Apocalypse Rats

While pondering the possibilities raised by Dean's latest post - which may deserve a separate post of their own - it occurred to me to wonder why, in lots of post-apocalyptic fiction and movies, there are so many rats. You know - when the sole survivors try to enter The Big City years and years later, they are forced out/killed by/battling endlessly the legions of rats therein.

Where do all the rats come from, and what the heck are they eating? I can see a huge boom in the rat population immediately post-plague/war/nuclear holocaust, when there are lots and lots of bodies lying around. Rats breed fast and often, and I could see the cities being a-swarm for maybe a year after the fact, max. Once all the people are consumed, there just ain't that much food in your average concrete jungle. Wander through NYC 10 years after all we're gone, and I'm betting there just won't be all that much wildlife. Certainly not endless billions of rodents. Nature is very good at wielding the twin swords of plague and starvation to keep populations in control - remove mankind from the picture, and she'll have things nicely balanced out again in no time.