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Attack of the Big-Headed Ants: Can No One Stop Them???
Volume 16, Number 43: 23 October 2013

In an intriguing paper published in Biological Invasions, Bertelsmeier et al. (2013) write that "climate change and invasive species are two of the most serious threats of biodiversity," noting that "a general concern is that these threats interact, and that a globally warming climate could favor invasive species," with the result that "many native species are displaced, leading to local extinctions of fauna and flora." More specifically, they say "several studies suggest that climate change could exacerbate the threat posed by invasive species, especially poikilotherms [animals having a body temperature that varies with the temperature of their surroundings], by removing thermal barriers and allowing them to establish at higher latitudes." But is this really the case?

In a study they designed to explore this important question, the three French researchers focused their attention on ants, because of the fact that (1) "ants are among the worst invasive species (Rabitsch 2011; Holway et al. 2002; Lach and Hooper-Bui 2010)," and that (2) "they are small, numerous and colonial" and can therefore "rapidly colonize a new habitat." More specifically, they chose to study the big-headed ant, Pheidole megacephala, which is considered to be one of the world's 100 worst invasive species. This they did using ecological niche models to estimate the species' potential suitable habitat in 2020, 2050 and 2080 with an ensemble forecast obtained from five different modeling techniques, including three Global Circulation Models and two CO2 emission scenarios, by the means of which they generated world maps with suitable climatic conditions and assessed changes, both qualitatively and quantitatively.

"Surprisingly," as Bertelsmeier et al. put it, their results suggested that "the invasion of big-headed ants is not only unlikely to benefit from climate change, but may even suffer from it," in that their projections showed "a global decrease in the invasive potential of big-headed ants as early as 2020 and becoming even stronger by 2080, reaching a global loss of 19.4% of area with favorable climate." And they go on to note that this finding is just the opposite of classical views of global climate change, wherein the ranges of many invading species are believed to increase in response to warming, especially those that are currently limited by climate. P. megacephala, on the other hand, "will experience very little shifts in potential habitat," according to the French scientists, as their niche envelope, presenting favorable climatic conditions, "will mostly shrink."

Interestingly, the analysis of Bertelsmeier et al. thus suggests, as they clearly state, that "the worst invasive species of today may not be the worst invasive species of tomorrow," if the earth ever begins to warm again, that is. And after the planet's close-to-two-decade climate-change hiatus, it's anyone's guess as to what it may or may not do.

Sherwood, Keith and Craig Idso

Bertelsmeier, C., Luque, G.M. and Courchamp, F. 2013. Global warming may freeze the invasion of big-headed ants. Biological Invasions 15: 1561-1572.

Holway, D., Lach, L., Suarez, A.V., Tsutsui, N.D. and Case, T.J. 2002. The causes and consequences of ant invasions. Annual Review of Ecology and Systematics 33: 181-233.

Lach, L. and Hooper-Bui, L.M. 2010. Consequences of ant invasions. In: Lach, L., Parr, C.L. and Abbott, K.L. (Eds.). Ant Ecology. Oxford University Press, New York, New York, USA, pp. 261-286.

Rabitsch, W. 2011. The hitchhiker's guide to alien ant invasions. BioControl 56: 551-572.