Some floated here on boats. Others flew. Still others arrived on the sole of a dirty boot. Many were invited, but some arrived unannounced. At this point, however, no one really cares how so-called alien species like the ash borer and the zebra mussel got here. Scientists are more focused on how to get rid of these pests.

Not every alien species becomes invasive, but those that do can wreak serious havoc. They eat or out-compete native species. They clog waterways and cover coral reefs. They reproduce wildly and turn once-diverse ecosystems into single species monocultures. In one extreme example the introduction of the invasive Nile perch into Lake Victoria in Africa in the 1960s led to the disappearance of as many as 200 native cichlids. In the U.S. a fast-growing Japanese vine called kudzu has blanketed large portions of the Southeast, knocking down trees and smothering native species under its dense canopy.

As international travel has become more common the problem has only gotten worse. People and cargo regularly cross mountain ranges, rivers, oceans and deserts—features that once kept plants, animals and insects apart. In the past century invasive species have infiltrated nearly every part of the globe. There are more than 1,500 invasive species in the U.S. alone, says Daniel Simberloff, an ecologist at the University of Tennessee Knoxville. No one has compiled a global list, but it would likely include 10,000 species or more, he says.

Scientists are trying to stem the influx of new invasive species as well as control the intruders that have already infiltrated native ecosystems. But these opponents are wily. They've gained a foothold and won't easily let go. Luckily, when it comes to combating invasive species ecologists these days are getting creative. You try everything you can to fight them, says Michael Massimi, invasive species coordinator at Barataria–Terrebonne National Estuary Program in Louisiana. "When you reach the end of your rope and nothing is working, you start trying the crazy stuff."

View a slide show of how scientists are dealing with some of these unwelcome interlopers