jdrakeh said:
There simply isn't (so far as I can tell) any good reason for why mundane animals in Faerun or Eberron should be mirror images of those that my neighbor keeps cooped up in his apartment all day. Should Carrion Crawlers not usurp Rattus Rattus in the foodchain of Waterdeep's many sewers? Should the skies over Sharn not be populated by something a bit more fantastic than pigeons?
Actually, I'd be pretty surprised if it were even possible for the rat to be dislodged from its niche. It's an amazingly well-adapted creature, and one that dominates every ecosystem it is introduced into. It's probably one of the most profitable lines of mammal evolution ever, since it's so easily able to adapt to different conditions, breeds quickly, is reasonably intelligent, and has a particular talent for leeching resources from (arguably) the single most profitable line of mammal evolution ever: humans.
While animals like bears might conceivably be outcompeted and driven to extinction by more fearsome and fantastic animals, I think you'd have to purge the earth with an
incendiary cloud to get rid of rats.
Also, keep in mind that one of the secrets to the success of the rat is that they are small. Carrion crawlers are not, and so require a hell of a lot more resources than a rat. I assume they probably get a lot of those resources by eating rats, considering that offal is not particularly nutritious for anything but a fungus, and rats are very convenient packages of protein, lipids, and energy that populate the carrion crawler's environment. Where rats are predated upon in the real world by ambush predators like cats and mustelids, a carrion crawler doesn't have to be fast to catch a rat; it just has to brush it with a tentacle. If I were to pick a creature to bet on for extinction, I'd go with the large ones first: otyughs, carrion crawlers, [everything else], and finally, rats. The only possible exception is the gelatious cube, which can survive on organic sludge. Decomposers are usually pretty robust, so long as the substrate stays the same, and I assume that mucky sewers are exactly what cubes are adapted to.