Where did North America pavement ants (genus Tetramorium) come from? (Joey, Mansfield, MA, USA)


I am junior that studies my local ants as a hobby. One question that has been bugging me recently is about  Tetramorium sp. E. I understand that until recently it has been considered to be T. caespitum and introduced from Europe like all other species of the genus except the desert species T. hispidum and T. spinosum. If there are only 2 desert species found in North America, how did they get there and why didn't the genus spread to other types of environments? I was wondering what evidence shows that T. sp E is an introduced species like T. bicarinatum or T. tsushimae, how and when it arrived, and if there is a possibility it isn't introduced.
                                                                                                                                                Joey, Mansfield MA

Dear Joey,

"Where in the world did this come from?" is the basic form of many questions in the field of biogeography. To answer it, scientists use evidence from the fossil record, molecular biology (usually DNA), and the structure and form of living organisms (morphology).

Fossil evidence is often the best, but most ant species don't have a fossil record (although there are many ant fossils, there are not enough to provide back-stories for the more than 14,000 described ant species!). Molecular biology can be very useful in tracing the ancestry of organisms, simply because it provides the most accurate way of re-constructing their family tree. If the most distantly-related members of the group of organisms you're studying all live in the same place, we often assume that place is the origin of the group. For example, the most diverse genus of ants, Pheidole, was shown by Dr. Corrie Moreau to have originated in the New World (North and South America). She used evidence from DNA sequences to show that the most distantly-related groups within the genus were all found there, and almost all of the Pheidole found everywhere else were part of one (relatively) closely-related group. Using this evidence, she reasoned that this is where they must all be from (actually, it was much more complicated than that--she used a lot of statistical analyses and sophisticated software to arrive at that answer. You can read more about it here).

In 1957, myrmecologist Bill Brown published a short piece entitled "Is the ant genus Tetramorium native to North America?"
in which he presents evidence that is more or less a well-reasoned series of anecdotes for the idea that Tetramorium sp. E (which was, at the time, T. caespitum) is not native to North America. For example, he argued that because it is only common in very urban settings in North America, whereas in Europe and Asia it is often found in more "natural" settings, the latter was more likely to be its natural habitat. Although not bullet-proof, this line of reasoning does make sense. As later authors have shown, Brown was probably not actually collecting the same species in all the places he travelled. It would be interesting to go back to his old collections and figure out what species he really got!

In 2006, Dr. Birgit Schlick-Steiner and several colleagues published a paper in which they provided evidence that individuals in what was previously considered Tetramorium caespitum (and the difficult to distinguish T. impurum) were actually members of at least seven different species. They used data from DNA sequences, chemicals found on the outside of the ants (cuticular hydrocarbons) and the way the ants look (morphology). Usually, in situations like this, authors will provide names for the newly erected species. But members of the Tetramorium caespitum group had been described under roughly 50 different names in the past, and the strict taxonomic convention dictates that they have to be assigned the first given name. Rather than attempting to find and re-evaluate all the relevant type material, they decided to simply go with a series of letters. (it should be noted that, at the time of this writing [January 2011], the majority of images of Tetramorium caespitum on AntWeb are actually of Tetramorium sp. E).

This study supported the idea that "pavement ants" aren't native to North America by showing that there were a cluster of closely-related, but distinct species in Europe, and, within that cluster, one very closely-related group in North America. Such close relationships over a very wide geographic area (across the US) strongly suggest a recent introduction. Although it is at a much finer taxonomic scale and grain, this reasoning is analogous to that used in Moreau's study of Pheidole described above.

There are collections of T. sp. E from the United States dating back to the early 1900s, and possibly before, so it was introduced before that, but (I would guess) probably not before the 1600s. Interestingly, there is also a species of ant that doesn't ever have workers and has to live as a nest parasite of Tetramorium sp. E, called Anergates atratulus. The most likely way that this species was introduced was with a whole or partial colony of Tetramorium, perhaps in some construction materials or ballast brought over from Europe. Both species may have even been introduced more than one time. In fact, Schlick-Steiner and colleagues' 2006 study showed DNA evidence of at least two separate introductions.

Regarding your questions about the truly native Tetramorium species in Americas, the desert relatives of the pavement ant, Tetramorium hispidum and T. spinosum are in the same genus, of course, but they are actually quite distantly-related, and have likely been present in North America for millions of years. The idea of a 'genus' is tricky, because it is not comparable in very different groups of organisms. Most of what we think of when we think about different species within a genus are mammals, and have only been separated for 10 million years or less. But really, many insect genera are actually about the same age as the majority of what we call "orders" within Mammalia (both are often around 65 million years old, give or take 15 million years). Thus, just because Tetramorium sp. E was suited for life as a pavement ant in cities across North America, doesn't mean that its very distant cousins are.

How do we know that they're native to North America? The best evidence is that they are distinct, recognizable species that have never been found anywhere else. Actually, for a long time, our native Tetramorium were classified in a different genus, Xyphomyrmex. In addition to the desert Tetramorium present in the US, there are three closely-related species in Mexico (T. bicolorum, T. placidum and T. mexicanum), and although there isn't any DNA evidence to prove it, based on the way they look, the closest relatives of the native New World Tetramorium (also formerly members of the genus Xyphomyrmex) are probably in Africa.

Why are our native Tetramorium in the desert and not in the Great Plains or the Boreal Forest? Well, why are cactuses and road-runners mostly in the desert? It is an unsatisfying answer, and like Bill Brown's paper, mostly based on anecdotes and reasoning, but most species, even when they disperse, tend to be most successful in habitats that closely resemble those of their ancestors (the formal, testable version of this statement is called the "Niche Conservatism Hypothesis"). Thus, most invasive species tend to do well in habitats that closely resemble their own. It is possible that the ancestors of the native New World Tetramorium were blown here, or washed ashore from a desert area in Africa. This may have even happened when the continents of Africa and South America were closer together--we could test that hypothesis with DNA sequence data. Tetramorium sp. E, on the other hand, was from a climate more similar to many of the non-desert areas of the United States, and this is where it has been successful.

Interestingly, we can't entirely rule out the idea that the New World Tetramorium never diversified into other biomes, only to subsequently go extinct, perhaps during an ice age. All we can do is shrug our shoulders and say, well, we have no evidence to suggest that they ever did. And when asked why, shrug our shoulders again and say, dunno, really. They just didn't. When the distribution of a species or group is not due to currently operating behavioral or ecological processes (like natural selection), we call this a(n) historical contingency (there are many other types of historical contingencies, perhaps most famously explored by Stephen J. Gould in his book, "Wonderful Life"). They're fun to think about, and propose explanations for (an activity often called "hand-waving," by academics), but ultimately unknowable, at least with the data and methods we have available today. "How did [Tetramorium] get there and why didn't the genus spread to other types of environments?" Is a really great example of such a question. With more molecular evidence, we could say roughly when they arrived, and where they were likely to be from, but we'll never know exactly what transpired when they dispersed here.

I hope this helps!
Jesse Czekanski-Moir & the AntAsk Team

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