Interesting thing to note about the demon populations: Each individual type is listed in the world population stats as 'unnumbered'.
The only thing that has a similar designation is some few, freakishly numerous, vermin. Ants and such.
Apparently any single demon species has enough numbers backing it to... match, in a strict number sense, the population of pretty much the rest of world's species, combined. Any single demon species. The smallest worldgen default has 20 of 'em.
Dorfworld is officially screwed... and that's considering the state it was in before.
....wow.
I see two possible causes of their unholy populations. One, they're
actually infinite, spawning from the glowing whenever needed. Two, my personal theory, is that the demons are technically finite. It's just that their biome (Hell) that spans all of creation, and there's nothing that can kill them if they stay down there. They don't fight each other, they have no predators, there are no sentient beings hunting them for food, sport, or vengeance, and they have an infinite lifespan and possibly the ability to reproduce.
Hell is... well, hellish, and I am beginning to truly appreciate just what sort of brutality our Toady Overlord has thrown at us. Any attempt to take hell will necessarily be incomplete, as there are far more demons that anyone can reasonably wipe out. The best dwarfkind can manage will be to enter and seal off a chunk of it, and make rude gestures at the marauding hellspawn from behind the safety of their walls.
I wonder if there are any further surprises waiting for us in DF 2010.