Energy passed along
Who Eats Whom?
A food chain traces sunlight as it moves from plants through animals, losing most of itself at each step.
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Step 1 of 5
It starts with plants, always.
Only plants and algae can turn sunlight into food. Every animal, however fierce, is ultimately living on what a plant captured.
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Step 2 of 5
Eating passes the energy along.
A rabbit eating grass takes some of the energy the grass captured. A fox eating the rabbit takes a share of that.
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Step 3 of 5
Most of it is lost at every step.
Roughly 90 percent is spent on moving, staying warm and living. Only about a tenth is available to whatever eats next.
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Step 4 of 5
Which is why chains are short.
After four or five links there is essentially nothing left. That is why there are no predators that hunt lions — the energy has run out.
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Step 5 of 5
Decomposers close the loop.
Fungi and bacteria break down everything that dies, returning nutrients to the soil for plants. Without them the chain would stall.
Real ecosystems are webs, not chains. Most animals eat several things and are eaten by several others.
The short version
About 90% of the energy is spent on living at every step, so only a tenth passes upward. After four or five links there is nothing left — which is why nothing hunts lions. Decomposers return the rest to the soil.
Try it yourself
Trace your dinner backwards to a plant, and then to sunlight. Every meal gets there within a few steps.