Operation: Kill Kermit

Most nights, armed men stomp through the Périgord-Limousin Regional Park in southwestern France with orders to shoot ... frogs. But not just any amphibians. They're after Rana catesbeiana — the North American bullfrog — introduced to France in 1968 by a French aviator who liked the idea of the critters croaking in his garden. They're now an ecological menace.

Weighing up to a kilo, these voracious predators gorge on crustaceans, fish, other frogs, salamanders and even the occasional bird. "It's capable of attacking anything it can swallow," says Tony Dejean, the naturalist at Périgord-Limousin leading...