Here's why:

* Photosynthesis: Plants and other photosynthetic organisms use sunlight to convert carbon dioxide and water into glucose (sugar) and oxygen. This process stores energy from the sun in the chemical bonds of glucose.

* Food Chain: Animals eat plants (or other animals that eat plants), obtaining the energy stored in the glucose. This energy is passed along the food chain.

* Decomposers: When organisms die, decomposers like bacteria and fungi break down their remains, releasing nutrients back into the ecosystem and completing the cycle.

Therefore, the sun's energy is the fundamental driving force behind all life and energy flow in terrestrial ecosystems.