1.11 Food Chains and Food Webs

Trophic levels and energy flow

A food chain shows a single path of energy transfer, starting with producers (autotrophs), then primary consumers (herbivores), secondary consumers, and tertiary consumers. Decomposers and detritivores break down dead matter, recycling nutrients. Energy flows one direction and decreases at each trophic level, mostly lost as heat via respiration.

Food webs and the 10% rule

A food web links multiple interconnected food chains, showing that organisms often eat and are eaten by many species. Only about 10% of energy transfers from one trophic level to the next (the 10% rule); the rest is lost as heat. This limits the number of trophic levels an ecosystem can support.

Ecological pyramids

Ecological pyramids (of energy, biomass, or numbers) visually represent this energy loss — each level is smaller than the one below. Energy pyramids are always largest at the producer base, explaining why top predators are relatively rare and why eating lower on the food chain is more energy-efficient.

Key terms

Trophic level

A position in a food chain/web, such as producer, primary consumer, or secondary consumer.

10% Rule

Only about 10% of energy is transferred from one trophic level to the next; the rest is lost as heat.

Food web

A model showing the interconnected feeding relationships among many organisms in an ecosystem.

Decomposers vs. detritivores

Decomposers (fungi, bacteria) chemically break down dead matter; detritivores (worms, insects) physically consume dead organic material.

Ecological pyramid

A diagram showing the decrease in energy, biomass, or number of organisms at each successive trophic level.

Try an AP-style question

Wolves were reintroduced to Yellowstone, and elk stopped overgrazing riverbanks. What is this an example of?

1. Primary succession
2. Commensalism
3. Competitive exclusion
4. A trophic cascade