Just because Sweettooth knows to avoid the shell doesn't mean that ALL creatures in the swamp do. It would be fairly responsible as a GM to have the party see quite a few small and large animal skeletons on the ground all around the barrier. Maybe even some of the trees and bushes sliced directly in half, growing on either side of the barrier, charred on the sides that face it.
A glowing shell is one thing, but a glowing shell surrounded by a pile of charred and shattered corpses? That's something to pay attention to. ;)
