It's a good question, in that it provides grounds for thought. I wouldn't say a S can be a D. S may compete with D, e.g., a fox eating a decaying roadkill rabbit; crocodiles eating a decaying drowned buffalo; sharks feeding on a rotting whale carcass.
S's seem to be higher order consumers, whereas D's are smaller organisms such as worms, termites, bacteria and fungi. I seem to recall that D's break down complex organic matter into smaller molecules
that plants can use directly, e.g., worm wee. A fox's manure has to be broken down further, regardless whether it has been feeding off fresh rabbit or decaying rabbit. I'd think of a beach crab as a S, whereas a beach worm would, I think, be a D.
This is just off the top of my head. If you want a good textbook answer, better consult a good textbook.
