Ant Battles – Why do Invasive Ants Overrun Native Ants?

Researchers have investigated the strategies used by ants when they enter battle against each other.

Invasive ant species have a number of common attributes that help them succeed when invading new areas. Their multi-queen, multi-nest colonies with large reproductive rates mean they can quickly amass large numbers in their colonies. But is it simply a numbers game? In Australia many native species are a lot larger than invasive species and also come with significant armaments such as large jaws and/or stings. They also have ‘home soil’ advantage – so how do invasive ants seemingly take over areas so quickly?

Australian researchers have been investigating whether warfare theory applies to ant battles with particular reference to the success of invasive ants colonising new areas. Warfare theory predicts that the outcome of a battle depends on the strength of individuals, the number of individuals in opposing armies, and the fighting strategies employed. Broadly speaking, in one-on-one battles, the stronger individuals win. But when the number of weaker individuals outweighs the number of stronger individuals, the advantage starts to swing in favour of the weaker individuals.

It is also hypothesised that the complexity of the battlefield influences success. With increasing complexity – the introduction of tunnels, tight spaces and obstacles – it becomes increasingly difficult for larger armies to use their numerical advantage. This is somewhat proven in the development of different worker castes in social insects. Several species have major and minor workers that are recruited to different combat situations; smaller workers are utilised in more open battlefield situations, where numbers count, while larger workers are utilised nearer the nest where they can use their size in the tight tunnels and entrances close to nests. This may also explain why the more numerous minor workers of Schedorhinotermes are found away from the nest and the fewer major workers tend to be found close to the nest.

The researchers first tested the hypothesis that battlefield complexity impacts success, by using the video game Age of Empires II. Battles were simulated using the strong Elite Teutonic Knights and the weaker Two-Handed Swordsman. The Knights won all one-on-one battles, and battles against small groups of Swordsman, but lost to groups of five or more Swordsman. However, as the battlefield environment became increasingly complex, greater numbers of Swordsman were required to beat a single Knight.

The researchers then decided to see if the results held true for staged battles between the Australian native meat ant, Iridomyrmex purpureus and the invasive Argentine ant, Linepithema humile (pictured above). Both species have mono-morphic workers and no specialised soldiers, and rely on jaws for defence as they have no stings. However, meat ant workers are significantly larger (8 mm long) than Argentine ant workers (2 mm long).

 

Iridomyrmex purpureus
Meat ant worker (Iridomyrmex purpureus) (photo credit: Alex Wild)

 

In one-on-one battles (where combat was initiated), the meat ant won all fights. In group battles, 20 meat ants were pitted against groups of between 5 and 200 Argentine ant workers in simple (open) arenas or complex arenas with laneways. In all battles, all the Argentine ants were killed. The number of meat ant fatalities increased with increasing numbers of Argentine ants, but the level of meat ant fatalities was always lower in the complex areas.

Whilst surprising that even the 10:1 ratio of Argentine ants to meat ants was not sufficient to produce an Argentine ant victory, the general results of these battles support the predictions. Namely that larger, stronger individuals have a far higher chance of success against a superior numerical force in complex environments. So, in their natural habitats larger native ants should have a better chance of outcompeting invasive ants.

Unfortunately, that’s not the case in disturbed areas, which are extrinsically less complex at the ground level. The fact that the areas have been disturbed will in itself displace some native species, but being a far more simple environment, the numerical advantage of invasive ants allow them to quickly overrun and eliminate native ant competition. This highlights the additional factor in the success of invasive ants: that humans have created large areas of disturbed land, which creates an environment ideal for invasive ant colonisation.

 

Further reading: Lymbery, S.J et al. (2023). Complex battlefields favor strong soldiers over large armies in social animal warfare. Ecology: 120(37)

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