Why care about ants?

My first proper article of the new blog just had to be about ants. Despite being ecologically vital parts of the natural world, ants are widely misunderstood – even hated – by some people. Hopefully this will set the record straight.

They march along the dense tangle of vines and branches of tropical jungle, across the undergrowth of the world’s boreal forests, over the scorching crests of sand dunes, and even on the hot tarmac of our city streets. Ants are to be found almost everywhere we care to look.

When it comes to diversity, however, ants are not especially impressive compared to other insect groups. Approximately 13,500 species are currently described, with somewhere between 25,000 to 30,000 species estimated to exist (AntCat, 2019). This is about 1% of known insect species!

No, the importance of ants comes from their sheer abundance. Very roughly, there are an estimated 10,000 trillion individual ants alive at any one time, weighing around the same as all human beings combined (Hölldobler and Wilson, 1994; Lach et al., 2010). To support such a high biomass, by definition, ants will exert important effects on the organisms around them. For example, ants are the primary predators of invertebrates in lowland tropical rainforest (Floren et al., 2002), important decomposers and conduits of energy in most measured terrestrial environments, and leaf-cutter ants are key herbivores in Neotropical forests (Wirth et al., 2013). But their roles within ecological communities – biocoenoses, if you will – can be more diverse. Ants are also key seed dispersers, soil turners, and nutrient movers. Plants growing near ant nests enjoy increased nutrient concentrations in the surrounding soil, e.g. the host plants of Azteca ants have >800% of nitrogen, phosphorus, and potassium in the soil surrounding their roots compared to leaf-litter, affording greater growth rates (Lucas et al., 2018).

Ants have also formed some of the most interesting and important mutualisms and associations with other organisms. Most species of ant, though to varying degrees, tap into the rich sugary reserves made available by aphids, scale-insects, and other exudate-producing insects – otherwise known as “honeydew”. This source of carbohydrate is vital for worker ants to remain highly active and is especially important in species that form larger colonies. Thus, the quest of the ants is to protect their honeydew-producing comrades from parasites and predators. Indeed, when these mutualists are found alongside ants, they can reach much greater numbers than those without their armoured myrmidons. In a sense, ants act as important herbivores via these mutualists.

Army ants, however, are simply next-level when it comes to mutualisms with other organisms. There are 557 recorded species associated with the army ant Eciton burchellii, which can form marauding colonies 700,000 individuals strong (so called ‘bivouacs’). These ants roam the tropical forests of South and Central America, predating upon anything they can catch and overpower, including mammals, amphibians, and birds in their nests. Many of the mutualists of army ants, and ants more generally, tend to be small arthropods (e.g. mites, beetles, bugs) that live within the bivouac and feed on the waste of the colony. Others are parasites of the colony that may follow the swarm and predate upon the ants themselves. Antbirds, creatively named, are famed for their association with army ants. These birds hop along behind the raiding front, picking up invertebrates flushed out by the ants. This is a form of parasitism called ‘kleptoparasitism’ (essentially theft). The ants have learned that large-bodied prey items need to be brought beneath the leaf-litter before butchering to prevent the antbirds from stealing them! Around twenty species of antbirds are obligately associated with army ants and many more facultatively. Ergo, army ants are at the centre of a tight-knit biocoenosis. What a shameless self-plug.

If their impressive ecological dominance or diverse associations aren’t enough for you, perhaps their unique life-histories will convert you to myrmecophily (myrmex- ant, phílos- love). The fact that ant workers readily sacrifice themselves for what seemed to be ‘the greater good’ of the colony posed a serious problem to Darwin when he was formulating the theory of natural selection (1859).

