I've always seen it as a consequence of CHANGE, rather than civilization or 'non-nature-plants-and-animals-look-at-the-darling-bunnies-sweetheart' sort of things. Province growth and development is a boulder thrown into Mebhaighl's pond, sending waves rippling through it that prevent it from being what it should be somehow - from flowing into the world, or from building up to usable levels, or whatever it is that it does. Elves don't maintain Source because they're more magical or because they're ecologically sustainable, they maintain it because they don't change the place very much - they build around what's already there, with a minimal impact. The province remains largely what it always was, and so they make a much smaller splash, smaller ripples.
By that token, anything that radically changes a province would reduce Source - an earthquake, a tidal wave, a massive uncontrolled forest fire. On the other hand, the damage, the change caused by natural disasters usually fixes itself pretty quickly. The forest grows back, the animals return, and the province becomes pretty much what it's always been - and the Source stabilizes back out. When people move in, though, they don't tend to leave again. The province never reverts. Who knows? Maybe in a thousand years, the Source in civilized provinces will begin to return, as the magic finishes stabilizing in the new form of the province? Maybe humanity (and other races) change things too quickly and too constantly to allow that to ever happen. But for now, all we know is we disrupt things and elves don't.
