If the producers were removed, there will be no species introducing energy in the ecosystem. As the producers are responsible for obtaining energy from the sun or chemical sources in the first trophic level and the consumers need this energy (that's why they feed on the producers and on other consumers that have already feed on the producers' energy), without this energy the entire ecosystem would be affected, as the amount of energy in all of the trophic levels would continuously reduce untill there were no more energy for them to acquire and they wouldn't survive.
On the other hand, the decomposers are responsible for returning to the soil/air/water the nutrients that were used by the species after the individuals are dead. Without them, the soil, the air and the water would be lacking nutrients, and the living individuals would have problems as they need those nutrients to keep the flow of energy through the ecosystem.