The ultimate source of energy for nearly all ecosystems on Earth is solar energy. Plants, through the process of photosynthesis, convert sunlight into chemical energy, forming the base of the food chain. This energy transfer sustains various organisms, from herbivores to apex predators. Without solar energy, ecosystems would collapse, as primary producers would die off, leading to a cascading effect on all trophic levels. Thus, solar energy is crucial for maintaining ecological balance and supporting life.