Posted by Darren Ward @nzhymenoptera

Ants are among the most invasive animals on the planet. Over 240 species have been recorded as being transported by humans to new geographic locations, and 19 of those are considered invasive.

Caption. The Argentine ant, a globally invasive species. Photo by Philip Herbst. Image available from Ant Web.

But not all invasions occur as the result of direct transport. Some species have managed to invade one place, survive, and then migrate to another—a process known as the “bridgehead effect”. In a new paper, just published in Proceedings of the National Academy of Sciences, we investigated these secondary invasions, and show bridgehead effects are a major driver of new invasions.

We looked at interception data, that is, records of what ant species have been intercepted at the border. Two large and long-term datasets were examined, one from the USA covering the years 1914 to 1984 and containing 51 ant species, and the second from New Zealand covering the years 1955 to 2013 with 45 ant species.

The most surprising result was that most of the interceptions did not originate from species’ native ranges but instead came from already invaded areas. In the United States, 75.7% of the interceptions came from a country where the intercepted ant species had been previously introduced. In New Zealand, this value was even higher, at 87.8%.

Interceptions also increased when they came from countries that were physically closer (Latin America for species intercepted in the United States and Oceania for species intercepted in New Zealand). Additionally, ant species that travelled the most tended to be more successful in invading a secondary location. This created a positive feedback loop between the introduction and establishment stages of the invasion process, in which initial establishments promote secondary introductions.

Overall, these results reveal that secondary introductions act as a critical driver of increasing global rates of invasions. Consequently, it is not enough simply to account for the original location of an invasive species. To better understand pathways of invasive species, we also need to follow the dynamics of spread throughout their entire range.

 

Cleo Bertelsmeier, Sébastien Ollier, Andrew M. Liebhold, Eckehard G. Brockerhoff, Darren Ward, and Laurent Keller. Recurrent bridgehead effects accelerate global alien ant spread. PNAS (2018). www.pnas.org/cgi/doi/10.1073/pnas.1801990115

 

and if you really like ants, then another really great related paper is

Cleo Bertelsmeier, Sébastien Ollier, Andrew Liebhold & Laurent Keller. Recent human history governs global ant invasion dynamics. Nature Ecology & Evolution (2017). doi:10.1038/s41559-017-0184

 

Darren Ward is an entomologist, Head Curator at the New Zealand Arthropod Collection at Landcare Research, and a senior lecturer at the School of Biological Sciences, University of Auckland.