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Science

Food chains

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Food chains, pyramids of numbers and pyramids of biomass are all ways of showing how energy is passed on from one organism to another.

Biomass is the food available for the next trophic level (feeding level) in a food chain. Biomass is used as a source of renewable energy to reduce our dependence on fossil fuels.

Food chains and pyramids

Food chains

Here is an example of a food chain. Grass is eaten by voles that are eaten by owls.

the vole eats the grass seed, the barnowl eats the vole

Food chain

A food chain shows how energy is passed on from one organism to another. The original source of energy is the Sun, which in this example provides the sunlight needed by the grass for photosynthesis. The grass is the producer in this food chain. The vole and the owl are consumers.

Pyramid of numbers

Shows the number present at each stage of the food chain, represented by a bar. Sparrowhawk has a thin bar representing a smaller number. Blue tit has a thicker bar - a greater number. Caterpillar has a thicker bar than the blue tit - again, an increase in number. The oak tree has a thinner bar again - around the same width as the sparrowhawk, representing that there are fewer trees in the food chain.

A pyramid of numbers

The number of organisms in a food chain can be represented graphically in a pyramid. Each bar represents the number of individuals at each trophic level (feeding level) in the food chain.

In this example a large number of caterpillars living in a single oak tree provide food for several blue tits, which in turn are consumed by a sparrowhawk.

The pyramid of numbers usually shows that the number of organisms at each trophic level gets smaller towards the top. This particular case is an exception – one tree provides food for many caterpillars.

It usually takes a large number of plants to provide sufficient energy for the consumers in the food chain.

Pyramid of biomass

Shows amount of biomass increasing as you go down the food chain, from sparrowhawk having a small amount, to blue tit having slightly more, to caterpillar having slightly more again, to oak tree producing the most amount of biomass.

A biomass pyramid

A pyramid of biomass is a more accurate indication of how much energy is passed on at each trophic level.

Biomass is the mass of living material in each organism multiplied by the total number of organisms in that trophic level. This makes it easier to compare the food value of a small number of large organisms with a large number of small organisms. Pyramids of biomass usually are a true pyramid shape (each level is smaller than the one below it).

The biomass in each trophic level is always less than the trophic level below. This is because biomass is a measure of the amount of food available. When animals eat, only a small proportion of their food is converted into new tissue, which is the food for the next trophic level. Most of the biomass that animals eat is either not digested, or used to provide the energy needed for staying alive.

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