|MadSci Network: Environment/Ecology|
This is a great question. Food chains are an interesting and important subject to understand about the world around us.
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 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 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 besides grass, and foxes eat many things besides rabbits -- and other things also eat grass and rabbits. When talking about the real world, it is more common to think of food webs.
Food chains are still an important concept to understand. In the 1960s, a pesticide called DDT became popular for its effectiveness in killing insect pests. It was sprayed in small concentrations, so as not to affect larger animals, but it never went away once it was sprayed. Eventually, rain washed it into rivers and lakes, where the concentration was still very small, but this is where the food chain took effect. DDT was taken up by plankton in the water, and accumulated in them. They were eaten by small fish, so each small fish accumulated all the DDT from the plankton it ate. Then the small fish were eaten by larger fish, and the larger fish were eaten by birds such as ospreys. Each large fish the osprey ate contained all the DDT from everything below it in the food chain, and that magnified the concentration of the DDT in the water by about 10 million times. Ospreys that were affected by it laid eggs with unusually thin and soft shells, and often their chicks didn't hatch properly. When ecologists noticed this and other similar problems, they knew that DDT was causing damage to the environment, and as a result, it is now banned.
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