How could evolution have favoured a strategy where individuals only very rarely reproduce themselves? In essence, viewing ants and other animal societies, and especially social insects, as ‘superorganisms’ has gone a long way to answer Darwin’s vexation. These superorganisms are comprised of individuals that are usually very closely related, and thus promote the inheritance of their genes indirectly (a theory called ‘kin selection’) by helping their relatives reproduce i.e. the queens and drones. Natural selection then works primarily not at the level of the individual, but at the level of the colony, and thus workers optimize the running of the colony to increase the success of their own genes via the reproductive members of the group (Lach et al., 2010). This also only works in an evolutionary sense because colonies are so much better at completing tasks than individuals. By using “series-parallel” operations, workers switch from one task to another when it’s efficient to do so, meaning that each task is completed more quickly and efficiently than if it was completed by the same number of uncoordinated individuals. This reasoning also scales up – larger colonies are more efficient at completing tasks than smaller colonies (Hölldobler and Wilson, 1990; 2009). Isn’t that an interesting metaphor for human societies?

I’m not done. Ever wonder why all ant workers are female? WELL! You’re in for a treat. Animals have a whole range of different methods to differentiate the sexes of offspring during development. Humans have the X-Y determination system; females have two X sex-chromosomes and males have one X and one Y sex-chromosome. Simple. Ants, on the other hand, have a system called ‘haplodiploidy’ where females arise from being diploid, i.e. they have a set of chromosomes from both their mother and their father, whereas males are haploid and only have a set from their mother. So males inherit their entire genome from their mother and never meet their father. How poetic. The differentiation between workers and queens is a complex melange of environmental and genetic factors which myrmecologists have yet to fully deconstruct, but factors such as diet and how genes are expressed at different times are likely factors that determine which females are fated to one day become queens.

Obviously, the study of these organisms in their own right is incredibly exciting (for some more than others), but specifically the investigation of their place in the wider ecological community as a group is essential in understanding terrestrial ecosystems and how they work, particularly in the tropics. As Lach and colleagues put it:

“… the study of ants has led to significant advances in our understanding of insect evolution, global biodiversity patterns, competitive interactions, mutualisms, ecosystem responses to change, and biological invasions. But ants are also important to study and understand because they are different…”

Indeed, the field of modern biogeography was furthered primarily by a mathematical ecologist (Robert H. MacArthur) and a myrmecologist (Edward O. Wilson) in their book The Theory of Island Biogeography (1967). Though much of the theory has evolved over the years, it has its roots in studying the ants of Melanesia (MacArthur and Wilson, 1963).

From their vast ecological importance and their interesting life-histories, to their use as model organisms to study sociality and species distributions, ants are certainly worth caring about just that little bit more.

References:

AntCat (2019): http://antcat.org/

Floren, A., Biun, A., & Linsenmair, E. K. (2002). Arboreal ants as key predators in tropical lowland rainforest trees. Oecologia, 131(1), 137-144.

Lach, L., Parr, C., & Abbott, K. (Eds.). (2010). Ant ecology. Oxford University Press.

Lucas, J. M., Clay, N. A., & Kaspari, M. (2018). Nutrient transfer supports a beneficial relationship between the canopy ant, Azteca trigona, and its host tree. Ecological Entomology.

Hölldobler, B., & Wilson, E. O. (1990). The ants. Harvard University Press. Cambridge, USA.

Hölldobler, B., & Wilson, E. O. (1994). Journey to the ants: a story of scientific exploration. Harvard University Press. Cambridge, USA.

Hölldobler, B., & Wilson, E. O. (2009). The Superorganism. W. W. Norton & Company, Inc., London, UK.

MacArthur, R. H., & Wilson, E. O. (1963). An equilibrium theory of insular zoogeography. Evolution, 17(4), 373-387.

MacArthur, R. H., & Wilson, E. O. (1967). The Theory of Island Biogeography. Princeton University Press. New Jersey, USA.

Wirth, R., Herz, H., Ryel, R. J., Beyschlag, W., & Hölldobler, B. (2013). Herbivory of leaf-cutting ants: a case study on Atta colombica in the tropical rainforest of Panama (Vol. 164). Springer Science & Business Media.