All animals and non-green plants cannot make their own food. They depend on green plants directly or indirectly for their food supply. All animals and non-green plants are called heterotrophs and their mode of nutrition is called heterotrophic nutrition. Heterotrophs can be classified into the following types depending on their feeding habits:
Animals that feed on plants are called herbivores, e.g., deer, cow, rabbit, sheep, giraffe, elephant, etc.
Animals that feed on the flesh of other animals are called carnivores, e.g., lion, tiger, lizard, hyaena, etc.
Some plants are also carnivores. Insects and small animals are digested with the help of digestive juices secreted by the cells of these plants. Sundew, venus’ flytrap and pitcher plant are some carnivorous (insectivorous) plants.
Animals that feed on both plants and flesh are called omnivores, e.g., cockroach, human beings, dogs, etc.
Organisms that depend on other organisms for food and shelter are called parasites (Fig. 8). The other organism is called the host. The parasite derives benefit from the host and harms it in the process. For example, roundworm and tapeworm are parasites living in the human intestine. They derive nourishment from the semi-cooked food present in the intestine and deprive us of the nourishment. Similarly, dodder is a parasitic plant that absorbs food material from host plants.
Organisms that feed on dead and decaying organic matter are called saprophytes, e.g., mushroom, earthworm and some bacteria.
Animals that feed on dead animals are called scavengers, e.g., vulture, jackal, crow, etc. All heterotrophs depend on autotrophs directly or indirectly for food. Autotrophs in turn depend on sun for preparing food. Thus, all living organisms ultimately depend on sun for obtaining nourishment. Thus, sun is the ultimate source of energy on the Earth.