Energy Flow & Trophic Levels

You’ll be able to

Trace energy from the sun through producers, consumers, and decomposers
Apply the 10% rule to calculate energy available at each trophic level
Explain why food chains rarely exceed four or five trophic levels

Energy flows; it does not cycle

The single most important idea in ecosystem ecology: matter cycles, but energy flows one way. Energy enters as sunlight, is captured by producers (autotrophs) through photosynthesis, passes to consumers that eat them, and leaves at every step as heat. Because heat cannot be re-fed into photosynthesis, an ecosystem needs a constant new input of sunlight to keep running.

Trophic levels are feeding positions

Each step in the flow is a trophic level. Producers occupy the first level. Primary consumers (herbivores) are the second, secondary consumers (which eat herbivores) the third, and tertiary consumers the fourth. Decomposers and detritivores feed on dead material at every level, releasing nutrients back to the soil and water.

Why so little energy moves up

Only a small fraction of the energy at one level reaches the next. Most is lost: organisms spend energy on cellular respiration (movement, growth, heat), some food is never eaten, and some is eaten but passed as undigested waste. As a rule of thumb, roughly 10% of the energy at one trophic level is stored as biomass available to the next — the remaining ~90% is lost, mostly as heat. Plotting the energy at each level therefore produces an energy pyramid, always widest at the base, and it explains why food chains rarely exceed four or five links: after a few 10% cuts, too little energy remains to support another level of predators.

The 10% rule

energy at next level ≈ 0.10 × energy at current level

A rule of thumb, not a law — real efficiencies range from ~5% to ~20%. AP problems assume exactly 10% unless told otherwise.

Worked example

Producers in a meadow capture 100,000 kcal·m⁻²·yr⁻¹. Using the 10% rule, how much energy is available to the tertiary consumers (the fourth trophic level)?

1.Producers (level 1) = 100,000 kcal·m⁻²·yr⁻¹.
2.Primary consumers (level 2) = 10% of 100,000 = 10,000 kcal·m⁻²·yr⁻¹.
3.Secondary consumers (level 3) = 10% of 10,000 = 1,000 kcal·m⁻²·yr⁻¹.
4.Tertiary consumers (level 4) = 10% of 1,000 = 100 kcal·m⁻²·yr⁻¹.

Answer: 100 kcal·m⁻²·yr⁻¹ — just 0.1% of the energy the producers originally captured.

Checkpoint

A population of producers stores 50,000 kcal. Applying the 10% rule, how much energy is available to the secondary consumers (third trophic level)?

Watch out

Count trophic levels carefully. "Secondary consumer" is the third trophic level, not the second — producers are level 1. Off-by-one level errors are the most common way to get a 10%-rule problem wrong, because each extra step changes the answer by a factor of ten.

Checkpoint

Why do most food chains contain no more than four or five trophic levels?

On the exam

On free-response questions, showing the chain of 10% multiplications earns the calculation points even if the final number is off. Always write the units (kcal·m⁻²·yr⁻¹) — AP awards a point for correct units, and dropping them is an easy way to lose it.

Answer the 2 checkpoints as you read.