Food Webs and Energy TransferAQA GCSE Biology: Revision notes
Section 1
Producers and Food Chains
Photosynthetic organisms (green plants and algae) are the producers of biomass for almost all life on Earth, since they make glucose using light energy in photosynthesis. Every food chain begins with a producer.
Section 2
Feeding Levels: Producers to Consumers
Food chains describe feeding relationships:
Producer → primary consumer → secondary consumer → tertiary consumer
- Primary consumers eat producers
- Secondary consumers eat primary consumers
- Tertiary consumers eat secondary consumers
Consumers that kill and eat other animals are called predators; the animals they eat are called prey. In a stable community, predator and prey numbers rise and fall in cycles.
Section 3
Trophic Levels
Feeding relationships can also be described using numbered trophic levels, starting at level 1 with producers:
| Trophic level | Organism type |
|---|---|
| Level 1 | Producers (plants and algae) |
| Level 2 | Primary consumers (herbivores) |
| Level 3 | Secondary consumers (carnivores that eat herbivores) |
| Level 4 | Tertiary consumers (carnivores that eat other carnivores) |
Organisms at the top of a food chain with no predators of their own are called apex predators.
Section 4
Pyramids of Biomass
A pyramid of biomass represents the relative amount of biomass present at each trophic level in a food chain or web.
- Trophic level 1 (producers) is drawn at the bottom of the pyramid
- Each bar's width represents the total biomass at that level, so the pyramid usually narrows towards the top
Unlike numbers of organisms, biomass pyramids are almost always a 'true' pyramid shape (wide base, narrowing upwards), because biomass is lost at each stage.
Section 5
Why Is Biomass Lost Between Trophic Levels?
Only around 10% of the biomass at one trophic level is transferred to the level above it. The rest is lost for several reasons:
- Not all the biomass an organism eats (ingests) is absorbed into its body — some passes straight through and is egested as faeces
- Some absorbed material is lost as carbon dioxide and water during respiration
- Some is lost as water and urea in urine
- Large amounts of glucose are used up in respiration to release energy for life processes, rather than being stored as biomass
The efficiency of biomass transfer between levels can be calculated as a percentage or fraction of the biomass passed on. Because so much biomass (and energy) is lost at each level, there is only enough left to support a smaller number of organisms — this is why food chains rarely have more than four or five trophic levels, and why there are usually far fewer top predators than producers.
If a food chain has 1000 kg of biomass in producers, only around 100 kg is available at the primary consumer level, and around 10 kg at the secondary consumer level.
Must Know
- Producers (plants and algae) start every food chain by making biomass through photosynthesis
- Feeding levels run: producer → primary consumer → secondary consumer → tertiary consumer
- Trophic levels are numbered 1 (producers) to 4 (tertiary consumers); apex predators have no predators
- Pyramids of biomass show biomass at each trophic level, narrowing from a wide base (level 1)
- Only around 10% of biomass is transferred between trophic levels
- Biomass is lost through egestion (faeces), respiration (CO2, water), excretion (urine) and glucose used up in respiration
- This loss limits the number of organisms that can be supported at higher trophic levels
That's the notes covered.
Carry on to the next subtopic.