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.Â The heart of the disagreement is this:Â 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.Â 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.Â
Unable to tell kin from foe, the introduced ants merged into a giant friendly supercolony spanning hundreds of miles.Â Thus changed, the mighty Argentine Ant dominates.Â 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.Â Â It turns out that the native range also contains supercolonies.Â 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.Â It seems the native range is a microcosm (or many microcosms) of the introduced range.Â 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.Â Â For one, they measure different spatial scales.Â Tsutsui et al collected from Buenos Aires north 1000 km to the Paraguayan border, sampling broadly but sparsely.Â The later studies traded geographic breadth for local depth, giving great detail for a very small part of the range.Â With this in mind it's hard to see why argument exists at all. Â 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.Â Is it to establish why the introduced colonies are so few yet so large?Â If so, the case is an elegant slam-dunk.Â 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.Â Yet, no one has done the work to show that colony size has anything to do with the displacement of native species.Â I'll repeat: no one has done the work.Â 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.Â 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.Â For instance, the presence of supercolonies might be used to demonstrate that the behavior in the native and introduced ranges is the same.Â Here is where semantics start tripping us up.Â 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.Â Â What is a supercolony?Â 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?Â We still don't know, but at least we can have fruitless arguments over what is and is not a supercolony.Â Phooey.Â What we need are experiments.
- Log in to post comments
I like your idea. Experiments to test assumptions, yes what a novel idea. It seems there are two here:
Ecological (removals and/or additions, perhaps even replicated across "supercolonies" of different genotypes and colony size).
Further testing of Vogel et al's measure of "supercolonialism" to determine if these are truly distinct breeding units, which I might go so far as to describe simply as colonies. This idea also applies to other invasives of interest, specifically Myrmica rubra here in the northeast.
Alex (and I'm being serious), I'd be more than happy to provide some ideas and muscle to help carry out such experiments (and I do have some experience with this sort of thing and will be doing some of this here with M. rubra, I think), although I'm sure the line is already forming. They said such a thing could never be done with fire ants...
There are so many questions here. What controls ant colony size in this species? Were the native ants in the introduced range already declining? Does climate shift favour the newcomers? Does lack of predators allow for major colony expansion? Lack of data and ecological knowledge can confound the picture, and confound management.
I think the main area of disagreement between the two research groups arises primarily from "semantic" issues. The Keller research group equates a colony (or supercolony) to a population. This stems from research that shows that supercolonies have many private alleles and therefore likely do not exchange genetic material (that is breed) with one another. However, in many / most native sites, supercolonies still come into contact with one another and compete for resources - so by definition, many coexist in a single population. Tsutsui's original argument was that there is something really different between ranges (genetically and behaviorally) at the population level. A single supercolony exists in most introduced areas while in most native sites there are many competing colonies (or supercolonies). The bottleneck argument only makes sense when viewed from the ecological or population level perspective.
While some experimental work has already been done in both the field and the lab to examine how intraspecific competition among Argentine ant colonies may influence growth and interspecific interactions, a lot of works still needs to be done.
I took a few worker ants from Northern CA to L.A. I put them together and they didn't even fight. Weird stuff.
But I want to know why sometimes Argentine ants kill thier own queens from the same area! I kept Argentine ant colonies, and adding more queens resulted deaths to them (pulling legs and stuff). :(
I read that this might be a population control, but no one knows for sure.
Limits to polygyny
Even highly polygynous ant species always eliminate a great lot of young queens.
One has only ot visit the Jura mountains in June and see how red Wood Ants (Formica lugubris) workers are merrily slaughtering hundreds of dealated females of their own species, athough several dozens of queens are present already in each of their large needle mounds.
Forcibly introducing supernumerary queens in a captive colony will almost always lead to the killing of these. It seems the workers want to decide by themselves how many queens they need and who they will choose. And I suspect genetic relatedness is only one of many factors involved in such choices.
Obviously this results in a way of controlling the growth of a given colony, but this is more a consequence than an intended purpose.
Not directly related to the question, but traditionally genetically similar populations are regarded as being at risk. If this super colony is genetically pretty well identical should it not be fairly easy to devise a targeted disease for them ?
No worry. Nature has a way of controlling when one species dominates and is concentrated, usually through disease or predation.
Also, since we have clearly entered a 100 year solar minimum after 20 years of strong sun fields, the climate will be dramatically cooler for the next 30-50 years, if paleohistory is a gauge. These cold cycles have presented survival challenges for eons and forever to every living creature, as it will toward homo sapiens. The climate is always changing, so that one species can never get too comfortable.
Cold, snow and ice have a way of cramping one's style. My advice to humans: forget about global warming hype (unless you make your lucrative living as an AGW "astrologist" as I call them) and get a wood stove. My advice to the ants? Go south, young pests, go south.
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8127000/8127519.stm linked to this story!! :D
Has the factor of same species competition been allowed for?
In South America if 'large' or 'giant' colonies form by taking over land from unrelated nests there will always be unrelated nests constraining the colony.
But in the larger colonies all the individuals are potentially free to interact and by implication free to spread disease. This would weaken the colony possibly allowing colonisation by unrelated queens breaking the large colony into an area of small nests. Could this be cyclical?
Is it possible that in South America megacolonies do not form because there is a balance between these factors whereas in other parts of the world one or both of these factors are missing so the colony keeps growing?