Energy Flow & Trophic Levels

You’ll be able to

Trace energy from producers through consumers and decomposers along a food chain
Apply the 10% rule to calculate energy available at each trophic level
Explain why energy pyramids taper and why food chains are short

Who makes the energy, and who spends it

Almost every ecosystem runs on sunlight. Producers (autotrophs) — mostly plants, algae, and cyanobacteria — capture solar energy through photosynthesis and store it as chemical energy in organic molecules. Everything else is a consumer (heterotroph) that must eat to get energy: primary consumers (herbivores) eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. Running underneath it all, decomposers (fungi and bacteria) break down dead organisms and waste at every level, releasing nutrients back to the soil and water so producers can use them again.

Trophic levels and the direction of energy

Each feeding step is a trophic level. Energy enters at the producer level and moves upward through the levels as one organism eats another. The single most important rule about this flow: energy moves in one direction and is never recycled. Nutrients (carbon, nitrogen) cycle in loops, but energy makes a one-way trip — it enters as sunlight and ultimately leaves as heat. That is why an ecosystem needs a constant new supply of solar energy but can keep reusing the same atoms indefinitely.

Why so little energy makes it up

When one organism eats another, only a small fraction of the prey’s stored energy becomes new body mass in the predator. Most is lost: it is used for the organism’s own cellular respiration and movement (released as heat), it is never eaten (bones, roots), or it passes out undigested as waste. On average, only about 10% of the energy at one trophic level is stored as biomass available to the next level. This "10% rule" is a rough but powerful accounting tool.

The 10% rule

energy at next level ≈ 0.10 × energy at current level

Roughly 90% of the energy at each trophic level is lost as heat (respiration), waste, and uneaten parts; only ~10% is stored as biomass the next level can eat.

Energy pyramids

Stack the trophic levels and plot the energy each one holds and you get an energy pyramid: a wide base of producers and ever-narrower tiers above. Because roughly 90% of energy is lost at each step, the amount of energy — and usually the total biomass and number of organisms — shrinks dramatically going up. This is exactly why food chains are short (rarely more than four or five levels): by the fifth level, so little of the original solar energy remains that it cannot support another level of predators. It is also why top predators are rare and need large territories.

Worked example

A grassland’s producers capture 45,000 kcal·m⁻²·yr⁻¹. Using the 10% rule, estimate the energy available to the primary consumers and to the secondary consumers.

1.Producers hold 45,000 kcal·m⁻²·yr⁻¹. Primary consumers receive about 10% of that: 0.10 × 45,000 = 4,500 kcal·m⁻²·yr⁻¹.
2.Secondary consumers eat the primary consumers, so they receive about 10% of the primary consumers’ energy: 0.10 × 4,500 = 450 kcal·m⁻²·yr⁻¹.
3.Notice the pattern — each step down to a tenth: 45,000 → 4,500 → 450. Two trophic transfers means multiplying by 0.10 twice (×0.01 overall).

Answer: Primary consumers get ≈ 4,500 kcal·m⁻²·yr⁻¹ and secondary consumers get ≈ 450 kcal·m⁻²·yr⁻¹.

Checkpoint

Producers in a lake store 60,000 kcal·m⁻²·yr⁻¹. Applying the 10% rule, about how much energy is available to the tertiary consumers (the third consumer level up)?

Watch out

The 10% rule counts transfers between levels, not the number of organisms named. Producers → primary → secondary → tertiary is three ×0.10 steps (÷1000 overall), not two. Always start counting from the producer level and multiply by 0.10 once for each arrow up.

Food chains vs. food webs

A food chain is a single linear path — grass → grasshopper → mouse → hawk. Real ecosystems are messier: most organisms eat several kinds of food and are eaten by several kinds of predator. A food web links many food chains into a network, showing all the feeding relationships at once. Food webs are more realistic and more stable: if one prey species disappears, a predator with several food sources can shift to another, so the whole system is less likely to collapse.

Checkpoint

Why do food chains rarely have more than four or five trophic levels?

On the exam

For any energy-flow free-response, lead with the one-way principle: energy enters as sunlight, flows up one direction, and is lost as heat at every step — it is never recycled. Then apply the 10% rule quantitatively. Naming that ~90% loss (respiration, heat, waste) is what earns the explanation point.

Answer the 2 checkpoints as you read.