The main source of energy for Earth's ecosystems is the sun. Solar energy drives the process of photosynthesis in primary producers, such as plants and phytoplankton, allowing them to convert sunlight into chemical energy. This energy is then transferred through the food chain as organisms consume one another, creating a complex web of life. Nearly all energy in ecosystems originates from sunlight, making it critical for maintaining the balance of life on Earth and supporting various ecological functions.