All animals need energy – to live, move, grow and reproduce themselves.
This energy comes from their food. Animals need complicated food that has been made by some other animals or plant.
Plants also need energy to live, grow and reproduce. However, they can produce their own food.
Green plants do this by taking:
(1) water and minerals from the soil
(2) the gas, carbon dioxide, from the air
They combine these in a chemical reaction using energy which they get from sunlight.
Animals cannot do this. Many animals obtain their food by eating the plants or plant products such as seeds or fruit.
Some animals are predators, obtaining their food by eating other animals.
Their food is a step farther away from the plants.
This sequence of events can be called a food chain.
There can be variable number of steps, but sunlight is always the original source of energy. At the end of the chain will be an animal with no regular predators of its own, eg a bird of prey.
Here are some examples:
(1) waterplant – invertebrate – fish – grey heron
(2) leaves – caterpillar – blue tit – sparrow hawk
(3) grain – rats – barn owl
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In reality, the chain becomes a complex web, for few predators feed exclusively on one prey animal and few prey animals feed exclusively on one plant.
Most plants will be eaten by several different herbivores. Most herbivores will feed on more than one type of plant. They in turn, will be preyed upon by more than one predator. Most predators will feed on more than one species of prey.
In reality, the result is a complicated system of interlocking food chains, called a food web.