Autotrophs do not have a digestive system because autotrophs include mainly the plants, trees, algae. They prepare their own food by the process of photosynthesis and store the food prepared in the form of starch for later use. Where as herbivores and carnivores include mainly the animals, which are heterotrophs and depend on plants directly or indirectly for their food. They are not able to prepare their own food. In order to breakdown the food they eat they need a well developed digestive system.