environment

Feeding Relationships — Edexcel Linear - OL Revision Notes & Topical Questions

10 min read 12 parts Lesson 8 of 10 1 quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Lesson summary

Trophic Levels, Food Webs, Pyramids and Energy Transfer — EDEXCEL LINEAR - OL notes: Describe trophic levels, food chains, webs and pyramids, and explain why energy is lost between trophic levels.The trophic level is the position of an organism in a food chain.

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Primary consumer
  • Decomposer

Section 1 of 12Trophic levels

The trophic level is the position of an organism in a food chain. An organism in a food web can occupy different levels in different chains. At every level organisms produce waste or die, and the energy in this matter passes to decomposers (bacteria and fungi), which recycle nutrients.

LevelOrganismsEnergy transfer
1stProducers (plants, algae)Convert light energy into chemical energy in glucose by photosynthesis
2ndPrimary consumersEat producers
3rdSecondary consumersEat primary consumers
4thTertiary consumersEat secondary consumers

Section 2 of 12Food chains and food webs

A food chain shows the transfer of energy between organisms as they feed; the arrows show energy flow.

1
All chains start with a producer because producers convert light energy into a usable chemical form.
2
A food web is a network of connected food chains.
3
A change in one population affects others: in the example web a decrease in rabbits could cause grass growth to increase (less grazing), fewer foxes (less food), more caterpillars and earthworms (more grass),
  • and fewer frogs and mice.
    • (foxes eat more of them to compensate)
4
Say populations "increase" or "decrease", not "die out".

To interpret a food web, first write out each separate food chain, then identify which organisms feed at more than one level.

Food chains and food webs
A hawk that eats both mice (primary consumers) and snakes (secondary consumers) is a secondary and a tertiary consumer in different chains.
When predicting changes, follow one step at a time: the change in the first population, then its direct predators and prey, then the indirect effects.
Remember that removing a predator often increases its prey, which then reduces the prey's own food supply.

Section 3 of 12Pyramids of numbers, biomass and energy

PyramidBar showsShape
NumbersNumber of organisms at each levelNot always pyramid-shaped; for example one oak tree supporting many caterpillars
BiomassMass of living matter at each levelAlways a pyramid
EnergyStored energy at each levelAlways a pyramid, since energy is lost at each level

Drawing rules

  • keep the same order as the food chain with producers at the bottom;
    • label each bar;
    • bar length shows the number, biomass or energy;
    • bars have equal height;
    • draw to scale on graph paper.

Section 4 of 12Energy transfer and loss

Light energy enters the food chain when producers make carbon compounds in their biomass.

Energy transfer and loss
When a primary consumer eats a producer it breaks down those compounds and uses the energy and substances to build its own tissues,
  • and so on up the chain.
Only around 10% of the energy available at each level becomes biomass at the next, which explains why food chains are short.
Causes of loss:
1
Not all of an organism is eaten (bones, fur, teeth, claws).
2
Not all that is eaten is digested and absorbed; some is egested as faeces.
3
Energy is lost as heat in respiration and movement.
4
Energy is lost in excretion, for example urea.
5
Energy is held in dead organisms that decomposers break down.

Link this to farming

  • because energy is lost at every step, eating plants directly (a short chain) supports more people from the same area of land than eating meat from animals that fed on those plants.
  • This is why crops give a higher yield of usable energy per hectare than livestock.

Section 5 of 12Worked examples

  1. Describe what is meant by a trophic level. [1]1 mark

  2. Explain why a pyramid of biomass is always pyramid-shaped. [2]2 marks

  3. Explain why food chains rarely have more than five trophic levels. [2]2 marks

  4. In a web, rabbits decrease. Predict the effect on grass and foxes, with reasons. [3]3 marks

Section 6 of 12The equation and why it works

Energy is stored in the biomass of organisms, so energy transfer and biomass transfer can be calculated in the same way. Efficiency is the percentage of what was available at one level that ends up in the next:

percentage efficiency = (biomass in the higher trophic level / biomass in the lower trophic level) x 100

Use consistent units and take the two levels that the question names. The loss of energy at each level results in a loss of biomass, so the answer is always below 100% and is commonly about 10%.

