Every week another study comes out documenting the tragic ecological consequences of an invasive species entering a new area and dominating the unsuspecting and unprepared native species. The history books, too, are filled with stories of places like Macquarie and Rat Island where species that tagged along on human travels decimated their new surroundings. But is it possible that the threat of invasives to the world at large has been overhyped?

Mark Davis says yes. Scientific American interviewed the contrarian ecologist from Macalester College last month, and Davis questions the some of the commonly-held wisdom among scientists who study these species. Specifically, he tells Scientific American, he doubts that invasive species are really the second-leading cause of threatening species, and that they impact 42 percent of the threatened or endangered species. 

That's not to say that some invasives aren't dangerous. Rather, Davis says ecologists should be focusing more on looking at harmful versus benign species rather than native versus foreign; you don't have to be from another area to be a damaging pest. And much of the money spent to fight invasive species, he argues, would be better spent saving habitat from destruction.

And this week, University of Illinois scientists found that the advantages invasive species have over native ones can wane over time. The team studied garlic mustard, which invaded the U.S. from Europe and injects a fungus-killing toxin into the ground. The species was wildly successful at wiping out competitors—in fact, too successful. Now that it only lives in patches with no other species, it's no longer worthwhile, natural selection-wise, to expend the energy to create the toxin. Thus, the researchers found, garlic mustard is actually de-arming its arsenal with successive generations.

Still, garlic mustard had to wipe out lot of other species to reach that point, and plenty of other invasives are just as nasty. But perhaps Davis is right: we should be more worried about how damaging a species is than where it came from. And, as usual, ecosystems are more complicated than they first appear.