Feeding relationships
BiologyΒ· 4.6β4.9 (2017 Issue 3 spec)Β· 15 min read
1. Trophic Levels in Feeding Relationshipsβ β ββββ± 3 min
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Trophic Level
The position an organism occupies in a feeding sequence, determined by its source of nutrition.
Producer: Autotrophic (usually green plant) that makes food via photosynthesis, forming the base of all feeding sequences
Primary consumer: Herbivore that eats producers
Secondary consumer: Organism that eats primary consumers
Tertiary consumer: Apex predator that eats secondary consumers
Decomposer: Bacteria/fungi that break down dead organisms and waste to recycle nutrients
Name the trophic level of each organism in the sequence: Grass β Caterpillar β Sparrow β Hawk
- 1
- Grass produces its own food via photosynthesis = Producer
- 2
- Caterpillar eats grass (herbivore) = Primary consumer
- 3
- Sparrow eats caterpillar = Secondary consumer
- 4
- Hawk eats sparrow = Tertiary consumer
- 5
- Decomposers break down all dead organisms and waste from every level in the sequence
Exam tip:
Decomposers are not usually drawn on standard food chains, but you should reference them explicitly if asked to name all trophic levels in an ecosystem.
2. Food Chains and Food Websβ β ββββ± 4 min
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Food Chain
A linear sequence showing the transfer of energy, biomass and nutrients between organisms, with arrows pointing from the organism being eaten to the organism that eats it.
A food web is a network of interconnected food chains that shows all the feeding relationships in an ecosystem. Most organisms have multiple food sources, so food webs give a more accurate representation of real ecosystems than simple linear chains.
A student draws the sequence: Fox β Rabbit β Grass. Explain the mistake and correct the sequence.
- 1
- The arrows point backwards, implying energy flows from fox to rabbit to grass, which is incorrect.
- 2
- Grass is eaten by rabbits, which are eaten by foxes, so arrows should point from the eaten organism to the eater.
- 3
- Correct sequence: Grass β Rabbit β Fox
3. Ecological Pyramidsβ β β βββ± 4 min
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Ecological pyramids show the relative amount of a given parameter at each trophic level in a feeding sequence. There are three core types you need to distinguish for your exam:
Pyramid Type | What it measures | Can it be inverted? |
|---|---|---|
Pyramid of number | Count of individual organisms at each level | Yes (e.g. 1 large tree supporting 1000+ insects) |
Pyramid of biomass | Total dry mass of organisms per unit area at each level | Rarely, almost always upright |
Pyramid of energy | Total energy available at each level per unit time | No, always upright |
Explain why a pyramid of energy can never be inverted, even if a pyramid of number for the same ecosystem is inverted.
- 1
- Energy is lost at every trophic level: as heat from respiration, in waste, or used for movement.
- 2
- The total energy available at a higher trophic level is always less than the total energy at the level below it.
- 3
- A pyramid of number counts individuals, so one very large producer can support hundreds of smaller consumers, making it inverted, but this does not apply to total energy available.
Exam tip:
If asked to compare pyramids, always reference energy loss as the core reason for the always-upright shape of energy pyramids.
4. Energy Transfer and the 10% Ruleβ β β βββ± 4 min
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10% Energy Transfer Rule
Approximately 10% of the energy stored in biomass at one trophic level is passed on to the next level, with the remaining 90% lost from the food chain.
Key reasons for energy loss between levels: energy used in respiration (released as heat to surroundings), energy used for movement, energy lost in undigested faeces (egestion), energy lost in urine (excretion), not all parts of the organism are eaten by consumers
Only energy used to build new biomass is passed on to the next trophic level
If 50,000 kJ of energy is stored in the producer level of a food chain, calculate how much energy is available to the secondary consumer level.
- 1
- 10% of producer energy is passed to primary consumers: kJ
- 2
- 10% of primary consumer energy is passed to secondary consumers: kJ
- 3
- Final answer: 500 kJ
Exam tip:
Exam questions asking for reasons for energy loss usually require 2β3 valid points for full marks, so learn at least 3 of the standard reasons listed above.
5. Common Pitfalls
Wrong move:
Drawing food chain arrows from predator to prey
Why:
Arrows represent energy flow from the eaten organism to the eater, not the direction of consumption action.
Correct move:
Always point arrows from the organism being consumed to the consumer (prey β predator).
Wrong move:
Stating pyramids of biomass are always upright
Why:
Rare inverted cases exist, e.g. aquatic ecosystems where phytoplankton biomass fluctuates seasonally below zooplankton biomass.
Correct move:
State that pyramids of biomass are usually upright; only pyramids of energy are always upright.
Wrong move:
Only referencing energy flow in feeding relationships, not substances
Why:
The spec explicitly requires you to understand transfer of both energy and substances (nutrients/carbon) along food chains.
Correct move:
Mention both energy flow and nutrient/substance transfer when describing feeding relationships.
Wrong move:
Listing only 1 reason for energy loss between trophic levels
Why:
Exam questions usually require 2β3 valid reasons for full marks.
Correct move:
Memorize 3+ standard reasons: respiratory heat loss, egestion of undigested food, not all parts of the organism are eaten.
Wrong move:
Including decomposers as a single level on energy pyramids
Why:
Decomposers break down waste from every trophic level, they do not occupy a single linear position in standard pyramids.
Correct move:
Only reference decomposers as separate to the linear producer-tertiary consumer sequence unless explicitly asked.
6. Quick Reference Cheatsheet
Concept | Key Exam Fact | Example |
|---|---|---|
Trophic level order | Producer β Primary consumer β Secondary consumer β Tertiary consumer | Grass β Grasshopper β Frog β Hawk |
Food chain arrows | Point in direction of energy flow (eaten β eater) | Grass β Rabbit (energy flows from grass to rabbit) |
Pyramid of number | Can be inverted | 1 oak tree β 1000 caterpillars |
10% energy rule | ~10% of energy passed between trophic levels | 10,000 kJ producers β 1000 kJ primary consumers β 100 kJ secondary consumers |
Energy loss reasons | Respiratory heat, egestion, excretion, uneaten parts | Warm-blooded animals lose large amounts of energy as heat via respiration |
7. Frequently Asked
Why are pyramids of energy always upright?
Energy is lost as heat, waste or for respiration at every trophic level, so there is always less total energy available at higher levels than the level below. No exceptions to this rule exist.
Do food chains include decomposers?
Most standard linear food chains do not draw decomposers explicitly, but they are present in all ecosystems, breaking down waste and dead organisms from every trophic level.
Why is eating a plant-based diet more energy efficient?
Plant-based diets mean humans eat directly at the producer trophic level, so far less energy is lost between the source of energy and the consumer, compared to eating meat from higher trophic levels.
Going deeper
What's Next
Now that you have mastered feeding relationships, you are ready to move on to nutrient cycling, which covers how decomposers recycle carbon and nitrogen through ecosystems β a closely linked topic that is often tested alongside feeding relationships in extended response questions. You should also practice applying the 10% energy transfer rule to exam-style calculation questions, and learn to interpret complex food webs to identify trophic levels for different organisms. Understanding these concepts will also prepare you for later topics on human impacts on ecosystems, including overfishing and deforestation, which draw heavily on energy transfer and feeding relationship principles.