Why Is the Number of Trophic Levels in an Ecosystem Limited?

Direct answer: The number of trophic levels is limited because energy is lost as it moves through a food chain. Eventually, too little chemical energy and biomass remain to support another consumer population.

Level: This explanation is required at both SL and HL in Topic 2.2, Energy and biomass in ecosystems. Calculating ecological efficiency is HL only.

How Energy Loss Limits Food-Chain Length

Producers transform light energy into chemical energy stored in biomass. When a consumer eats an organism, only part of that biomass becomes new consumer biomass that can be transferred to the next trophic level.

These losses reflect the second law of thermodynamics: energy transformations are inefficient and increase the dispersal of energy. Energy therefore flows through ecosystems rather than being recycled. Matter can cycle, but energy ultimately leaves the ecosystem as heat.

For an HL illustration, if producers contain $10{,}000\text{ kJ}$ and ecological efficiency is hypothetically $10\%$, primary consumers receive $1{,}000\text{ kJ}$, secondary consumers $100\text{ kJ}$, and tertiary consumers only $10\text{ kJ}$. This explains why top predators are relatively scarce and food chains are usually short.

A common misconception is that the “10% rule” is a fixed law. It is not: ecological efficiency varies among ecosystems, trophic levels, and species. SL students should explain energy losses qualitatively without assuming a percentage.

Exam technique: For an explain question, identify specific energy losses, link them to declining available biomass, and conclude that insufficient energy remains to support additional trophic levels. Do not simply state that “energy decreases.”