There are only a limited number of monsters of each type in Lightning Returns: Final Fantasy 13, and if you're diligent (or just careless about deforestation, pollution and poaching) you can wipe them out entirely.
According to a Famitsu interview translated by Gematsu, the number of each kind of monster in the world is pre-determined.
When the player encounters the ominously capitalised Last One of a species, it will be much tougher than the others, and may drop rare loot.
Speaking of rare loot, players who complete the game will unlock a harder difficulty; in this mode, enemies will drop better abilities, and items otherwise unobtainable will appear in shops.
Square Enix expects the game to last between 20 and 30 hours, but up to 50 for really thorough players. It's due on PlayStation 3 and Xbox 360 in February 2014.