Too late for a border fence

Feral hogs of the species Sus scrofa live on every continent but Antarctica, and also on many islands and archipelagoes. Except in the original range of the Eurasian wild boar, feral hogs are non-native everywhere...

The current American style of real-estate development—expanding horizontally, taking over rural areas, mallifying farmland, leaving only the soggy places and creek beds and river valleys more or less the same, and then passing ordinances prohibiting the discharge of firearms in the new municipalities—suits the hogs just fine. 

(link) New Yorker, 2005