They're often the most ubiquitous examples of man's failure against nature. These creatures spread quickly, resist any attempt at control and are wildly undesirable.
They are the invasive species. Many have become so pervasive and destructive that an entire government agency, the National Invasive Species Council, has been working to combat their spread since 1999.
Yet the arrival of many of these plants and animals are almost exclusively the byproduct of human will. One man's dream to populate America with every bird mentioned in the works of Shakespeare (yes, a novel idea) has led to million-member swarms of starlings that rain poop over less-than-pleased neighborhoods. Australia's efforts to control pests by introducing cane toads onto the continent has resulted in populations numbering into the billions -- none of which actually eat the bugs they were supposed to.
So take a look at some of the world's most loathed species, but remember, it's not always their fault they're around. It's ours.