A blog post highlighting the article written by Calcaterra, Cabrera and Briano in Insectes Sociaux

Written by Luis Calcaterra

The world’s worst invasive ants originated in the same place as the tango- the River Plate basin in Argentina. Step by step, naturally, helped by humans and/or global warming, these ants then proceeded to move from their native range in all directions over the last 150 years. Finally conquering the world, they have changed human life on Earth irreversibly. But why are invasive ants, as the tango, more notorious abroad than in their homeland?

Of the approximately 14,000 ant species on earth, only a handful are invasive and threatening to the planet. Like tango dancers, these species gained fame abroad, but not for positive reasons. These ants cause billions of dollars in losses by affecting agriculture, human health and wildlife, replacing and reducing native species, and even pushing some species to the edge of extinction.

Main source of global invaders

Like the tango dancers, these versatile ants have evolutionary adaptions to move freely around the world to establish and dominate successfully in new places, with devastating consequences. Most of their invasions originated in the basin of the River Plate, as in the case of the invasive red and black fire ant (Solenopsis invicta and S. richteri), the Argentine ant (Linepithema humile), the little fire ant (Wasmannia auropunctata), and the South American big-headed ant Pheidole obscurithorax, whose source populations have been recently discovered in Argentina.

But are these evil invaders as dominant and problematic in their homeland as they are in the invaded regions? Little is known about the performance of invasive ants in their native land or how they interact with other native ants. Previously, we thought that invaders were not dominant in their homeland, but studies conducted during the last decade revealed that some of them can be just as ecologically dominant in their home as in their introduced range. Such is the case of the aggressive red fire ant (Calcaterra et al. 2008), now spread in North America, Australia, China, Hong Kong, Taiwan, the Philippines and some Pacific and Caribbean islands (Ascunce et al. 2011).

Local coexistence of several highly invasive ants

In our study, we investigated how different invasive ants fare against other native ant species as part of an effort to learn more about the behavior of invaders in their homeland. We studied interactions between several of the most highly invasive ants and other native aboveground foraging ants that locally co-occur in four habitats of a protected area (Otamendi Natural Reserve) next to the city of Buenos Aires. We used a combination of pitfall traps and baits to study day-to-day activity in ant communities to determine ant abundance at the sites, and this showed the ability each species had to discover and dominate food resources.

Of the 49 ant species that locally coexisted in the reserve, five were well-known global invaders: the black fire ant S. richteri, the Argentine ant L. humile, the little fire ant W. auropunctata, the tawny crazy ant Nylanderia fulva, and the rover ant Brachymyrmex gaucho. But only two of them, the black fire ant and the Argentine ant were ecologically dominant, though their supremacy was much lower than in the invaded regions, likely due to the presence of much more skilled competitor ants in their homeland. It´s as if local acceptably good dancers were treated as demigods abroad, something that in fact often happens in the tango world.

Factors promoting ant coexistence

Coexistence among so many invasive ants and other co-dominant native, but non-invasive, ants was apparently helped by niche and competitive differences, different habitat preferences, different abilities to scout and discover the baits, and in the ability to immediately recruit enough workers to monopolize them. But the high ant species diversity and the fact that invasive ants are mostly randomly distributed in space (not segregated) suggest that competition only played a secondary role in organizing the ant communities.

The other invasive ants were not dominant in the reserve- much like the tango dancers that appear mediocre in their very competitive local environment. Thus, it is possible that their dominance in native and introduced regions of the world is more associated with their high colonization capacity to harsh environments (mostly anthropic) rather than to their superior competitive abilities. For example, the little fire ant is able to monopolize bait only when a large number of workers are recruited to it, which can rarely be achieved owing to its slow performance in scouting and recruiting workers to defend it from better contenders (unless the bait is within its nesting territory). The little fire ant is mostly problematic in places where there are very few competitor ants, such as islands (e.g. Hawaii) or anthropic sites (e.g. cities, where the weather is also more extreme).

Pheidole workers removing little fire ant workers from a bait. 
Video credit: Lucila Chifflet

As for the ant tango, a question remains without answer: what factors enable the appearance in the River Plate basin of so many successful invaders? To our knowledge, this region represents the main exit door of known and potential invasive ants. As global trading expands, the spread of the nastiest ants from this region of the planet will continue threatening life on Earth.

References

Calcaterra, L. A., Livore, J. P., Delgado, A., & Briano, J. A. (2008). Ecological dominance of the red imported fire ant, Solenopsis invicta, in its native range. Oecologia, 156(2), 411-421.