PART - 2 CH - 13 Our Environment — Science STD 10 — Question

The solar energy enters in an ecosystem through the process of photosynthesis performed by green plants. The energy that is captured by green plants does not revert back to the sun.
From green plants energy passes to upper trophic levels of consumers. Energy obtained by herbivores does not come back to the green plants. As it moves progressively through the various trophic levels it is no longer available to the previous level.
Thus, energy flow is always unidirectional in an ecosystem.