What is the ultimate source of energy for all ecosystems on Earth?

Sun is the ultimate source of energy on Earth. 

Plants are the primary source of food for other organisms in an ecosystem. They produce food through the process of photosynthesis. The process of photosynthesis requires sunlight. Sunlight is absorbed by plants. Then, through a series of complex processes, the CO₂ is converted into sugar. Food is synthesized in plants through the process of photosynthesis only in the presence of sunlight.

ATPs are generated and utilized in this process.

Creatine phosphate is a molecule that stores energy.