Environmental Science / Unit 2: Ecosystems and Energy Flow
2.3 Trophic Levels and the 10% Rule
Notes
Each step in a food chain is a trophic level: producers (level 1), primary consumers (2), secondary consumers (3), tertiary consumers (4).
- Only about 10% of the energy at one level is passed to the next. The rest is used for life processes (respiration) and lost as heat or in waste.
- Energy at the next level = energy at current level $\times 0.10$.
- Because energy shrinks, there are fewer top predators, and food chains rarely have more than 4 or 5 levels.
- An energy pyramid shows this decrease. Net primary productivity (NPP) is the energy producers store after their own respiration.
- Eating lower on the food chain (plants rather than meat) supports more people per area of land.
- Example: producers hold 50,000 kcal. Primary consumers get 5,000 kcal, secondary consumers 500 kcal, and tertiary consumers 50 kcal.
Practice
Producers in a meadow contain 20,000 kcal of energy. About how much reaches the primary consumers using the 10% rule?
$20{,}000 \times 0.10 = 2{,}000$ kcal.
A food chain has 8,000 kcal in the secondary consumers. About how much energy was in the producers (two levels below)?
Going down each level multiplies by 10: $8{,}000 \times 10 \times 10 = 800{,}000$ kcal.
Why are there usually far fewer top predators than herbivores?
Only about 10% of energy moves up, limiting top-level biomass.
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