When you think of invaders, you might think of extraterrestrial life or the unwanted creatures that occasionally scramble across your countertop. But what about the invaders that wreak more havoc than an average household nuisance? Invasive species -- or organisms that overtake and negatively affect existing plants, animals or humans after being introduced to a new environment -- cause an estimated $1.4 trillion in damages per year [source: Global Invasive Species Programme]. Often, an animal that evolved in one place can be invasive in another if it has enough food and few or no natural predators.
If you've seen kudzu's blanketing grasp on the southeastern U.S., the annoying presence of fire ants in your backyard or maybe chytrid fungus's devastating impact on amphibian populations, you're familiar with invasive species. These species are not only hard-hitting in the economic and biodiversity departments, they can also spread diseases and harm human health. So who's responsible for these invasions? In most cases humans are to blame, especially with increased travel, trade and exotic pet ownership in recent decades.
Of the thousands of invasive taxa to choose from, the five species on this list will surely penetrate your thoughts and -- who knows -- maybe your backyards, too.
See which animal spurred Australia to build a 1,139-mile (1,834-kilometer) long pest-exclusion fence on the next page.