Decomposers and scavengers break down dead plants and animals. They also break down the waste  of other organisms. Decomposers are very important for any ecosystem. If they weren't in the ecosystem, the plants would not get essential nutrients, and dead matter and waste would pile up.When a plant or animal dies, it leaves behind nutrients and energy in the organic material that comprised its body. Scavengers and detritivores can feed on the carcasses, but they will inevitably leave behind a considerable amount of unused energy and nutrients
Decomposers are the organisms that consume dead organisms and break them down into simple chemical components like carbon , nitrogen and other nutrients that are essential for living organisms.By eating up the dead materials they clean the environment.Without them the world would have filled with dead bodies of plants and, they are called natures cleansers.