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.