Question

give reason-energy flow through an ecosystem is one way

Answer 1

you can simply consider it as energy always flows from higher potential to lower potential

Answer 2

The best example to understand that energy flow through an ecosystem is one way is through the simple food chain.

The primary source of energy for the ecosystem is from the sun. Producers transform the sunlight into either food or chemical energy. The food is in turn eaten by consumers to gain energy. At no point of time, does this cycle happen in reverse.

This is the reason why the energy flow through an ecosystem is always one way.