A producer creates energy, primary consumers eat producers, secondary consumers then eat the primary consumers, and the tertiary consumers eat secondary consumers.

Producers produce or create energy using either energy from the sun () or chemical energy (). Consumers then consume or eat the producers, either in part or the entire producer, and obtain their energy through consumption.

Primary consumers consume producers. Secondary consumers consume either only primary consumers or they consume both primary consumers and producers. Tertiary consumers consume secondary consumers, and they may also consume primary consumers and producers.

Thus, the secondary consumer eats primary consumers but the primary consumer only eats producers. The tertiary consumer eats secondary consumers (and possibly primary consumers), but the secondary consumer does not. If the secondary consumer ate secondary consumers, it would be a tertiary consumer!

An example could be: grass is eaten by a rabbit, rabbit is eaten by a snake, snake is eaten by a hawk. Throughout this process, energy is transferred. Only from one level passes though to the next organism. Less and less energy is available as one moves higher up the or , causing the lower organisms to have a larger population. As you move up the food chain, the population size shrinks due to the amount of energy being transferred.