@Deon
Really? I noticed it happens just before the town goes desert, and in an instant (1 worldgen year). Makes it feel like if something exploded in the world map.
@Zangi
Yes, they seem to be conquered/destroyed by either megabeasts (from all the samples I just made, ettins) or humans.
Ironically despite their abuse of trees, elves seem to ignore them and even share regions, not my intention at all.
I didn't modify any existing entity or creature from vanilla, but with the mod on, EVEN HUMANS fail to survive at times, leaving huge areas as "towns" (not human towns). Only dwarves and whatever lives in a dark fortress seem to survive in a regular basis, or huge human civs with multiple towns. Several worlds without remaining elves have been seen.
For the record I am generating "small region" worlds with the default settings to test (because of gen speed). Pocket and small worlds end worldgen so early that they are still there, though.
@darkflagrance
It IS a possibility, but if I do it like that, the exact opposite might happen. Just having one playable group of them in civ mode, consistently, is good enough. At the time it's 30, three times more breeding couples (?) than humans. I will try it now for the lulz and see what happens though.