3.4Unit 3 — Life on Earth
Energy in Ecosystems — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 2 parts Lesson 4 of 6
Section 1 of 2Food chains, food webs, and energy loss
- Food chain: Represents the feeding relationships and energy transfer between organisms:(Arrows indicate the direction of energy flow).
- Producer (Oak tree) → Primary Consumer (Caterpillar) → Secondary Consumer (Blue tit) → Apex Predator (Sparrowhawk)
- Energy transfer between trophic levels:
- In any food chain, only approximately 10% of energy is incorporated into new biomass and passed to the next trophic level.
- Approx. 90% of energy is lost at each link in the chain through:
- Heat generated during cellular respiration.
- Movement and physical cellular activity.
- Undigested waste materials (faeces, urine) and unconsumed parts (bones, roots, bark).
- Because 90% of energy is lost at every step, food chains rarely exceed 4 or 5 trophic levels.
Section 2 of 2Pyramids of numbers vs Pyramids of energy
| Feature | Pyramid of Numbers | Pyramid of Energy |
|---|---|---|
| What is measured | The total population number of individual organisms at each trophic level | The total quantity of energy contained within biomass at each trophic level (measured in kJ m-2 yr-1) |
| Typical shape | Usually broad at base and narrows upwards; but can have irregular / inverted shapes | Always a true regular pyramid shape (never inverted) |
| Irregular examples | A single large producer (e.g. one oak tree supporting thousands of caterpillars) or a host supporting many microscopic parasites (e.g. fleas on a fox) | None — energy is continuously lost as heat and movement at each successive level, so each bar is strictly smaller than the one below. |
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