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How 16th Century Trade Made Fire Ants an Early Global Invader

By inadvertently stowing away in Spanish ships, one pesky little insect quickly spread throughout much of the world

The tropical fire ant is the first known ant to travel the world by sea. (Alex Wild/Visuals Unlimited/Corbis)
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Today, tropical fire ants can be found in nearly every warm and humid region on Earth, including Africa, India, Southeast Asia, Australia and the Americas. But the species didn’t always so thoroughly dominate the world. A new study using genetic analysis and historical data has traced the little critter’s spread to the rise of Spain’s global trade routes—making them the perpetrators of one of the earliest known biological invasions.

In a paper published by Molecular Ecology, a team of scientists set out to determine just how the tropical fire ants (Solenopsis geminata) spread and from where they originated. They hypothesized, as New Historian reports, that the “original ant population would have had the greatest genetic diversity where it was native.” So they analyzed the genomes of fire ants from 192 different locations and found that Southwestern Mexico was likely the source of the invasive population.

Eventually, a distinct pattern of fire ant infiltration emerged, and it matched almost perfectly to Spain’s first global trade route. As University of Illinois at Urbana-Champaign explains:

The researchers were able to date the ants' invasion of the Old World to the 16th century. At this time, the Spanish had just established a regular trade route between Acapulco [in Southwestern Mexico] and Manila, Philippines, not only setting up the first trade route across the Pacific Ocean but also effectively globalizing commerce.

Tropical fire ants were likewise globalized, it appears, by unknowingly hitching rides on Spanish ships. “A lot of these ships, particularly if they were going somewhere to pick up commerce, would fill their ballast with soil and then they would dump the soil out in a new port and replace it with cargo,” said Andrew Suarez, one of the study’s authors. “They were unknowingly moving huge numbers of organisms in the ballast soil.”

As Spanish trade routes and influences widened, so did fire ants’ domain. They are the first ants known to travel the world by sea and one of globalization’s earliest ecological impacts. Known to negatively affect native species and agriculture, today, they are often regarded as a pest species and, according to one of the study’s researchers, can cost millions a year to control.

Today, biological invasions are almost routine, but a new route for dispersal is opening: the newly expanded Panama Canal, scheduled for completion in 2016. As National Geographic reports, a new study warns that the increased volume of ships coming through the canal will open the door for a greater number of alien species.

Just as in the 1500s, ballast could be a big part of the problem. These days, ships often use water to help provide stability; when this water is released upon arrival in a new port, so are the small, potentially invasive species.

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