Food Chains
If an ecosystem is to be self-sustaining it must contain a flow of energy.

One way of representing the flow of energy through the living components of an ecosystem is through the use of a food chain.

A food chain indicates the transfer of energy from producers through a series of organisms which feed upon each other.

A Food Chain
Note that the arrows in the food chain point to the organisms which are doing the eating.

Thus the arrows in the food chain represent the flow of energy through the ecosystem.

The algae and floating plants are the producers in this food chain.

The aquatic crustaceans are the primary consumers which eat the producers.

Fish are secondary consumers eating the primary consumers.

A food chain may also contain third level or other consumers as indicated by the raccoons in this food chain.