Energy Flow Through an Ecosystem
In 1942, American ecologist Raymond Lindeman measured energy flow in a lake and found that only about 10% passed to each trophic level — a finding that explained why top predators are always rare.
Printable Worksheets
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Q1 · When a kangaroo eats 100 kJ of grass, how much of that energy do you think ends up as kangaroo body? Where does the rest go?
Q2 · Why do you think there are usually MANY more rabbits than foxes in any place where they both live?
● Know
- The 10% rule — only about 10% of energy passes to the next trophic level
- Energy pyramids narrow upwards (less energy at higher levels)
- Food chains rarely have more than 4–5 levels
● Understand
- Where the missing 90% of energy goes (heat, movement, droppings)
- Why energy pyramids cannot ever be the wrong way up
- How energy pyramids and biomass pyramids are similar (and slightly different)
● Can do
- Apply the 10% rule to calculate energy at each level
- Draw a labelled energy pyramid for a 4-level Australian chain
- Explain why top predators are rare
- Energy
- 10% rule
- Energy pyramid
- Biomass
- Heat loss
- A diagram showing energy at each trophic level
- Energy released by respiration that leaves as heat
- Measured in joules (J); the ability to do work
- Total mass of living material at one level
- ~10% of energy passes to the next level; ~90% is lost
Imagine a kangaroo eating a kilogram of grass — you might expect the kangaroo to grow by a kilogram, but instead it only gains about 100 grams, while the rest is lost as heat, droppings, and the energy cost of moving around and staying warm. Roughly 90% is lost. Let's follow 10 000 kJ of sunlight captured by grass:
| Trophic level | Organism | Energy in body |
|---|---|---|
| 1st — producer | Grass | 10 000 kJ |
| 2nd — primary consumer | Kangaroo | 1 000 kJ |
| 3rd — secondary consumer | Dingo | 100 kJ |
| 4th — tertiary consumer | Wedge-tailed eagle (eating a young dingo) | 10 kJ |
Where does the other 90% go at each step? Three main places:
- Heat from respiration — the biggest loss. Every cell in the kangaroo burns food and releases heat.
- Movement and life processes — hopping, breathing, growing fur, fighting predators.
- Waste — bits the kangaroo can't digest (passed out in droppings) and urine.
The heat that leaves the kangaroo's body is gone — it cannot be eaten by the dingo. That's why each level has less energy than the one before.
Only about % of the energy at one trophic level is passed to the next. The other % is lost — mostly as from .
Wrong: "Energy is destroyed when it flows up the food chain." Energy is never destroyed — it's converted to heat which leaves the ecosystem. Conservation of energy still holds.
Right: Energy is not destroyed — it is transferred to the surroundings as heat. It just becomes useless for the next consumer.
Wrong: "An energy pyramid can be the wrong way up if there are lots of predators." Energy pyramids can NEVER be inverted, because of the 10% rule. There must always be more energy at the bottom.
Right: Energy pyramids always narrow upwards. (A biomass pyramid can occasionally look inverted in oceans, but energy pyramids never can.)
Wrong: "Food chains could easily have 8 or 9 trophic levels." By level 5, almost no energy is left (10 000 kJ → 1 kJ). That's not enough to support a top predator, so chains run out.
Right: Real food chains rarely have more than 4–5 trophic levels because so much energy is lost at each step.
An energy pyramid is just a stacked bar chart of how much energy is at each trophic level. The producer level goes at the bottom, and each higher level is narrower (because it has less energy).
Using the grass → kangaroo → dingo → eagle chain from earlier:
| Level (top to bottom) | Organism | Energy (kJ) | Bar width |
|---|---|---|---|
| 4th | Wedge-tailed eagle | 10 | ▍ |
| 3rd | Dingo | 100 | ███ |
| 2nd | Kangaroo | 1 000 | ████████████ |
| 1st | Grass | 10 000 | ██████████████████████████ |
The pyramid has to narrow as you go up. The widest level is always the producer. The narrowest is always the top predator.
The 10% rule explains one of nature's most striking patterns: food chains rarely have more than 4 or 5 levels. Look what happens if we start with 10 000 kJ in grass:
| Trophic level | Energy |
|---|---|
| 1st | 10 000 kJ |
| 2nd | 1 000 kJ |
| 3rd | 100 kJ |
| 4th | 10 kJ |
| 5th | 1 kJ |
| 6th | 0.1 kJ |
By the 6th level there is barely any energy left — not enough for a big animal to live on. That's why an ecosystem cannot support a "super-predator" that eats wedge-tailed eagles. There simply isn't enough food energy by then.
