How does the events of the series affect the environment?

I'm looking a lot of at the environmental story telling of Dark Souls, which is done beautifully, but I can't help but wonder how it got like that? We see giant castles that were clearly neglected with weeds and giant trees growing through them. There are barely any green areas and most of the trees feel that they are dying. All the swamp areas are poisoned. Most "natural beasts" like the hounds are definitely in some way corrupted.

I'd like to ask how did this happen? I assume that the early stages of the age of fire were not like this. Did the fading fire cause this?

Admittedly I have only finished the third game, but I am looking to finish the series soon. If there are any environments in the first and second game that made you go "wow" let me know.