Re: Are single climate/habitat worlds really possible?
Exactly. Naturally we can conceive of ways to combat contraband fauna. The question is, would such measures even be worth it? With tens of billions of people coming and going, countless folks will eventually bring their pet rats and cats and birds and bugs and whatever else and some of them will get loose. (And that's discounting illegal transports, which will likely be uncountably numerous.) People are messy and there will be plants to eat and the animals will gain a foothold. At what stage does somebody do the math and figure out that quarantine measures can be reduced without an unacceptable increase in wildlife? What's the price point at which it's no longer worth it to attempt a vermin-free planet?
Sure, we can talk about scenarios where custom plagues eradicate everything nonhuman or a giant computer monitors every life form or whatever. Personally I'm talking about a city that acts like a real city, chaotic and organic and messy and bigger than every other city in history combined. I don't see that measure of control working in such an environment.