Continuing with the thread-jack a bit, global warming is simply our latest term for climatic change. The planet has seen lots of climate swings during it's long existence. The cause of the observable and documented warming trend currently underway is somewhat open to debate, but the fact of the warming isn't argued by anyone except those who might profit from pretending it isn't happening.
California is an example of species fitting in to their environment. There are lots of things that people can grow in Cali most of the time. There are not nearly so many things that grow there naturally. Every once in a while, the things people try to grow there get killed off. The reason is, as previously noted, the extremes that happen only once in a while. It is these occasional extremes in conditions that limit where a species can survive. Up here in Ohio, we have to figure for a frost depth of 3 feet when putting in a foundation. Only rarely do we get a winter with temperatures cold enough for long enough with little enough snow cover to allow the ground to freeze anywhere near that deep, but we have to allow for it or see foundations heave. Same with your fire ants, which are actually a wingless wasp.
You need to factor evolution into the equation a little. Insects reproduce very quickly compared to humans. The appearance of variations that will allow them to deal with colder temperatures, whether it's behavioral and causes them to burrow deeper into the soil or physiological and allows them to experience colder temperatures without dying, might allow some of them to survive a winter that would kill their southern cousins. These survivors then reproduce their own kind, and the process gradually leads to a more cold tolerant population. It's the same process that has allowed pesticide resistant populations of numerous insects to develop, although I didn't know the critters had developed a tolerance for Sevin. Up here, it's really tough on bees, wasps, and the other Hymenoptera. -- Except for the dadgummed carpenter bees that keep trying to eat my house. /forums/images/graemlins/mad.gif