Which organisms represent the second trophic level in a food chain?

Explain the role of decomposers in the environment.

Flow of energy in an ecosystem is always

In a certain study conducted on the occurrence of $DDT$ along food chains in an ecosystem,the concentration of $DDT$ in grass was found to be $0.5\, ppm$ (parts per million),in sheep it was $2\, ppm$,and in man it was $10\, ppm$. Why was the concentration of $DDT$ maximum in the case of man?

$(a)$ What is an ecosystem? List its two main components.
$(b)$ We do not clean ponds or lakes,but an aquarium needs to be cleaned regularly. Explain.

In an ecosystem,the $10\%$ of energy available for transfer from one trophic level to the next is in the form of