In Spain and other countries there have benn problems with T.scripta elegans infesting rivers and ponds and competing with the native emys and mauremys. to the point of forbidding the sale of this species. Dealers went straight to other sliders that adapt as well as T.s. to our climate.
I think they have even found Pelomedusa subrufa in southern Spain (not sure, I think I have read somewhere)
Most of those animals are not scaped but given "freedom" for people who are fed up with the huge, sometimes agressive terrapin that gets the water dirty and smelly.
In Catalonia, where I live, you can´t posses any T.hermanni, and untill recently, neither T.graeca. I suppose this is because of the risc of mixing subspecies (T.hermanni hermanni is native in some parts of Catalonia and there are reintroducing programs) and the risc of spreading illnesses of captive animals. The problem is that this conveys that people have animals of other countries, which also can scape with even greater illness dangers for wild populations.
I myself keep species that adapt quite well to our climate. This is because it allows me to maintain them in outdoor ponds or pens, with all the advantages that this has. But I can´t avoid thinking what would happen if those species became more common as pets. The same as with trachemys? What would be the solution, let us keep only the native subspecies? Or only letting us posses species that cannot adapt? I don´t know...
Jordi