(b) Feeding relationships
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
4.6Trophic levelsUnderstand the names given to different trophic levels, including producers, primary, secondary and tertiary consumers and decomposers.4.7Concepts of food chainsUnderstand the concepts of food chains, food webs, pyramids of number, pyramids of biomass and pyramids of energy transfer.4.8Food chains and energy transferUnderstand the transfer of substances and energy along a food chain.4.9Ten percent energy transferUnderstand why only about 10% of energy is transferred from one trophic level to the next.
Assign organisms to trophic levels
A trophic level is an organism's feeding position in a food chain; the position depends on what the organism eats in that particular chain.
| Trophic position | Name and role |
|---|---|
| 1 | producer: makes organic substances, usually by photosynthesis |
| 2 | primary consumer: feeds on a producer |
| 3 | secondary consumer: feeds on a primary consumer |
| 4 | tertiary consumer: feeds on a secondary consumer |
| across levels | decomposers such as bacteria and fungi digest dead organisms and waste, recycling substances |
In a food web, one organism can occupy more than one trophic level because it may eat prey from different levels. Trace each route separately from its producer.
Arrows point from food to feeder—the direction in which substances and energy transfer. A top predator is not automatically tertiary if the chain has a different number of steps.
Read chains, webs and ecological pyramids
Food chains show one transfer route; food webs join many routes. Ecological pyramids compare trophic levels using number, biomass or energy.
| Representation | What width means | Shape and limitation |
|---|---|---|
| food chain | not a width model; each arrow is a feeding transfer | one linear route |
| food web | linked feeding routes | shows alternatives and indirect effects |
| pyramid of numbers | number of organisms | may be inverted: one tree can support many insects |
| pyramid of biomass | dry mass at one time, often per unit area | usually upright, but fast-turnover producers can give an inverted snapshot |
| pyramid of energy transfer | energy transferred per area per time | always upright because usable energy decreases between levels |
Label every trophic level and keep the producer at the base. Use the quantity named in the question: organism size affects biomass but not the count used in a pyramid of numbers.
Pyramids of number and biomass are not interchangeable. Biomass is a standing mass snapshot; energy transfer includes a time interval and cannot increase up a chain.
Track substances and energy through food chains
Producers store light energy as chemical energy in organic substances; feeding transfers some of those substances and their chemical energy to consumers and decomposers.
| Route | Substance or energy change |
|---|---|
| photosynthesis | light energy becomes chemical energy in producer biomass |
| feeding | carbon compounds, mineral-containing molecules and stored chemical energy enter the consumer |
| assimilation | digested, absorbed food can be built into new biomass |
| respiration | chemical energy is released for cellular work and ultimately dissipated as heat |
| egestion and excretion | substances and their remaining energy leave as faeces or metabolic waste |
| death and uneaten material | organic matter passes to decomposers, which respire and recycle mineral ions |
Transfer efficiency (%) = energy or biomass transferred to the next level ÷ energy or biomass available at the previous level × 100. Compare like units and the same area and time interval.
Substances cycle through organisms and the environment, but energy flows one way and is dissipated as heat. Decomposers recycle matter; they do not return used energy to producers.
Account for the roughly 10% energy transfer
On average, only about 10% of the energy at one trophic level becomes biomass available to the next; the exact percentage varies among organisms and ecosystems.
| Energy route not transferred as new consumer biomass | Mechanism |
|---|---|
| organisms or parts not eaten | roots, bones, fur or whole organisms remain for decomposers |
| food not digested or absorbed | energy remains in faeces and is egested |
| excretory products | some chemical energy leaves in substances such as urea |
| respiration and activity | energy supports movement, active transport and other cell processes |
| heat | energy released in respiration is ultimately dissipated to surroundings |
If producers store 20,000 kJ m⁻² yr⁻¹, an approximate 10% model predicts 2,000 at primary consumers and 200 at secondary consumers. Reapply the fraction at every transfer rather than subtracting one fixed amount.
Energy is not destroyed: much is transferred to decomposers or dissipated as heat and is therefore unavailable to the next consumer level. '10%' is a useful approximate pattern, not a universal constant.