OK, what kind of critter munches off the flowering heads of marigolds and dianthus? And sometimes leave the flowers behind? I don't see any rabbit spoor, I see only casual digging (as in no warrens being made) near the victims. I work all day, so can't keep an eye out for neighborhood cats, but these plants are in a very open spot; usually cats want more privacy.
Snails and slugs most likely if its stem damage. I have a theory that they don't eat the flowers because of the pyritheroids (sp) which is used in insecticides. Go out their at night with a flashlight and take a peek!