Xenophobia justified in ant kingdom

By John Yeld Time of article published May 12, 2013

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Cape Town - They may be small, but in boxing parlance, they’re punching way above their weight – and there’s a real threat they may land a knockout blow to some fynbos plant species.

These aggressive little fighters are Argentine Ants, probably introduced from South America in 1901 through fodder imported for British army horses during the Second Anglo-Boer War.

They’ve made themselves right at home in many regions across the country, but particularly in the fynbos region of the Western Cape.

A Stellenbosch University PhD candidate has confirmed that this alien invasive ant is flourishing in its adopted surroundings, at the cost of some local ant species – including the Pugnacious Ant that is clearly not pugnacious enough – and, possibly, many fynbos species.

Natasha Palesa Mothapo, who is to submit her doctoral thesis, “Resource limitations to the invasion success of Argentine Ants within the fynbos”, next month, says the invading ants are affecting fynbos by displacing local ant species that promote germination by burying fynbos seeds, and by preventing these local ants from enjoying access to fynbos nectar as a food source.

The invader Argentine Ant is described as a “tramp” species that thrives in Mediterranean-type regions around the world, including the Western Cape and coastal California. Although found mostly in human-disturbed areas, it is steadily advancing into natural veld, and by 2007 there were records of it in Limpopo and Lesotho.

The dangers posed by the ant to fynbos are well known. In 1984, University of Cape Town botany Professor William Bond and his friend and fellow fynbos enthusiast, mapmaker Peter Slingsby, published a seminal scientific paper in which they spelled out the problem.

Fynbos is pollinated and sustained through seed dispersal by a range of indigenous creatures, including birds, butterflies, beetles, mice and ants.

Seed dispersal by ants is called myrmecochory, and while common in the plant kingdom, in Africa it is almost exclusively confined to fynbos.

Native ant species are believed to disperse the seeds of more than 20 percent of the 8 600-odd fynbos plants, including more than half of the huge protea family.

The seeds of these plants that attract ants contain small, white, oily appendages called elaiosomes, which are highly nutritious. In exchange for these “rewards”, ants carry the seeds into their underground nest middens where they consume the elaiosomes.

The bare seeds are discarded, and because they are now in locations that are safe from rodent seed predators and fire, are able to germinate and maintain the population of that plant.

Although some invasive species may replace the role of the species they eliminate, Argentine Ants do not disperse seeds.

There is serious concern their presence will ultimately result in the loss, or at least serious decline, of ant-dispersed fynbos plants, especially after fire.

 

Mothapo grew up in Alexandra in Joburg, where she attended primary school.

A recruiting road show by Stellenbosch University while she was at Northview High, also in Joburg, persuaded her to move to the Boland town for her university career.

Mothapo became interested in Argentine Ants while working on her Master’s degree under Professor Theresa Wossler, in the department of botany and zoology.

“(Professor Wossler) had a project on insect communication and invasive ants, and so I looked at the colony structure and behaviour of the Argentine Ant,” Mothapo explains.

Most of the previous research involving Argentine Ants had focused on their presence and abundance, and “nobody looked at the real behavioural interactions between the ant species – they were not actually testing out how the ants compete for food”.

Mothapo worked specifically in the Jonkershoek and Helderberg nature reserves, but said “my findings can be applied everywhere else”.

Why are the invaders so successful? Argentine Ants seem to have an inherent aggression, and they also have a numerical advantage when they defend a food source, Mothapo says.

 

“They have multiple queens – as many as 50 – that reproduce at the same time, so there can be millions of workers out-competing the much smaller colonies of indigenous ants.

“The Argentine Ant colonies will be growing, and in late autumn they start moving into a single, really huge, wintering nest – often at the base of the Protea nitida (blue sugarbush or waboom) trees that dominated my study site. Then this whole huge nest will start to spread in spring again, so there is this constant movement.”

The Argentine Ant doesn’t displace all local ant species – the Black Garden Ant, for example – but unfortunately, it does displace two key indigenous ant species that play an important role in seed dispersal: the Pugnacious Ant and the Cape Nutcracker Ant.

 

The problem is not just that Argentine Ants don’t practise myrmecochory; they are also very aggressive towards other pollinators like butterflies and beetles, especially during flowering time, and disrupt their activities, Mothapo adds.

This affects plants species like the common sugarbush protea (Protea repens) and the blue sugarbush or waboom, because the ants are particularly attracted to the rich nectar in their flowers.

Mothapo’s disturbing conclusion is that the invasion success of the Argentine Ant is likely to increase exponentially because of the high resource availability and the lack of competition from native ant species.

“We found that Argentine Ants consistently out-competed the native ant species when it came to finding food, and prevented the native species from having access to the food through high levels of aggression,” Mothapo says. - Cape Argus

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