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Biomass
Animals tend to be bigger further up the food chain. Think about all the creatures in a woodland again. There may be 40 blue tits and only one sparrowhawk in a wood. But a sparrowhawk weighs about 20 times more than a blue tit. So the hawk takes up about half as much living material as all the blue tits combined.
Sounds tricky? This diagram might help:
The measurement of living material in an ecosystem is called biomass. A pyramid of biomass shows how much energy there is at each level of a food chain. It shows you how much prey a predator needs in order to survive.