The ants wars

yes, better than Starwars and World of Warcraft together, the wars of ants. Last year in our lab, we set up wars between different species, among the most aggressive in the world.

I’m sure you can imagine. Monstrous armies of millions of Unsullied warriors, impervious to danger, dedicated to the death, working together with the efficacy given by millions of years of evolution, all entirely bent to one single purpose, destroying the other armies. I’m certain to are picturing this. Well, you are picturing it wrong, you immature brutes. So, what did we do and why did we do it?

It was a time when a Ph D student (Cleo Bertelsmeier) was studying the effect of climate change on invasive ants. I’ve told you already why we study invasive ants. If you’ve missed it, you can read it here. The first part of the PhD thesis was to build up species distribution models to try and predict where invasive ants would find favorable regions with climate change (ants are very sensitive to climate, and milder winters may mean higher probability of establishment). And the result was that some of the most problematic invasive ant species were predicted to arrive at the same place in several regions. And because the most obvious characteristics of all these invasive ants is that they are extremely efficient at removing other arthropods, starting with local ant species, we naturally wondered what would happen if two of such Hun armies were to clash in newly invaded territories. Or in other words, is there among these tiny berserk beasts one that would take over all the others (and the rest of the world with it).

So we set up colonies of four of the worst of the worst. These were the invasive garden ant Lasius neglectus, the Argentine ant Linepithema humile, the big-headed ant Pheidole megacephala and the electric ant Wasmannia auropunctata. The experiment set up by Cleo was not really the wars you pictured, but they were enough for our purposes: boxes with colonies of 300 workers and one queen, put into contact by a tiny tube, and days of counting the dead and the survivors. And these taught us a lot. First, that the experiments of one worker versus another in a Petri dish – often set up to establish dominance hierarchies among ant species – are not well suited, because some ants species need other workers to kill others. Some ants hold the enemy while it is being cut into pieces, and you can’t do that when you’re alone, and you’ll systematically lose in duels but not necessarily a battle. It also mean that classical experiments of 10 vs 10 workers in a Petri dish are also problematic, because the lack of natural conditions can bias the results. These ants are very stressed, more or less forced to fight, and with no territory, nest or queen to defend (which was not the case in our experiment). Last, it taught us that ants adapt their strategies according to their opponents. Some species that are very aggressive and kill everything were less so when confronted to potentially stronger adversaries. Some even escaped or feigned death. And some raided the other colonies D-Day style improved with chemical weaponry, with many losses but an eventual conquest while some others remained in their strongholds and privileged defense. And eventually it taught us that when you increase complexity, for example by putting all four species together, you increase… well complexity. Here, the species that systematically lost against any of the three others won half the time when all four were fighting simultaneously.

Now I’m sure you’d like to know who was the meanest of the four. The tiny electric ant, so named for its terribly painful sting? Or the scary big-headed ants, which soldiers can cut in two any of the other species? Well, I guess that to know that you’ll have to read the paper (and perhaps that one too about their strategies)… Yes, I know, I’m mean. That’s what the ants say too.

Of course, the best fighter of all remains the Ant-man

Why would one study invasive ants?

Yes, I mentioned that earlier: we study invasive ants. We do all kind of fun things we them, from models trying to predict where they could be invasive, now or with climate change, to lab experiment trying to see which species are better at finding and monopolising resources. There are a bunch of papers now that we have published on that (and more to come, that I will probably advertise here, so I won’t say too much now), but the point is, they are very interesting, and quite problematic for biodiversity and economics alike.

There are about 20 000 ants species out there (40% of which remaining to be described by science). One percent of these species are exotic, meaning they have been introduced outside their native range, and seem to have been established there. About a tenth of these 200 exotic species are known to be highly invasive. That mean they are mean. And I do mean mean. Not the mean mean, really mean. Ok, I stop. They are highly aggressive and exceedingly efficient. When they invade, they destroy the native entomofauna, starting by the native ant communities, but also affecting plants, other invertebrates, amphibians, reptiles, birds and even mammals. Humans are known to be victims of the Red Imported Fire Ant by the thousands in the USA (with hundreds of death – that is more than by sharks). They really affect biodiversity and doing so they disrupt ecosystem functioning and services (like pollination and seed dispersal). And they cause billions of dollars of damage every year to agriculture, forestry, real estate and public health.

Oh, and they also go by scary names, like the fire ant, the electric ant, the crazy ant, the destroyer ant, the ghost ant… And before you ask, yes there are zombie ants and vampire ants as well, only they are not invasive ants. No werewolf ant though. Kinda disappointing.

Anyway, invasive ants are about to take over the planet, and who is working their ass off trying to save the day, in the general ingratitude? To whom will you turn when they try and make you their slave? Biodiversity Dynamics, thank you.

So, to answer the question in the title, we study invasive ants, simply because invasive ants are importants. Import-ants. Get it? Ok ok, I thought that was a good one…

 

Why ants live in big colonies

Ever wondered why ant colonies are so big? One talks about millions of individuals in some colonies. And for people working on invasive ants, like us, this thought can be both fascinating and frightening (BTW, we work on invasive ants, because they are importants. Sorry.). Take the Argentine ant for example. It forms “supercolonies” (related colonies) that can encompass tens of thousands of nests over thousands of kilometres. This amounts to perhaps hundreds of millions of tiny, mean brothers-in-arms that will attack and effectively kill almost anything they encounter and that is small and stupid enough to hang around. Sweet little things. We love them. Later on, I’ll tell you how we compare the nastiness of the various invasive ant species, and believe me, Mother Nature had loads of fun – and bursts of creativity – when She created ants.

But for now, how did they manage to evolve such immense social colonies? One answer could come from our study of Allee effects. An Allee effect is a positive relationship between the size of a population and its capacity to persist and grow. In a word, in some species, the more individuals there are, they better they do, often because they help each other in some various ways. So, we studied Allee effects in ants; just because we study all we can about the Allee effect (because we are a bit monomaniac) and because we like ants. We like rhinos too, but that’s less practical in a lab. Oh, and of course because, very surprisingly, nobody ever had the idea (or the madness?) to look into that.

So, we set up large experimental designs to record the survival and reproduction of colonies of various sizes. By various sizes, I mean different number of workers and different numbers of queens. Because, yes, some ant species live with several (sometimes hundreds of) queens in the same nest. And after a few years of hard work, the results were worth it: there are indeed Allee effects in ants; at least the two species we studied. The corresponding paper is here. The more ants, the better they do. And it even gets better: the more queens, the more workers are produced per queen (that’s not that obvious, they could compete for food). And also, the more queens, the better the workers survive (don’t ask me why, I’m supposed to be concise here). And the fun continues: the more workers, the higher the queens productivity! So in fact, each cast (workers and queens) benefit the other cast, so that it creates a mad feedback loop leading to ever growing colonies (that I think can possibly only end in them taking over the world). So this mechanism of Allee effect might have played an important role into making large colonies of eusocial species. Nice no? Oh, come on!

 

What, no funny image? Yes, this one IS funny too, just watch closer!