Who Invited You? - When Species 'Invade'

Marauding camels from Afghanistan, India, and the Middle East overrun Australia. The "invaders" flatten fences, topple into wells, and devour native plants. Meanwhile, toxic toads originally from Central and South America multiply without mercy and poison Aussie critters to near-extinction.

- By Lynn Ermann

Marauding camels from Afghanistan, India, and the Middle East overrun Australia. The "invaders" flatten fences, topple into wells, and devour native plants. Meanwhile, toxic toads originally from Central and South America multiply without mercy and poison Aussie critters to near-extinction.

The prickly pear cactus is the state plant of Texas.

In Kenya, though, it threatens the livelihood of tribal herders. The cactus sucks water from the soil, making it tough for native plants to thrive.

Nine thousand miles away (14,500 km) in Texas, toothy Eurasian wild pigs with shovel-like snouts decimate crops and backyards. Huge flocks of European starlings strip clean farmland across the United States and shower main streets with guano.

Stealthy Burmese pythons swallow most of the mammals in the Everglades in Florida.

This is a small sample of a big problem. Nonnative species cost more than $1.4 trillion in damages worldwide per year. The distance traveled in these continent-hopping journeys is just as staggering.

Have you ever wondered exactly how your local invader got from the other side of the globe to your backyard? Do the pythons slither thousands of miles? Do the camels walk on water? Are they all alien "super villains" imbued with superpowers? Nope. In almost every case, people are the ones to blame.

Invaders!