How Humans Helped Ants Invade the World

Waves of globalization brought these warriors to new shores, where certain species spread like wildfire

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The fire ant has spread like wildfire around the world, thanks to a winning combination of traits and a little help from humans. Grant Heilman Photography / Alamy

If you’ve never been stung by a fire ant, consider yourself lucky. Known for their fearlessness and painful, venom-laden butt pinches, these wee warriors can easily take down a chicken, kitten and occasionally even a human (usually by anaphylactic shock). It’s no wonder that the appearance of floating rafts teeming with these horrors was considered a “terrifying threat” to the Gulf Coast in the wake of Hurricane Cindy.

In addition to inciting fear, fire ants have also been particularly successful at spreading around the world. Since tropical fire ants rode Spanish trade ships to new continents in the 16th century, the tenacious critters have taken hold across the Southern United States and reached as far as Taiwan and Australia. And once they invade, they can significantly reshape their new environments—sometimes in catastrophic ways.

What qualities have made them so successful? That was the question that drove Cléo Bertelsmeier, an ecologist at the University of Lausanne in Switzerland, to chart the global spread of ants for a study published last week in the journal Nature Ecology and Evolution. Her study documents how the history of ant migration has largely been driven by waves of human globalization—and asks how we might be able to predict the next great ant invasion.

Ants are far more than just a nuisance for picnics and pantries, Bertelsmeier points out. "Invasive ants are really a huge problem for biodiversity," she says. Besides displacing native species, invasive ants can also cause harm by eating valuable agricultural crops, attacking people and even shorting out electrical systems.

"I think ants globally really are one of the bigger and more problematic invasive taxa," says Andrew Suarez, a University of Illinois at Urbana-Champaign entomologist who has long studied invasive ants. He points to aggressive and durable fire ants as a prime example of a harmful invasive ant genus. Their aggression in colonizing new areas and attacking rival insects helps them push out native insects and even nesting birds and reptiles.

While prior research has traced the paths of some invasive ant species, Bertelsmeier wanted to find out whether there was a pattern to when, and how extensively, certain ant species spread over time. She took to scouring various public databases covering the more than 13,000 known ant species for information on the 241 ant species that have been identified as "aliens," or introduced to environments they aren't native to.

Among those 241 species, Bertelsmeier categorized ants into four different groups based on how well they seemed to take to invading foreign environments. Some alien ant species had barely spread beyond their native ranges, while others had spread throughout a continent. A few ant  managed to make footholds around the world in relatively low numbers. The final, most effective group—which includes fire ants—has been able to spread globally with verve.

Bertelsmeier was able to identify a handful of traits that were associated most strongly with ants that were exceptional invaders. Those included body size, number of queens, how their colonies are organized and other traits.

It turns out that the best invaders tend to be smaller ant species, with multiple queens who bring worker ants along with them to found new colonies instead of going it alone. Other helpful factors include the ability to settle in ecologically disturbed habitats—often those that have been shaped by humans—and the ability to build new nests in many different kinds of environments. Cooperation, hardiness and versatility: These are the traits helps that make groups like fire ants and Argentine ants ruthless invaders.

For the 36 species that she managed to find enough historical data on, Bertelsmeier was also able to track when exactly these alien species typically spread. Unsurprisingly to her, the ant invasions of the last 200 years correlated with the two peaks of human globalization, from the Industrial Revolution and age of European colonization to the Great Depression, and then the global post-war boom starting in the mid-20th century until today. Wherever people went, it seemed, ants followed.

"Human activities have left a fingerprint on the distribution of these alien species," Bertelsmeier says.

"I this is a pretty amazing study," says Suarez, who was not involved in the research. He is particularly impressed, he says, by the amount of data Bertelsmeier was able to collect for the study by scouring public databases and collecting data from many separate studies done over time, and sees it as a useful resource for future research on invasive ants worldwide. "That's something that people have been trying to do for a long time."

Next, Bertelsmeier plans to focus on different countries that have harbored invasive ants and those that haven't, to see what factors make one place more appealing than another. Meanwhile, Suarez says he hopes to see more research expanding on this study that could help scientists predict which ant species are most at risk of causing harm as invaders, and how likely they are to spread in the first place.

In the meantime, if you see a floating raft of fire ants, run far, far away.

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