Question

avilable energy goes on decreasing at each trophic level in a food chain

Answer 1

energy gets on decreased as it is used in every trophic level by organisms this phenomenon is based on 10 percent law..

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Answer 2

Producers, ie, plants are eaten by herbivores animals. Some of the energy is used by producers and the rest is passed on to herbivores. The animals store this energy and use some of this energy. Again, the carnivorous animals eat the herbivores and like this energy keeps on decreasing at each trophic level in a food chain.
Hope this answer helps.