We humans think we're so smart. But boy, do we do some not-so-smart things. Case in point: kudzu. The Japanese introduced it to the United States at the 1876 Centennial Exposition. Americans thought the leafy, sweet-smelling plant was so lovely, they began using it for ornamentation. Then as forage for livestock. The government even got into the act, instructing the Civilian Conservation Corps to plant kudzu for erosion control. Yet no one spent a millisecond pondering how this non-native plant might react in its new environment. Big mistake.
Kudzu loved the climate in the Southern U.S., and took off like wildfire there, growing up to 60 feet (18.3 meters) per year. It began climbing up buildings and telephone poles, smothering cars and homes and becoming a general nuisance. And it's still around today.
Unfortunately, that's hardly an isolated incident. For years, people have brought non-native species into their countries because they're pretty, or because they may be able to solve a problem. For instance, some might have imported amphibians or birds to eat insects that were destroying local crops. Except things didn't exactly work out as planned. Lacking natural predators, the non-native species often thrived in their new surroundings to the point where they became problems -- sometimes, rather big ones.
Today, such invasive species are found around the globe, and their presence outside of their native areas is damaging the world's ecosystems and threatening biodiversity, costing people billions of dollars in reparations, eradication efforts and preventive measures [source: EarthTrends]. Although many governments have wised up and enacted tight controls on travelers, imported goods, plant nurseries and more, plenty of non-native species continue to be tracked around the globe unintentionally, by hiding in people's shoes and luggage or hitchhiking on boat hulls, for example. Some -- like the five on this list -- seem almost unstoppable.