"Long before it's in the papers"
February 20, 2015

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Fire ants spread globally on 17th-century ships, study finds

Feb. 20, 2015
Courtesy of the University of Illinois
at Urbana-Champaign
and World Science staff

Hu­mankind’s Age of Dis­cov­ery was a boon not only for many hu­man traders, pirates and con­quis­ta­dors, but al­so for some lit­tle ant con­querors, a study has found.

Thanks to some ge­net­ic sleuthing, re­search­ers say they now know that the events of the 1600s en­abled the trop­i­cal fire ant, Solenop­sis gem­i­nata, to col­o­nize al­most eve­ry con­ti­nent.

Historical Spanish trade routes (Cour­tesy Ju­lie Mc­Ma­hon)


Their stu­dy, re­ported in the jour­nal Mo­lec­u­lar Ecol­o­gy, con­cludes that Span­ish galleons shut­tled trop­i­cal fire ants from Aca­pul­co, Mex­i­co, across the Pa­cif­ic to the Phil­ip­pines, and from there to oth­er parts of the world. 

To­day, the spe­cies is found in vir­tu­ally all trop­i­cal re­gions, in­clud­ing in Af­ri­ca, the Amer­i­cas, Aus­tral­ia, In­dia and South­east Asia.

Many of these ships “would fill their bal­last with soil and then they would dump the soil out in a new port and re­place it with car­go,” said Uni­vers­ity of Il­li­nois en­to­mol­o­gist and an­i­mal bi­ol­o­gy de­part­ment head An­drew Suarez, an au­thor on the stu­dy. 

“They were un­know­ingly mov­ing huge num­bers of or­gan­isms in the bal­last soil.”

“In­va­sive ants are a huge prob­lem. Once they ar­rive, they es­tab­lish really high dens­i­ties in new habi­tats, with neg­a­tive con­se­quenc­es for ag­ri­cul­ture, na­tive spe­cies and hu­man qual­ity of life,” said Uni­vers­ity of Ver­mont bi­ol­o­gist Sara Helms Ca­han, who par­ti­ci­pated in the stu­dy. “Con­trolling them costs mil­lions of dol­lars an­nu­al­ly.”

The re­search­ers an­a­lyzed the genomes of trop­i­cal fire ants from 192 lo­cales, look­ing at pat­terns of ge­net­ic di­vers­ity. The team al­so an­a­lyzed the trad­ing pat­terns of Span­ish sail­ing ves­sels go­ing to and from the New World in the mid-1600s.

“If you look at the records, you look at the his­to­ry, you look at the old trad­ing routes and you look at the ge­net­ics, it all paints this pic­ture that this was one of the first glob­al in­va­sions, and it co­in­cid­ed with what could be the first glob­al trade pat­tern of the Span­ish,” Suarez said. “The ants from the in­tro­duced ar­eas in the Old World are ge­net­ic­ally most si­m­i­lar to ants from south­west­ern Mex­i­co, sug­gest­ing that their source popula­t­ion came from this re­gion.”

The Span­ish had just es­tab­lished a reg­u­lar trade route be­tween Aca­pul­co and Manila, Phil­ip­pines, not only set­ting up the first trade route across the Pa­cif­ic Ocean but al­so ef­fec­tively glob­alizing com­merce, Suarez said.

“A­ca­pul­co was a big stop­ping point for the Span­ish,” Suarez said.

“From there, Span­ish galleons brought sil­ver to Manila, which served as a hub for trade with Chi­na,” the au­thors wrote.

The re­search­ers hy­poth­e­sized that the orig­i­nal ant popula­t­ion would have the high­est ge­net­ic di­vers­ity and that any ants tak­en from that orig­i­nal popula­t­ion to a new en­vi­ron­ment would have a sub­set of that orig­i­nal vari­abil­ity. And that’s what they found.

“There was this very clear pat­tern where there was the most ge­net­ic di­vers­ity in the New World, where it’s na­tive, and then you see these step­ping stones of nest­ed sub­sets of di­vers­ity as you move away from the New World in­to the Old World,” Suarez said. And the pat­tern of ge­net­ic changes over time “al­ways overlaps the tim­ing of when the Span­ish trade was go­ing on,” he added.

“Unco­vering events that hap­pened long ago, be­fore the age of dig­it­al track­ing codes and cus­toms en­force­ment, is of­ten a dif­fi­cult task,” Helms Ca­han said. “Luckily for us, how­ev­er, it turns out that in­va­sive spe­cies keep their own records of their his­to­ry, en­cod­ed in their genomes.”


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Humankind’s Age of Discovery was a boon not only for many human traders and conquistadors, but also for some little ant conquerors, a study has found. Thanks to some genetic sleuthing, researchers say they now know that the events of the 1600s enabled the tropical fire ant, Solenopsis geminata, to colonize almost every continent. Their study, reported in the journal Molecular Ecology, concludes that Spanish galleons shuttled tropical fire ants from Acapulco, Mexico, across the Pacific to the Philippines, and from there to other parts of the world. Today, the species is found in virtually all tropical regions, including in Africa, the Americas, Australia, India and Southeast Asia. “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 University of Illinois entomologist and animal biology department head Andrew Suarez, an author on the study. “They were unknowingly moving huge numbers of organisms in the ballast soil.” “Invasive ants are a huge problem. Once they arrive, they establish really high densities in new habitats, with negative consequences for agriculture, native species and human quality of life,” said University of Vermont biologist Sara Helms Cahan, who participated in the study. “Controlling them costs millions of dollars annually.” The researchers analyzed the genomes of tropical fire ants from 192 locales, looking at patterns of genetic diversity. The team also analyzed the trading patterns of Spanish sailing vessels going to and from the New World in the mid-1600s. “If you look at the records, you look at the history, you look at the old trading routes and you look at the genetics, it all paints this picture that this was one of the first global invasions, and it coincided with what could be the first global trade pattern of the Spanish,” Suarez said. “The ants from the introduced areas in the Old World are genetically most similar to ants from southwestern Mexico, suggesting that their source population came from this region.” The Spanish had just established a regular trade route between Acapulco and Manila, Philippines, not only setting up the first trade route across the Pacific Ocean but also effectively globalizing commerce, Suarez said. “Acapulco was a big stopping point for the Spanish,” Suarez said. “From there, Spanish galleons brought silver to Manila, which served as a hub for trade with China,” the authors wrote. The researchers hypothesized that the original ant population would have the highest genetic diversity and that any ants taken from that original population to a new environment would have a subset of that original variability. And that’s what they found. “There was this very clear pattern where there was the most genetic diversity in the New World, where it’s native, and then you see these stepping stones of nested subsets of diversity as you move away from the New World into the Old World,” Suarez said. And the pattern of genetic changes over time “always overlaps the timing of when the Spanish trade was going on,” he added. “Uncovering events that happened long ago, before the age of digital tracking codes and customs enforcement, is often a difficult task,” Helms Cahan said. “Luckily for us, however, it turns out that invasive species keep their own records of their history, encoded in their genomes.”