There’s a lot of talk today about foreign insect species being introduced to new regions through international trade and travel, but this is nothing new. One of the best-known cases has to do with honey bees, which were first brought to North America around 1622. Native Americans, who had never seen them before, reportedly referred to them as “white man’s flies.”
Now researchers have documented an even earlier example. According to a study published in the journal Molecular Ecology, tropical fire ants (Solenopsis geminata) were stowaways on Spanish ships as early as the mid-1500s, which brought them from Acapulco, Mexico to the Philippines, and from there to other parts of the world. Today, the ant species is found in virtually all tropical regions, including 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 entomology Andrew Suarez, one of the co-authors. “They were unknowingly moving huge numbers of organisms in the ballast soil.”
He and his colleagues 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 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 and Manila, not only setting up the first trade route across the Pacific Ocean but also effectively globalizing commerce.
“Acapulco was a big stopping point for the Spanish,” Suarez said.
The researchers hypothesized that the original ant population would have the highest level of 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 is 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.
“Uncovering events that happened long ago, before the age of digital tracking codes and customs enforcement, is often a difficult task,” said University of Vermont biology professor Sara Helms Cahan. “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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