New animals will also spawn in existing worlds, but all animals are persistent (this means they never despawn automatically). Npc spawn depends on the overall population, so if there are already "enough" animals on your existing island, no new animals will respawn. This only happens if animals either wander off, or if they die.
To delete npcs, you don't have to modify the database. Instead you could just use the deletenpcs console command
By default it removes all npcs in your world, but optionally you can also restrict it to a specific type (or only delete npcs within a certain radius).
it would be cool if there was an option to only delete the ones a set distance from buildings, so you could essentially reset the wilderness but not lose any pets/livestock you are keeping.