There’s been a debate simmering among Argentine Ant researchers about the difference between the ant’s ecology in its native South America and in the introduced populations. The heart of the disagreement is this: is the introduced Argentine ant dominant because its biology changed during introduction, or because the ecologies of the native and introduced ranges are different?
Like most scientific debates, some aspects are factual in nature while others are semantic. Sometimes the semantic and the factual become confused in a way that makes it difficult to tease the arguments apart without careful parsing of words, and I think this debate is one of those cases with much needless confusion.
The classic story, raised by Neil Tsutsui et al, is that Argentine Ants passed through a genetic bottleneck at introduction (a natural consequence of founding a new population from a few transported individuals) and the resulting homogenous population lacked the genetic diversity needed to recognize nestmates from non-nestmates. Unable to tell kin from foe, the introduced ants merged into a giant friendly supercolony spanning hundreds of miles. Thus changed, the mighty Argentine Ant dominates. This version captured the attention of the media and remains a common explanation.
Several other studies, including a set by Nicole Heller, Jes Pedersen, and now a new paper by Valerie Vogel dispute the classic story. It turns out that the native range also contains supercolonies. Indeed, these newer studies document large areas- up to several hundred meters- covered by genetically uniform interconnected nests, within which the ants peaceable cooperate but among which the ants brutally contest territory and resources. It seems the native range is a microcosm (or many microcosms) of the introduced range. Is it time to stick a fork in the Tsutsui genetic diversity hypothesis?
In my opinion, that depends.
The first complication is that the studies don’t fundamentally conflict on the science. For one, they measure different spatial scales. Tsutsui et al collected from Buenos Aires north 1000 km to the Paraguayan border, sampling broadly but sparsely. The later studies traded geographic breadth for local depth, giving great detail for a very small part of the range. With this in mind it’s hard to see why argument exists at all. The big picture is the same regardless of who conducted the study: the native range contains more genetic diversity, and this diversity is linked to a much greater number of supercolonies.
The problem for everyone comes in the interpretation of what Tsutsui’s genetic diversity hypothesis is supposed to explain. Is it to establish why the introduced colonies are so few yet so large? If so, the case is an elegant slam-dunk. Or is it bigger than that, providing the explanation for how the introduced Argentine ants are able to eliminate the natives and dominate the landscape?
There’s only one way to resolve whether supercolony size bred ecological dominance, and that is field experiment. Yet, no one has done the work to show that colony size has anything to do with the displacement of native species. I’ll repeat: no one has done the work. It may well be that an Argentina-style mosaic of smaller colonies would be just as devastating to the native Californian fauna as the single supercolony. Or not. The point is, we don’t know.
In the absence of truly relevant data, scientists with opposing ideas carry out disagreements using indirect arguments resting on various assumptions. For instance, the presence of supercolonies might be used to demonstrate that the behavior in the native and introduced ranges is the same. Here is where semantics start tripping us up. Slightly different meanings of “supercolony” will be carried to quite different conclusions, with one side defining them smaller (Vogel et al even label a meter-wide assemblage as a “supercolony”!) and the other side fudging the distinction larger. What is a supercolony? Is 10 meters enough? 100 meters?
So to answer the question, are Argentine ants dominant because they’ve got larger colonies in the introduced range? We still don’t know, but at least we can have fruitless arguments over what is and is not a supercolony. Phooey. What we need are experiments.