Energy Flow, Food Chains, Food Webs and Pyramids
BiologyΒ· 19.1, 19.2Β· 15 min read
1. Energy Flow in Ecosystems (Core)β β ββββ± 4 min
Energy Flow
The unidirectional transfer of energy from one organism to the next in an ecosystem, starting with light energy from the Sun absorbed by producers via photosynthesis.
Only ~1% of the Sunβs light energy reaching Earthβs surface is captured by producers. This energy is converted to chemical energy stored in glucose and other organic molecules, which is then passed between organisms when they feed on other organisms.
State the original source of energy for a grassland ecosystem, and name the process that producers use to capture this energy.
- 1
- Identify the original energy source: Light energy from the Sun (not producers, which only capture the energy).
- 2
- Name the capture process: Photosynthesis, which converts light energy to stored chemical energy in organic molecules.
Exam tip:
Always name the Sun as the original energy source for standard ecosystems in Core questions to gain full marks; answers listing only producers will be marked incorrect.
2. Food Chains and Food Webs (Core)β β β βββ± 5 min
Food Chain
A linear sequence of organisms showing the transfer of energy from one organism to the next, always starting with a producer at the first trophic level.
Example:
Grass β Grasshopper β Frog β Hawk
A food web is a network of interconnected food chains that shows all possible feeding relationships in an ecosystem, which is a more accurate representation of real-world feeding patterns than isolated food chains.
From the food chain: Algae β Zooplankton β Small Fish β Heron, identify the secondary consumer and state its trophic level.
- 1
- List trophic levels in order: Level 1 = Algae (Producer), Level 2 = Zooplankton (Primary Consumer), Level 3 = Small Fish (Secondary Consumer), Level 4 = Heron (Tertiary Consumer).
- 2
- Secondary consumer is the small fish, occupying the 3rd trophic level.
3. Energy Transfer Efficiency (Extended Only)β β β β βExtended onlyβ± 6 min
Approximately 90% of energy is lost between consecutive trophic levels, so only ~10% of energy is passed to the next level. Energy is lost as heat from respiration, in waste products (faeces, urine), and in uneaten body parts (e.g. bones, woody plant stems). To calculate percentage energy transfer, use the formula:
If 25,000 kJ of energy is stored in grass (producers) in a grassland ecosystem, calculate the amount of energy available to tertiary consumers in the food chain: Grass β Rabbit β Fox β Eagle.
- 1
- Calculate energy at primary consumer (rabbit) level: 10% of 25,000 kJ = 2,500 kJ.
- 2
- Calculate energy at secondary consumer (fox) level: 10% of 2,500 kJ = 250 kJ.
- 3
- Calculate energy at tertiary consumer (eagle) level: 10% of 250 kJ = 25 kJ.
- 4
- Final answer: 25 kJ of energy is available to the eagle population.
Exam tip:
You may be given actual energy values instead of the 10% rule in exam questions; always use the values provided to calculate transfer efficiency for full marks.
4. Ecological Pyramids (Core + Extended)β β β βββ± 5 min
Ecological Pyramid
A diagram that represents the relative amount of energy, biomass, or number of organisms at each trophic level in an ecosystem.
Pyramid Type | Tier Requirement | Key Shape Notes |
|---|---|---|
Pyramid of Number | Core | Can be irregular/inverted (e.g. one large tree supports thousands of insects) |
Pyramid of Biomass | Core | Almost always upright, rare exceptions for aquatic ecosystems |
Pyramid of Energy | Extended | Always perfectly upright, units are kJ/mΒ²/year |
Explain why a pyramid of energy can never be inverted, but a pyramid of number can be inverted.
- 1
- Energy is always lost between trophic levels, so the total energy at a higher trophic level can never exceed the total energy of the level below it, making energy pyramids always upright.
- 2
- A pyramid of number counts individual organisms, so a single large producer (e.g. an oak tree) can support hundreds of primary consumer individuals (caterpillars), making the first trophic level bar smaller than the second, inverting the pyramid.
Exam tip:
If asked to draw an ecological pyramid in an exam, always label each trophic level clearly and write the units for the pyramid type you are drawing.
5. Common Pitfalls
Wrong move:
Drawing food chain arrows pointing from predator to prey
Why:
Arrows represent the direction of energy flow, not the direction of feeding action.
Correct move:
Always draw arrows pointing from the organism being eaten to the organism that consumes it.
Wrong move:
Stating producers are the original source of energy in an ecosystem
Why:
Producers only capture light energy from the Sun, which is the ultimate original source of energy for almost all ecosystems.
Correct move:
Name the Sun as the original energy source for all standard ecosystem questions.
Wrong move:
Assuming all ecological pyramids are always upright
Why:
Pyramids of number and biomass can be inverted in specific cases, so this generalisation is incorrect.
Correct move:
Only state that pyramids of energy are always upright, and note possible exceptions for number and biomass pyramids.
Wrong move:
Calculating energy transfer as 10% of the higher trophic level to get the lower level value
Why:
Energy decreases as you move up trophic levels, so you always multiply the lower level energy by 0.1 to get the higher level value.
Correct move:
Use the percentage energy transfer formula to cross-check your calculations for Extended questions.
Wrong move:
Counting primary consumers as the first trophic level
Why:
Producers are always the first trophic level, so primary consumers occupy the second trophic level, secondary consumers the third, etc.
Correct move:
Always start counting trophic levels from the producer at level 1, moving up one for each consecutive consumer.
6. Quick Reference Cheatsheet
Concept | Key Fact | Exam Answer Reminder |
|---|---|---|
Original Energy Source | Sun, 1% captured by producers | Always name the Sun first when asked for energy source |
Food Chain Arrows | Point in direction of energy flow | Draw arrows from prey to predator, never reverse |
Energy Transfer | ~10% passed between levels, 90% lost | Use 10% rule if no values are given in Extended questions |
Pyramid of Energy | Always upright, units kJ/mΒ²/year | Never state energy pyramids can be inverted |
Trophic Levels | 1=Producer, 2=Primary Consumer, 3=Secondary, 4=Tertiary | Count levels starting at producers, not consumers |
7. Frequently Asked
Why do most food chains have a maximum of 4 trophic levels?
Only ~10% of energy is passed between each trophic level, so after 3-4 transfers, there is insufficient energy left to support a viable population of higher consumers.
What is the difference between a pyramid of number and a pyramid of biomass?
A pyramid of number counts the total individual organisms at each trophic level, while a pyramid of biomass measures the total dry mass of organisms at each level. Pyramids of number can be inverted, but biomass pyramids are almost always upright.
What's Next
Now that you have mastered energy flow, food chains, webs and pyramids, you are ready to move on to nutrient cycles and human impacts on ecosystems, the next core topics in the Organisms and Their Environment unit for CIE IGCSE Biology 0610. Practice constructing food webs from given organism lists, calculating energy transfer efficiency (for Extended tier), and interpreting pyramid diagrams to answer exam-style questions, as these are frequent high-mark questions in both Core and Extended papers. Make sure you can clearly explain the reasons for energy loss between trophic levels to gain full marks in extended response questions.