How fire ants invaded North America, then the world, in just a few decadesS

The insect known as S. invicta, or the fire ant, will bite the crap out of you, join up with its buddies to swarm everywhere, and ruin everything in its path. And they're everywhere! The truly scary part is that they didn't used to be so ubiquitous - in fact, fire ants used to live in a relatively small area in Argentina. But about a century ago, they made it to the southern United States - most likely as stowaways on boats. And that's when things got weird. Once the ant invaders created a successful colony in the southern U.S., they launched successful invasions of California, the Caribbean, China, Taiwan and Australia. A group of researchers have identified at least nine separate invasions launched by those original fire ant invaders. How did they do it?

The researchers, who figured out the invasions had all sprung from one original invader population by analyzing the DNA of fire ants around the world, aren't entirely sure what made this group so hardy. They speculate in a paper published today in Science that the first fire ant invaders were uniquely suited to the invader lifestyle - perhaps they were hardier, or had colony structures that made adaptation to a new location easier for them. Regardless of what made this ant group's offspring so fit as colonizers, this scenario is one of the few examples we have of what evolutionary biologists call the "invasive bridgehead effect." That's when one invading force forms a "bridgehead" from which to launch further invasions of new territories.

All thanks to hitching rides on human transport, from trains to planes. That's right - we brought the ant invasion on ourselves.

The researchers conclude:

Our study indicates that fire ants have been introduced on no fewer than nine separate occasions to California, Asia, and Australia from the southern United States, where S. invicta populations previously were confined for decades. We consider this a minimum estimate, because our newly-invaded area sample collections were not geographically exhaustive, and analyses of a few individuals obtained in Trinidad and New Zealand (from intercepted colonies) suggest that these ants also originated in the United States. Although we find little evidence for serial introductions among regional newly-invaded area populations, long-distance human-mediated transport of S. invicta after an initial introduction probably explains the dispersion of a single genetic cluster across several hundred kilometers in China. Such long-distance transport evidently was responsible for much of the early spread of the ant within the southern United States.

Read the full scientific paper via Science

Image by Kenneth G. Ross