All living organisms need energy.

Animals obtain energy by eating other plants or animals. They are called consumers.

Green plants obtain light energy from the Sun to make their own food by photosynthesis.

They store the energy in chemicals inside the plant cells.

Green plants are called producers, because they produce energy in a usable form for all other organisms.

The ultimate source of energy for all organisms is therefore the Sun.

A food chain shows how energy is transferred through a living system.

For example:

maize		cow		human
producer		primary consumer		secondary consumer

On average, only about 10% of the light energy from the Sun actually ends up stored in the plant tissues.

If an animal eats the plant, only 10% of this is used to build new animal tissues. The rest is used by the animal to stay alive, keep warm and move about, and some is lost in its faeces and urine.

Large mammals and birds lose a lot of energy as heat to their surroundings, because their bodies must be kept at a constant temperature, which is usually higher than that of their surroundings.

This diagram shows how much energy is transferred through a food chain:

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Pyramids of Numbers  

The information in a food chain can be shown in a pyramid of numbers.

Each block in the pyramid represents the number of organisms at each stage in the food chain.

A pyramid of numbers for the food chain above would look like this:

The number of organisms at each level decreases because there is less energy available.

However, sometimes a pyramid of numbers is not pyramid-shaped.

For example, this food chain would give a pyramid of numbers shaped like this:

         oak tree            caterpillars           shrews         owl

Here, one large oak tree can provide enough energy for many caterpillars, which provide energy for fewer shrews and these provide energy for one owl.

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Pyramids of Biomass

Biomass is the total mass of the organisms at each stage in a food chain. This always gets less as you move along a food chain.

The biomass at each stage can be drawn to scale as a pyramid of biomass.

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Food Production and Energy Efficiency

At each stage in a food chain, less material and less energy are contained in the biomass of the organisms.

Long food chains are therefore inefficient.

If a farmer has one hectare of land, he could either use the land to grow food for cows, which would then be fed to humans, or use the land to grow maize to feed humans directly.

It would be more efficient to grow maize for humans, because less energy is wasted in the shorter food chain. In fact, ten times as many people could be fed from the same area of land.

Shorter food chains are more efficient for food production.

The efficiency of food production can also be improved by reducing the amount of energy lost by animals by limiting their movement and controlling the temperature of their surroundings.

However, many people oppose these methods because they say they are cruel.

The efficiency of producing fruit for human consumption is increased by using hormones to regulate their ripening on the plant and during transport to consumers, so less is wasted.

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