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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

Part 1 of 5Trophic 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

Part 2 of 5Food 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.

Part 3 of 5Pyramids 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.

Part 4 of 5Energy 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.

Part 5 of 5Worked 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

Key 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.

Avoid these mistakes

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.

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