Herbivores eat producers and are classified as primary consumers. Producers make their own food from a primary energy source such as the sun or boiling-hot deep sea vents. Plants, for example, are producers that use the sun's energy, carbon dioxide and water to make their own food, a process known as photosynthesis. Chemosynthetic bacteria produce their own food from chemicals and energy within hydrothermal vents. Primary producers are at the bottom of the food chain. Grass, algae and phytoplankton are examples of the many producers eaten by herbivores. Herbivores can be large or small and include organisms such as mosquito larvae, grasshoppers, rabbits, cows and deer.
Carnivores are consumers that eat other animals and are classified by what those other animals ate prior to being eaten. Carnivores that eat herbivores are secondary consumers, those that eat secondary consumers are tertiary consumers, and those that eat tertiary consumers are quaternary consumers. In a typical pond environment, for instance, the dragonfly that eats mosquito larvae is the secondary consumer, the fish that eats the dragonfly is the tertiary consumer, and the raccoon that eats the fish is the quaternary consumer. A carnivore's classification is not fixed. For example, if a snake eats a grass-eating rabbit, it is considered a secondary consumer, but if that same snake eats a rat that ate a plant-eating grasshopper, it is a tertiary consumer. The food chain ends with an animal, or top predator, that has no natural enemies and is not at risk of being eaten by another animal in its biological community. Top predators include animals such as alligators, polar bears and hawks.
Consumers that eat both plants and other animals as part of their typical diet are called omnivores. An omnivore's diet can include plant materials such as fruits, nuts, seeds, leaves and twigs, as well as animals such as insects, birds' eggs, fish, pigs or cows. Chickens, chimpanzees and humans who are not vegetarians are examples of omnivores.
Detritivores are secondary or higher consumers that feed on dead animals. Vultures, crabs, worms and other scavengers are examples of detritivores.