Section 7 of 12Worked example

A food chain has four trophic levels with total biomass shown at each level: clover 1450 kg, snails 138 kg, and further levels. Calculate the efficiency of biomass transfer from the first to the second trophic level, to 3 significant figures.

Identify the values: first level (clover) = 1450 kg; second level (snails) = 138 kg.
Substitute: (138 / 1450) x 100.
Calculate: 138 / 1450 = 0.09517..., so 9.517...%.
9.52%
Round to 3 significant figures: 9.52%.

Always show the working because method marks are given even if the final number is wrong, and include the % sign.

Second example for practicea food chain has grass 20000 kJ, grasshoppers 2400 kJ and frogs 360 kJ.

Worked example
Grass to grasshopper efficiency = 2400 / 20000 x 100 = 12%.
Grasshopper to frog efficiency = 360 / 2400 x 100 = 15%.
The overall efficiency from grass to frog is 360 / 20000 x 100 = 1.8%, which shows how little of the original energy reaches the top of the chain.

Section 8 of 12Why the efficiency is low

The result near 10% matches the typical value.

Why the efficiency is low
The reasons are the same as for energy loss: parts of the organism are not eaten (bones, fur, teeth), some food is not digested and is egested as faeces, energy is released as heat in respiration and movement,
  • and energy is lost in waste such as urea or remains in dead matter taken by decomposers.
Plant material is difficult to digest, so herbivores egest much of what they eat.
Loss routeExample
Uneaten partsBones, claws, roots
EgestionUndigested plant fibre
RespirationHeat and movement
ExcretionUrea, carbon dioxide

Section 9 of 12More practice patterns

More practice patterns
Check that the higher level is the numerator; if your answer is above 100% you have inverted the fraction.
  • Efficiencies are normally lower for the later steps in many chains, but the method does not change.
Example: a producer level holds 24000 kJ and the next level holds 2400 kJ, so efficiency = 2400 / 24000 x 100 = 10%.
  • If the producers hold 5000 kg and efficiency is 8%, the primary consumers hold 5000 x 8 / 100 = 400 kg.
Typical exam variations: given a diagram of numbers, calculate the efficiency using biomass, not numbers of organisms;
  • or compare two transfers and state which is more efficient and why.
    • (for example a carnivore eating meat digests more of its food than a herbivore eating plant fibre)
  • Always state the equation, substitute, calculate, round as instructed and give the unit or % sign.

Section 10 of 12Worked examples

  1. Biomass is 1450 kg in clover and 138 kg in snails. Calculate the efficiency to 3 s.f. [2]2 marks

  2. A level holds 8000 kJ and the next holds 640 kJ. Calculate the percentage efficiency. [2]2 marks

  3. Producers have 6000 kg of biomass; transfer to the herbivores is 12%. Calculate the herbivore biomass. [2]2 marks

  4. Give two reasons why efficiency is below 100%. [2]2 marks

Section 11 of 12Key terms

TermMeaning
Trophic levelPosition of an organism in a food chain.
ProducerOrganism that makes its own food by photosynthesis.
Primary consumerHerbivore that eats producers.
DecomposerBacteria or fungi that break down dead matter.
Food webNetwork of linked food chains.
BiomassMass of living material.
Pyramid of energyDiagram of stored energy at each trophic level.
TermMeaning
EfficiencyPercentage of energy or biomass passed to the next trophic level.
BiomassMass of living matter.
EgestionRemoval of undigested food as faeces.
ExcretionRemoval of metabolic waste such as urea.
Significant figuresDigits that carry the precision of a number.

Section 12 of 12Exam technique

1
Drawing arrows from predator to prey — arrows show energy going to the eater.
2
Saying energy is destroyed — it is transferred as heat to the environment.
3
Assuming numbers pyramids are always pyramid-shaped.
4
Saying a species dies out when the web changes — populations increase or decrease.
1
Dividing the lower level by the higher — the higher goes on top.
2
Forgetting to multiply by 100.
3
Not rounding to the required significant figures.
4
Saying energy disappears — it is transferred to the surroundings as heat.

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