A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. Primary producers lie at the base of the food chain. The primary producers are autotrophs and are most often photosynthetic organisms such as plants, algae etc. The organisms that eat the primary producers are called primary consumers. Primary consumers are usually herbivores or plant eaters. The organisms that eat the primary consumers are called secondary consumers. Secondary consumers are generally meat eaters or carnivores. The organisms that eat the secondary consumers are called tertiary consumers. These are carnivore eating carnivores, like eagles or big fish. Organisms at the very top of a food chain are called apex consumers. One example of the food chain on land is Grains - mouse - snake - hawk. Here grain is the primary producers, the mouse is the primary consumer, the snake is the secondary consumer and hawk is the tertiary consumer.