Autotrophic and heterotrophic are the classifications made by biologists according to the way organisms obtain nutrition. All animals are the same. They must eat to survive. They are called eaters. Fungi cannot be eaten. They obtain nutrition by decomposing organic matter in animal and plant carcasses, and are called decomposers. They are collectively called heterotrophs, and autotrophs are organisms that rely on energy to make their own nutrients. The main autotrophs on the earth are plants and some bacteria. Plants are the most important autotrophs. They use light energy to make food, so plants are also called photoautotrophs, which are the most important autotrophs on the earth now. They are the main producers of nutrients and the main source of oxygen on the earth.