Trophic levels in an ecosystem — AQA GCSE Biology
Trophic levels, pyramids of biomass, and how biomass is lost between levels of a food chain, including efficiency calculations.
Revision notes
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Trophic levels are the feeding levels of a food chain, numbered from level 1. Level 1 is producers (plants and algae), level 2 is herbivores (primary consumers), and levels 3 and 4 are carnivores (secondary and tertiary consumers). Apex predators are carnivores with no predators. Decomposers secrete enzymes onto dead matter, then soluble food molecules diffuse in.
A pyramid of biomass represents the relative amount of biomass in each trophic level of a food chain. Trophic level 1, the producers, is at the bottom. Each bar is narrower than the one below it, because biomass decreases at each higher trophic level. To draw one, make each bar's length proportional to its biomass and centre the bars.
Only approximately 10% of the biomass at each trophic level is transferred to the level above. Biomass is lost because some ingested material is egested as faeces, some absorbed material is lost as waste such as carbon dioxide, water and urea, and large amounts of glucose are used in respiration. So fewer organisms can be supported at higher levels.
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Quick check questions
What is trophic level 1, and what are those organisms called?
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Plants and algae, called producers.
What is an apex predator?
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A carnivore with no predators.
How do decomposers break down dead material?
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They secrete enzymes into the environment and small soluble food molecules diffuse in.
Approximately what percentage of biomass passes to the next trophic level?
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About 10%.
Give two ways biomass is lost between trophic levels.
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Egested as faeces; lost as carbon dioxide and water in respiration, and as water and urea in urine.
Frequently asked questions
Why is energy lost between trophic levels?
Energy is lost between trophic levels because biomass is lost: only about 10% of the biomass passes to the next level. Some ingested material is egested as faeces. Some absorbed material is lost as waste, such as carbon dioxide and water from respiration and water and urea in urine. Large amounts of glucose are also used in respiration.
What is a pyramid of biomass?
A pyramid of biomass is a diagram that shows the relative amount of biomass at each trophic level of a food chain, with the producers at the bottom. Each bar is narrower than the one below, because biomass decreases at each higher trophic level. The length of each bar is drawn to scale, proportional to the biomass of that level.
Why are food chains short?
Food chains are short because so much biomass is lost at each step. Only about 10% of the biomass at one trophic level is transferred to the level above, as the rest is egested, lost as waste or used in respiration. So less biomass is available at each higher level, and fewer organisms can be supported.
How do you calculate the efficiency of biomass transfer?
Efficiency of biomass transfer is the biomass transferred to the next level divided by the biomass available at the previous level, multiplied by 100. For example, if caterpillars eat 250 g of leaves and their body mass increases by 20 g, the efficiency is 20 ÷ 250 × 100 = 8%. It can also be written as a fraction, 2/25.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026