Energy Loss Between Levels
Explain how energy is transferred between trophic levels in a food chain, why energy is lost at each stage, and use pyramids of biomass/numbers to represent this
What a learner can do afterwards
- Explains that only about 10% of energy passes from one trophic level to the next
- Constructs a pyramid of biomass from data and explains its shape
- Identifies where energy is lost at each trophic level (heat, movement, waste)
- Explains why food chains rarely have more than five trophic levels
1 · Read
You already know a food web shows who eats whom. But when energy moves from one animal to the next, it does not all get passed along. Only a small part makes it to the next level.
Scientists have a rule of thumb: only about 10% of the energy at one trophic level passes on to the next. The other 90% is lost along the way.
Picture a field with 10,000 kg of grass. Insects eat the grass, but most of that energy is used for movement and lost as heat, so only about 1,000 kg becomes insect body mass. Frogs eat the insects and only get about 100 kg worth. By the time a hawk eats the frogs, there is only about 10 kg of energy left.
You can also read the rule backwards. If a level ends up with about 20 kg of biomass, the level below it must have started with about ten times as much, around 200 kg. Each step down the pyramid multiplies by ten, just as each step up divides by ten.
That is why a food chain almost never goes past four or five levels: there just is not enough energy left to feed one more level. It is also why a field has thousands of plants but only a few foxes.
Energy shrinks to about a tenth at every step, which is why pyramids narrow fast and food chains stay short.
More to read
46.2 Energy Flow through Ecosystems
Picture a food chain as a staircase of eaters. Plants stand on the bottom step, plant eaters on the next, and hunters above them. Each step is called a trophic level. Energy climbs the stairs as each level eats the one below.
But the climb is wasteful. At every step, most energy is burned on movement, warmth, and waste, and only about a tenth passes on. That is why the bottom steps must be huge to support the few hunters at the top.
20.1 Energy Flow through Ecosystems
A pyramid of biomass shows how much living material sits at each level. The base is wide because plants and plankton weigh far more altogether than all the top hunters. If you count individuals instead, you get a pyramid of numbers, wide at the bottom for the same reason.
Both pyramids have the same shape because energy is lost on the way up. So much is lost at each transfer that long chains run out of fuel. That is why food chains almost never stretch past five levels.
Learn: Trophic levels and energy loss
Where exactly does the missing energy go? Some escapes as body heat that warms the air. Some is spent on swimming, running, and hunting. The rest leaves the body as waste that decomposers claim.
Test the idea with numbers. If plants hold 10,000 units of energy, plant eaters keep roughly 1,000, and the next hunters keep roughly 100. This ten percent rule explains why a field holds crowds of grasshoppers but only a handful of hawks.
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2 · Watch
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Where it sits
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.