Because it will create balance on the earth. For example if there are only autotrophs there will be competition for nutritious and space for the roots to grow .Also for the heterotrophs, the harbivous will be hard for them to find plants to eats eventually it will lead to starvation then death and their death will affect carnivores because the organisms that they used to hunt are all dead now. So when there is a balance between autotrophs and heterotrophs they will help each other to survive.
