Photo courtesy of Jonas Stenstrom

If you’ve ever seen ants on the sidewalk, you’re probably already familiar with one of the most commonly encountered ants in North America. Now researchers with the University of Florida say that the story of how this non-native ant spread across the continent can be found in its genes.

In a new study, the researchers found that populations of this ant are genetically near-identical to one another. This suggests that the ants’ spread was likely aided by people unintentionally transporting them via railroads, highways and other transportation infrastructure.

Known as pavement ants, this species gets its name from its habit of building nests under sidewalks and alongside man-made structures, which are similar to the gravelly areas they prefer in their native habitat. Originally from Europe, this species of ant is now found in 39 states and three Canadian provinces. Its scientific name, Tetramorium immigrans, refers to its tendency to immigrate beyond its native range and thrive in new territory.

“These animals are thriving in an increasingly paved world. You might say that we’ve rolled out the red carpet for them with our rails, roads, and cities,” said Andrea Lucky, one of the study’s authors and assistant professor of entomology with the UF Institute of Food and Agricultural Sciences.

Pavement ants are well suited to urban and suburban life, often feeding on the food scraps we leave behind. In the spring, worker ants can often be seen fighting for territory with neighboring ants in what are known as “sidewalk battles,” where thousands of ants engage in mortal combat at miniature scale.

Experts have theorized that pavement ants were introduced to North America about 200 years ago. The authors of the new study wanted to know how the pavement ant spread across the continent and became so pervasive.

To answer this question, they looked at the genetic makeup of ants collected by citizen scientists across the United States.

They extracted DNA from nearly 100 ants from 26 states. The goal, explained Miles Zhang, a post-doctoral researcher in Lucky’s lab and the study’s first author, was to evaluate differences in genetic make-up of ant populations in different parts of the country and, in turn, better understand the history of their spread.

“If the ants genes were very different across large distances, this shows that they only spread and mate over short distances, and suggests that they gradually expanded their domain westward from the initial site where they first became established on the east coast,” Zhang said.

In contrast, genetic similarities across the whole country— for example, if California ants were very similar to those in New York—meant the ants were able to jump from one location to another, even if those locations were far apart.

To make these jumps, the ants were probably moved around in plants, soil or building materials transported by railroad or highway.

To analyze the genetic diversity of ants across the country and compare the different populations, the study’s authors used two techniques. First, they sequenced a single gene (Cytochrome Oxidase I, or COI) to confirm that all the samples were from just the Tetramorium immigrans species of pavement ant and not others. Next, they used a newer and more in-depth genomic technique (ddRADseq) that allowed Zhang and colleagues to compare tens of thousands of DNA fragments from across each ant’s genome.

The researchers were surprised by just how similar even very distant populations of pavement ants were.

“Both analyses show there is very little genetic diversity overall within North American pavement ants,” Zhang said. “What’s more, from one area to another, these ants don’t show a signature of having been isolated from one another.”

This finding enhances scientific understanding of how non-native species expand into new regions, Lucky said.

“We know that pavement ants arrived here around 200 years ago, right when North America was beginning to urbanize,” she said. “Our findings tell us that pavement ants in North America likely originated from a single lineage, that is, several closely related colonies that became established here, and spread across the continent in concert with the expansion of railroads, highways and cities.”

Lucky emphasized that this study also highlights the power of citizen science to shed light on common, yet poorly understood species like pavement ants.

“This research combines original data generated by motivated public participants — literally, kids collecting ants with cookie crumbs in their backyards — with very sophisticated genomic analyses,” she said. “Crowdsourcing data allows us to answer these big questions in ways we haven’t been able to before. I hope this study served as a positive model for other researchers interested in public engagement.”

In addition to Zhang and Lucky, the study’s co-authors include:

• Tyler Vitone, a former master’s student in Lucky’s lab
• Caroline Storer, assistant research scientist with the Florida Museum of Natural History
• Adam Payton, a laboratory technician with Stuart McDaniel, associate professor of biology with UF
• Robert Dunn, professor of applied ecology at North Carolina State University, Raleigh
• Jiri Hulcr, associate professor of forest entomology at UF
• Stuart McDaniel, associate professor of biology with UF

The study is published in the journal Frontiers in Ecology and Evolution.

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