All those small republics were swallowed up into one large one, and gradually the world population declined : we have only to see what Italy and Greece were before and after the Roman victories.
“I will be asked,” says Livy, “where the Volsci were able to find enough soldiers to wage war after being so often defeated. There must have been a limitless population in those places, which today would be just a wasteland were it not for a handful of soldiers and Roman slaves.” 
“The oracles have ceased,” says Plutarch, “because the sites where they spoke are destroyed ; scarcely would you find three thousand men of war in Greece today.”
“I shall not describe,” says Strabo, “Epirus and its surroundings, because these regions are entirely deserted. This depopulation, which began long ago, continues daily, so Roman soldiers camp in abandoned houses.”  He finds the cause for this in Polybius, who says that Paulus Æmelius, after his victory, destroyed seventy cities in Epirus, and left with fifty thousand slaves.