Explain why energy in most ecosystems must flow from the sun, then to autotrophs and finally to heterotrophs, in that order

It must flow like that because sun is one of the most used energy. Also, it flows like that because the herbivores eat off the autotrophs, and then the carnivores eat the herbivores. Also, the heterotrophs are a small fraction that is taken away from the energy.

Because the sun produces energy which gose to autotrophs such as plants then the heterotrophs eat the autotrophs since they cannot produce enegry themselves