Heterotrophs are non-producing organisms of the ecosystem. They obtain their food from autotrophs; hence, they are called consumers. They are chiefly animals, that depend directly or indirectly on autotrophs (green plants).
Energy used by heterotrophs is derived from the chemical energy stored in food. They eat food to obtain energy for their activities and the building up of their body tissues (growth). Excess food is stored as fat or glycogen (animal starch) in the various parts of their bodies for re-use during critical situations (e.g., starvation). Heterotrophs that feed directly on green plants are called primary consumers, e.g., caterpillars, fish, tadpoles, grasshoppers, zooplankton, goats, cattle, sheep, etc. Animals that are primary consumers are also called herbivores (plant eaters), e.g., goats and cows. Heterotrophs that feed on primary consumers are termed secondary consumers, e.g., birds, lizards and small fish, cats, lions, and dogs. Animals that prey on herbivores are called carnivores (flesh-eating), e.g., cats, lions and dogs. Those animals that feed on secondary consumers are called ternary consumers. Organisms in this group (tertiary consumers) depend directly or indirectly on green plants. Animals that feed on both plants and other animals are called omnivores, e.g., man.
Heterotrophs include non-producer organisms such as herbivores, insectivorous plants (pitcher), carnivores, omnivores, scavengers and parasites.