1.9 Trophic Levels

Levels in the energy pyramid

Trophic levels describe an organism's feeding position. Producers (autotrophs) make energy via photosynthesis or chemosynthesis. Primary consumers (herbivores) eat producers. Secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. Decomposers/detritivores break down dead matter, recycling nutrients back to producers.

Energy flow and the 10% rule

Energy transfer between trophic levels is inefficient — only about 10% of energy is passed to the next level; the rest is lost as heat (via respiration/metabolism) or unconsumed biomass. This is why energy pyramids are wide at the base (producers) and narrow at the top, limiting the number of trophic levels an ecosystem can support.

Ecological efficiency and biomass

Ecological efficiency is the percentage of energy transferred from one trophic level to the next, typically 10% (range 5–20%). This limits biomass at higher levels, meaning fewer top predators can be supported. A food chain shows a single energy path, while a food web shows the many interconnected feeding relationships in an ecosystem.

Key terms

Trophic level

An organism's position in a food chain based on its feeding role (producer, consumer, decomposer).

10% rule

On average, only about 10% of energy is transferred to the next trophic level; the rest is lost as heat.

Primary consumer

An organism that eats producers (herbivore).

Ecological efficiency

The percentage of energy transferred from one trophic level to the next, usually 5–20%, averaging 10%.

Food web vs. food chain

A food chain shows one linear energy path; a food web shows the many interconnected feeding relationships in an ecosystem.

Try an AP-style question

A hawk eats a snake that ate a mouse that ate grass. What trophic level is the hawk?

1. Secondary consumer (3rd trophic level)
2. Primary consumer (2nd trophic level)
3. Quaternary consumer (5th trophic level)
4. Tertiary consumer (4th trophic level)