In Gygaxian Naturalism, monsters form an internally consistent ecosystem. When the PC's aren't there, the monsters lead rich, full lives—weaving baskets, cultivating mushrooms, raising families, etc.
Monsters of the Mythic Underworld coalesce out of darkness and nightmares. What are the goblins doing when the PC's aren't there? They're don't exist at all, or they're just sitting in the darkness waiting for a PC to open the door, or they're doing something completely alien to human experience. It doesn't matter. Monsters are just an expression of the mystical underworld's primal animosity to the player characters.
I recently articulated to myself a slightly different ecology that I favor. It's a decontextualized alien ecology. Such monsters are natural, but not naturally occurring on this world. They lead lives when the PC aren't around, but their activates are not familiar to men. These are visitors from outside—invaders, castaways, refugees, and fugitives. They're the invasive species of the dungeon.