As many as 23 million feral pigs have taken over Australia, causing an untold amount of damage. These highly intelligent invaders are endangering native species, spreading disease, destroying farmland and even attacking people. They were deliberately introduced by people centuries ago; now it's up to locals to keep their populations in check. Scientists and citizens are resorting to a range of methods to defend their country against these "pigzillas."

Giant swamp rats are taking over the U.S. and destroying fragile ecosystems from Maryland to Oregon. The rodent, known as nutria, is native to Argentina, but was introduced to the U.S. during a "get-rich-quick" scheme to farm the animals for their fur. Now there are 20 million in Louisiana alone, and Nat Geo WILD is on the front lines with experts trying to curtail the invasive species.

A moose in the country is no big deal ' but a moose in a New York suburb is a recipe for disaster. At upwards of 6 feet tall and weighing more than ton, moose have a reputation as gentle, clumsy giants. But with large racks of antlers and sharp hooves, they can be dangerous. Nat Geo WILD examines why moose are invading the Northeast, infiltrating populated areas including regions just outside of New York City.