A food chain is what living things eat in an ecosystem. It starts with a producer like plankton. Then you have a consumer like Krill. Producers are eaten by consumers. Then you have a predator like a penguin. The predator eats the consumer. An Orca also a predator, they eat the penguins. For example, an Orca eats the penguin, the penguin eats the krill, the krill eats the plankton.