It's also why top predators are always rare. A pack of dingoes needs hundreds of kangaroos, which need a huge area of grass. Larger and rarer as you go up the pyramid.
You'll meet two similar diagrams. Both narrow upward, but they measure different things:
| Pyramid type | What's measured | Units | Can it be inverted? |
|---|---|---|---|
| Energy pyramid | Energy stored at each trophic level | kJ / m² / year | No — never. Energy must always decrease going up. |
| Biomass pyramid | Total mass of living things at each trophic level | kg or tonnes / m² | Very rarely. In oceans, fast-reproducing plankton may have less biomass at one moment than the fish that eat them. |
For Year 7, the key point is: both pyramids show producers as the widest base, and both show why energy/food becomes scarce at the top.
Grass in a paddock stores 50 000 kJ of energy. A kangaroo eats the grass. A dingo eats the kangaroo. Predict how much energy is in the kangaroo and how much is in the dingo (use the 10% rule). Lock your prediction, then reveal.
How close was your prediction?
Great — you applied the 10% rule across two steps correctly.
Good — the trick is to do ×0.1 once per step, not once for the whole chain.
At the start of the lesson you were asked: why don't food chains have ten levels? Why does nature stop at about four or five?
Now you know the 10% rule, write your full answer. Use the numbers — if grass captures 1000 units of energy, how much is left by level 4? Does that explain why a level 5 predator is almost impossible?
Q1. Explain the 10% rule. Where do the missing 90% of the energy go? (3 marks)
Q2. Grass in a paddock stores 20 000 kJ of energy. Using the 10% rule, calculate the energy in the kangaroo, then in the dingo that eats the kangaroo, then in the wedge-tailed eagle that eats the dingo. Show your working. (4 marks)
Q3. Explain why an energy pyramid can NEVER be drawn upside down (with more energy at the top than the bottom). Use the 10% rule and the idea of heat loss in your answer. (4 marks)
Answers
▾MCQ 1
B — 10% of 8 000 kJ = 800 kJ. The other 90% (7 200 kJ) is lost as heat, movement and waste.
MCQ 2
D — The missing 90% is mostly lost as heat from respiration, plus what's used in movement and what comes out as droppings/urine. Energy is not destroyed (A is wrong), it's transformed into heat that leaves the ecosystem.
MCQ 3
A — Energy pyramids always have producers at the widest base and narrow upward, because of the 10% rule. B, C and D are wrong.
MCQ 4
C — Because only ~10% of energy passes to each next level, after 4–5 steps there is barely any energy left to support a larger animal. A, B and D are not real reasons.
MCQ 5
D — A biomass pyramid measures total mass of living material at each level (kg or tonnes). It usually also narrows upwards but measures mass rather than energy.
Short Answer 1
Model answer: The 10% rule says that only about 10% of the energy stored at one trophic level is passed on to the next. The other 90% is lost — mostly as heat from respiration in the organism's cells, plus what is used to move, breathe and grow, plus what is passed out as waste (droppings and urine). Energy isn't destroyed; it just leaves the ecosystem as heat and so cannot be eaten by the next level. 1 mark for the 10% statement, 1 for heat from respiration, 1 for any second loss (movement, growth or waste).
Short Answer 2
Model answer: Grass: 20 000 kJ. Kangaroo = 20 000 × 0.10 = 2 000 kJ. Dingo = 2 000 × 0.10 = 200 kJ. Wedge-tailed eagle = 200 × 0.10 = 20 kJ. 1 mark for each correct value (3 marks) plus 1 mark for clearly showing the ×0.1 working at each step.
Short Answer 3
Model answer: An energy pyramid can never be drawn upside down because the 10% rule means each higher trophic level has LESS energy than the one below — about 90% is lost between levels, mostly as heat from respiration. Heat that leaves an organism is gone from the ecosystem and cannot be eaten by the next level. So the producer level always has the most energy (it captured it from the sun) and the top predator level always has the least. There is no way for energy to "go back up" the pyramid, so the shape must always narrow upward. 1 mark for the 10% rule, 1 for "heat is lost from respiration", 1 for "heat cannot be re-eaten", 1 for clear conclusion that the shape must narrow upward.