A food chain is an idea developed by a scientist named Charles Elton in 1927. He described the way plants get energy from sunlight, plant-eating animals get their energy from eating plants, and meat-eating animals get their energy from eating other animals. The idea of a chain means that all these animals are linked together, so anything that affects one link in the chain affects everything in the chain.
]The key concept that we need to learn from food chains, food webs and food pyramids is the transfer of energy. For all life on earth there is only one available source of energy, the sun. Furthermore, only plants can change this energy into a form that can be used by animals. All plants rely either directly or indirectly on plants for their energy. Plants feed upon sunlight. Only plants have the ability to convert sunlight into food that they and animals can use.
Feeding levels are also called trophic levels. The word "trophic" is a Greek word for nourishment. To determine the trophic level, we count the number of energy transfers. Green plants are at the first trophic, or nourishment, level because there has been one transfer of energy, from the sun to the plants.
The first link in the chain, the plant, is called the producer, while all the links above it are called consumers. For example, look at a simple chain in which grass uses sunlight to produce sugars and proteins so it can grow. Rabbits eat the grass, and get energy from it. Foxes eat rabbits and get energy from them. Nothing eats foxes, so they are said to be at the top of that particular food chain. If something happens to the grass, suppose a farmer plows up some of the grass to plant a field. Then the rabbits have less food and some of them will die. Then because there are fewer rabbits, some of the foxes will die, too, even though they don't eat the grass directly.
Of course, in the real world, there are no simple food chains like this. Rabbits eat many things...