In the Avernum Series I had always wondered where the Avernites would get their source of vitamin D due to lack of sunlight. Then I learnt some mushrooms varieties are a good source of vitamin D. Cue Fridge Brilliance.
This started as Fridge Logic, looped around and became Fridge Brilliance again, then drifted into Fridge Horror. The Empire has an ongoing crusade to rid the world of all fantastic creatures (e.g., dragons, griffins, nephilim), but how do they know what creatures are fantastic? We know know that they're fantastic because they don't exist in our world, but they were a normal thing in Aversenum prior to aforementioned efforts to drive them extinct. How do they know that they should kill unicorns, but not that they should kill deer? The answer is that they don't. The Empire wants to exterminate all non-human animal life that's not useful to humans. Bears and wolves are hard to kill, but when was the last time you read a description of a large docile wild critter like a deer outside of someplace like a Dryad's grove?
I'm not sure if it's all non-human animal life but it's definitely all non-human intelligent life (Unicorns for example are intelligent while deer are not). The lack of deer and such might just be conservation of detail, there are a few places in the games where it does make a point about the lack of general wildlife such as birds to indicate that the area is a dead zone for one reason or another.
(OP here) While it's true that unicorns are intelligent, the better-known grey ones are thought to be about as intelligent as a particularly belligerent goat; that's why it's noted that the signs of intelligence in them is disturbing to your party in A3. Although it is possible that the grey unicorns are an exception to the rule because of the fact they're horrible pests and/or because of their association with the smart unicorns. I've actually gone back and found a couple traces of deer in Valorim, but they were in relatively remote places. Songbirds are pretty (or at least sound pretty), and some smaller woodland critters like squirrels probably fall under Rule of Cute. So I guess I could see it go either way.