What is the ultimate source of energy for all ecosystems?

Sun

Sunlight is needed in photosynthesis, a process in which autotrophic organisms such as plants make their own food. Autotrophs are then eaten by heterotrophs such as animals including humans. The process of eating and being eaten is where energy passed on from autotrophs to heterotrophs. Thus, sun which drives photosynthesis that enable plants to produce food where generally most of the organisms get their energy is the main source of energy for all ecosystems.