Energy Flow and the 10% Rule
Introduced| English |
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| energy pyramid/ˈenədʒi ˈpɪrəmɪd/ |
Energy leaks away
- Energy enters an ecosystem from the Sun and flows up the food chain.
- But at every step, most of it is lost.
- Only a small fraction reaches the next level.
- This steady loss shapes the whole structure of an ecosystem.
The 10% rule
- The 10% rule says only about 10% of energy passes to the next trophic level.
- If producers hold 10,000 units, primary consumers get about 1,000.
- Secondary consumers get about 100, and so on.
- Each step up loses about 90% of the energy.
According to the 10% rule, how much energy passes to the next trophic level?
The 10% rule: only about 10% of the energy at one level passes to the next.
Where the energy goes
- Most lost energy leaves as heat during respiration.
- More is lost in waste and in parts that are never eaten.
- None of it is destroyed — it just becomes unusable heat.
- So energy makes a one-way trip and is not recycled.
The 10% rule
Drag the energy the producers capture and watch only about a tenth pass up each level.
Where does the other ~90% of the energy go?
Most energy is lost as heat during respiration, and in waste and uneaten material.
If producers hold 10000 units of energy, about how many reach the secondary consumers (two levels up)?
$10000 \times 0.1 \times 0.1 = 100$ — two steps of the 10% rule.
The energy pyramid
- Stacking the levels gives an energy pyramid 能量金字塔 — wide at the bottom, tiny at the top.
- Each level holds roughly a tenth of the level below.
- This is why there is so much grass but so few lions.
- And it is why food chains rarely reach more than four or five levels.
The 10% rule is why food chains rarely have more than four or five levels.
So little energy remains after a few steps that higher levels cannot be supported.
Select all true statements about energy flow.
Energy flows one way and is lost, unlike nutrients which cycle. The other three are correct.
Energy is not recycled like nutrients. Nutrients (carbon, nitrogen) cycle round and round, but energy flows one way — in from the Sun, out as heat — and must be constantly resupplied. Never say energy "cycles" through an ecosystem.
Following 10,000 units of energy:
- Producers capture 10,000 units.
- Primary consumers receive about 1,000 (10%).
- Secondary consumers receive about 100, and a fourth level would get only ~10 — too little to support many top predators.
The 10% rule: only about 10% of energy passes to each higher trophic level, the rest lost mostly as heat. Stacking the levels gives an energy pyramid — huge at the base, tiny at the top. Because energy flows one way and is lost at each step, food chains stay short.