Grade 11 · Biology
Energy flow & trophic levels
Why it matters
Food chains, food webs, pyramids and the 10% rule.
Last reviewed 2026-09-03 · accuracy standard
How does energy flow through an ecosystem?
Short answer
Energy enters an ecosystem as sunlight, is captured by producers through photosynthesis, and flows to consumers as they eat one another along a food chain. At each trophic level only about 10% of the energy is passed on; the rest is lost mostly as heat. Because energy is not recycled, ecosystems need a continual input from the Sun.
Concept preview
What you will be able to do
- 01Distinguish producers, consumers and decomposers and describe their roles in an ecosystem.
- 02Build food chains and food webs and identify the trophic level of any organism.
- 03Explain the 10% rule, why energy is lost between trophic levels, and read energy pyramids.
- 04Distinguish gross and net primary productivity, and explain bioaccumulation and biomagnification.
Every ecosystem on Earth runs on a single dazzling power source: the Sun. Green producers capture a thin sliver of the sunlight that reaches the ground and lock its energy into the chemical bonds of sugar, and from that moment the energy begins a strictly one-way journey through the living world. This is the master idea of ecology and it is worth stating carefully: energy flows, but matter cycles. Energy enters an ecosystem as sunlight, is handed from organism to organism, and is steadily bled off as heat until nothing useful remains; it never loops back and never returns to the Sun. Atoms of carbon, nitrogen and phosphorus, by contrast, are used over and over again, endlessly recycled between living things and the physical environment. Grasping the difference between a flow and a cycle is the key that unlocks the whole chapter.
School Plan
Energy flow & trophic levels
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Related topics
Frequently asked questions
- What is the 10% rule?
- Only about 10% of the energy at one trophic level is passed to the next; the rest is used for life processes or lost as heat.
- Why are food chains usually short?
- Because so much energy is lost at each level, there is rarely enough left to support more than four or five trophic levels.