The original UO, the first few months, had a "true" ecology. Monsters & animals could be hunted out, they hunted each other, and they respawned based on a global table of resources or "energy". Regs were sold but were rare and meant to be gathered in the wild.
It was a wonderful idea and would have worked except that more people showed up to play than the devs had anticipated, there was never anything to kill, and the players turned on each other.
This idea would work pretty well on player run shards, but GMs would have to make sure they encourage enough crafters & gatherers to show up, or at least intervene now and then.
I started UO in the 1997 beta - I saw it all except for the last year and a half...
