Heterotrophs are organisms that obtain energy by feeding off of other organisms, because they are unable to make their own food. They are also called consumers.
All living organisms require nutrients and energy to survive and can obtain these in two broad ways. Organisms that can produce their own nutrients are called autotrophs, while the organisms that cannot produce their own food are called heterotrophs. Autotrophs include plants, bacteria and fungi. Most of the autotrophs produce carbohydrate nutrients through a process known as photosynthesis.
Heterotrophs include all types of organisms, such as animals, fungi, bacteria and protists. There are four types of heterotrophs, classified based on their source of food. Herbivores consume only plants and are called primary consumers, since they directly obtain nutrients from the autotrophs. Carnivores consume other animals, including herbivores, thereby getting the nutrients indirectly from the autotrophs. Carnivores are called secondary consumers if they eat herbivores and tertiary consumers if they eat other carnivorous organisms.
Omnivores consume both plants and animals and can be considered as primary, secondary and tertiary consumers, depending on their diet. For example, humans not only consume plants in the form of vegetables and fruits, but also meat from various sources. Detritivores are heterotrophs that consume dead organic matter and can include bacteria, fungi, insects and worms.