Decomposers take away all of the earth natural “trash” and turns it back into nutrients for the plants. So in the food chain we have plant->prey->predator->then the decomposers take the (dead) predator and turn it into to nutrients for the plants. Of course you can count all the other dead things as well, not just predators. Imagine if every time you killed a bug in your house and you didn’t pick it up how many dead bugs there would be laying around, it would be like this but on a much much larger scale.