Last night, Richard said that a lot of the world simulation aspects of Ultima Online had to be taken out because players didn't behave as expected. I have another theory.
The example he gave was that they originally set things up so that surrounding a village were sheep and deer and further out were wolves and dragons that ate those sheep and deer. The idea was that if players killed off the population of sheep and deer too quickly, the wolves and dragons would have to venture closer to the village and the village would cry out for a hero to kill the wolves/dragons. Hence the system itself would create the need for kill-the-dragon-type hero quests.
Unfortunately, players killed everything and so the system never had time to restore balance and so the concept had to be removed. Richard said there were lots of examples of this.
I wonder if the concept might have worked had human populations been susceptible to the same forces. It's a classic example where a free-market-like system won't lead to equilibrium because there are constraints which prevent the system from operating on each participant.
The moment you set up some 'protection' for one participant in the system, the natural balance will be lost.
So if you want to simulate plants and animals in an ecosystem realistically, you've got to include humans and allow for disease, famine and player death.