The heat and light from the sun are critical parts of an ecosystem. The sun's heat helps water evaporate and return to the atmosphere where it is cycled back into water. The heat also keeps plants and animals warm. Without light from the sun there would be no photosynthesis and plants wouldn't have the energy they need to make food. Without water there would be no life.
Water is a large percentage of the cells that make up all living organisms.
In fact, you may have heard that humans can go longer without food than they can without water. Without water all life would die. In addition to being an important part of cells, water is also used by plants to carry and distribute the nutrients they need to survive. As I Live and Breathe Ecosystems have lots of different living organisms that interact with each other. They are all important parts of an ecosystem.