The flow of energy is unidirectional.The energy that is captured by the autotrophs does not revert back to the solar input and the energy that is passed on through the herbivores does not come back to autotrops. as it moves progressively through the various tropic lavel it is no longer available to the previous lavel.
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A food chain represents a single directional (or unidirectional) transfer of energy. In a food chain transfer of energy takes place from producer to primary consumer then from primary consumer to secondary consumer. It cannot take place in a reverse direction from secondary consumer to primary consumer to producer.
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