Food Chains and Food Webs

周越

How did you get to school today？ Did you walk， ride your bike， or take a bus？ Did you walk from class to class during the day？ Did you write in your notebook today？ All of these activities require energy. We obtain energy from the food we eat. But how did energy get into our food in the first place？

The path energy takes in an ecosystem begins with the Sun. All plants grow by absorbing energy from the Sun and turning it into foods such as sugars and starches. Th e energy of the Sun is stored in the plant. When an animal such as a chicken eats grain （seeds of various grass plants）， the energy stored in the plant is transferred from the starches in the grain to the chicken. When you eat chicken for dinner， some of the energy is passed on to you. So， energy starts from the Sun and passes from producers to consumers.

A food chain is a model that shows how energy and nutrients flow from one organism to another in an ecosystem. Arrows show the direction of the energy and nutrient flow. The number of "links" in a food chain can vary， but the food chain always starts with a producer and ends with a consumer. A food chain can have just two components， a producer and a consumer. Other food chains are more complicated. A producer may be consumed by a herbivore （primary consumer）， which is then eaten by a carnivore （secondary consumer）. In some cases， another carnivore （tertiary consumer） eats the first carnivore.

Food chains show one producer being eaten by one consumer and perhaps another consumer eating the fi rst consumer. Food chains are not that simple in real ecosystems. Producers are usually eaten by many different consumers， and most consumers are eaten by more than one predator. For example， a squirrel eats several diff erent types of seeds， fruits， and nuts. Th e squirrel may be eaten by a fox， a hawk， or a raccoon. The raccoon also feeds on frogs， clams， birds' eggs， and corn. The fox will also eat mice and grasshoppers; the hawk will also eat frogs， mice， and snakes. Most organisms are part of several food chains. A model that shows the connections between several different food chains is called a food web. A food web starts with the producers in the ecosystem and then branches off into interconnected food chains that show who eats whom in the ecosystem. Food webs can quickly become very complex.

If one species is eliminated from a food chain or web， it aff ects the other species in that chain or web. For example， consider the simple food chain： clover → rabbit → fox. If rabbits are removed from the food chain， then the population of foxes in that ecosystem will decrease because they will have no food. If one species disappears from a food chain， organisms that eat that species may eat other species instead. Th is will also affect the food web that the food chain is part of. The foxes， for example， may begin to eat mice and insects instead. When any part of a food web changes， it aff ects the flow of energy throughout the web.