Decomposers and scavengers break down dead plants and animals. They also break down the waste (poop) of other organisms. Decomposers are very important for any ecosystem.
The absence of decomposers in Earth would cause irreversible damage to the planet's wildlife. Carcasses and dead leaves will cover the surface the planet without getting broken down.
If there were no decomposers. Wastes and the remains of dead organisms would pile up and the nutrients within the waste and dead organisms would not be released back into the ecosystem. Producers would not have enough nutrients.
If they weren't in the ecosystem, the plants would not get essential nutrients, and dead matter and waste would pile up and life will end for ever and our beautifull will be destroy.