As soon as an animal eats a plant, a food chain is formed. All animals are heterotrophs ("other-feeders"), since they find energy by consuming other living things.

• Herbivores eat plants, becoming primary or first-order consumers.
• Carnivores eat herbivores, becoming secondary or second-order consumers.
• Other carnivores may eat the second-order carnivores, so they are third-order consumers. Some carnivores may eat both herbivores and other carnivores, making them both second- and third-order consumers on different food chains.
• At every trophic level, decomposers (like bacteria, fungi, moss, lichen) make use of the wastes and remains of dead organisms. They use the energy in these decaying

  A pyramid of energy shows where the available energy lies in an ecosystem. The greatest amount is found in the plants; the least amount in the top-level consumers.

  remains to fuel their own metabolism. They also break down organic compounds into inorganic ones, making substances available for reuse.

Food chains can have as few as one link, or as many as six. Very few food chains have more than six links. Why? The food web consists of the transfer of energy from one species to another. Most of the energy transferred is used up by the consumer—only 10% is available for the next animal. By the sixth trophic level, very little energy remains to be consumed.

The animal at the top trophic level of a food chain faces no predators, but these species often face the greatest chance of extinction, since they depend on so many other species below them on the food chain.

But food chains show only part of the story. The true complexity of these energy transfers is shown in a food web.