05 1.4 Energy transfer between levels (extension with calculations)
- Only about 10 % of the energy at one trophic level passes to the next; the rest is used in respiration (lost as heat), movement, and not eaten or not digested.
- This is why food chains are short (usually 4–5 levels) and why there are far fewer top predators than producers.
Worked example 1.
- Problem. Phytoplankton in a lake fix 50 000 kJ of energy per square metre per year.
- Using the 10 % rule, find the energy available at the levels zooplankton, small fish and large fish.
- Why can the lake support only a few large fish?
Solution. Zooplankton (primary consumers) get 10 % of 50 000 = 5000 kJ. Small fish (secondary consumers) get 10 % of 5000 = 500 kJ. Large fish (tertiary consumers) get 10 % of 500 = 50 kJ. Only 0.1 % of the producers' energy reaches the top level; since a large fish needs a lot of energy to survive, only a small number can be supported.
Worked example 2 (predator removal).
- Problem. A reef has 200 predatory fish that eat 40 000 small fish per year.
- The small-fish population (about 100 000) is stable because its births (minus natural deaths) add exactly 40 000 fish per year.
- What happens if overfishing reduces the predators to 50?
- (Assume each predator eats the same number of fish.)
Solution. Each predator eats 40 000 ÷ 200 = 200 small fish per year. With 50 predators the removal falls to 50 × 200 = 10 000 per year. Births still add 40 000, so the population now grows by 40 000 − 10 000 = +30 000 per year (+30 %). Within a few years the small fish overgraze the plankton and algae, and the food supply collapses, so the whole system is out of balance.