]]>It's important to remember that he's not trying to be realistic. Just as some authors write about wizards and elves, and some authors write about faster-than-light spaceships, McCarthy writes about hard men walking bleak landscapes where strangers are likely to kill them. In The Road, he has pushed bleakness into the realm of fantasy by creating a world where nothing at all lives but his two protagonists and the dying or murderous humans they encounter.
In reality, if there are dead trees, there will be grubs and insects eating the wood, and if there are dead humans, or living humans leaving shit, there will be flies, and if there are insects, there will be birds eating them, and feral cats eating the birds, and coyotes eating the cats. If there is enough sunlight to scan distant cities with binoculars, there will be enough for plants adapted to living in dense forests. There will be mosses, lichens, beetles, earthworms, and crows. McCarthy has excluded all these creatures for purely literary reasons.