Microfauna seem to be distributed at random across the face of the Earth, with very few of them (relatively speaking) dependent on particular climates.
A problem that's really starting to get out of hand now comes from tropical fish farms that raised a lot of exotic varieties in special ponds.
No problem until flooding washed the whole collection into the nearest stream/river, and you can see where that leads.
By the way, you CAN catch a piranha in Florida, but they aren't like you see them in the movies.