Question

why are decomposers important part of a food chain

Answer 1

Decomposers are the small microorganisms like parasites or saprophytes which decompose the dead organisms.
Thus the nutrients which were acquired by the organism throughout its life are consumed by these microbes and then again restored or put back in the environment.
Thus the nutrients taken by the organisms after they for do not get wasted and are again put back into the soil water etc.