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Army ants are creators not destroyers of worlds
By Matt Walker
Editor, Earth News

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Army ants swarm in search of prey, while other species follow them for their own survival

Army ants have a reputation for annihilating everything in their path as they march through the jungle.

But the most complete study of its kind has found that army ants are creators of whole worlds, not destroyers.

More than 300 species, ranging from birds to tiny mites, depend in part on a single species of army ant for their survival, scientists have discovered.

That means army ants support a greater number of other life forms than any other known species.

It is a bit of a paradox that E. burchellii, bringing death to so many species, has this other role
Biologist Stefanie Berghoff

This revelation about army ants is the culmination of more than 50 years of scientific research into their behaviour conducted by naturalist Carl Rettenmeyer and his wife Marian, based at the University of Connecticut, US.

Together with fellow biologists, they generated a comprehensive list of animals known to be found in the company of a single army ant species Eciton burchellii.

This New World species is one of two army ant species that sends out large swarming raiding parties above ground, and it is the only one that creates temporary bivouacs above ground in piles of brush, or inside logs or tree trunks.

Antbird at the swarm front, waiting for the ants to flush out arthropods. Image: Stefanie Berghoff
Antbird at the swarm front, waiting for the ants to flush out arthropods

Around half a million worker ants belong to a colony.

Each colony sets up home for a period of 20 days, which gives time for ant pupae and newly laid eggs to develop.

During this time, the workers send out daily raids, each time proceeding in a new direction to avoid raiding the same area twice.

Then once the eggs have hatched and the larvae have become adults, the colony becomes nomadic.

During this wandering phase, the army ant colony again raids every day, setting up bivouacs at night.

But while it is well known that army ants kill and eat numerous insects and other arthropods on their raids, less well known is how many animals actually depend on the ants for their survival.

Emigration column, with workers holding a male E. burchellii. On the thorax of the male rides a Cephaloplectinae beetle. Image: Daniel Kronauer
Workers holding a male ant, which carries a beetle on its thorax

"Only a few studies have focused on collecting army ant 'guests'," says Dr Stefanie Berghoff, who worked with the Rettenmeyers to complete the study.

Some of these "guest" or associate species, as biologists call them, are already known.

So-called antbirds, for example, follow ant raiding parties, picking up arthropods flushed out by the marauding ants.

One species of African army ant even has a snake that follows it.

A bristletail running in an E. burchellii column. Image: Daniel Kronauer
A bristletail running in an E. burchellii column

But until now, no-one has recognised the huge diversity of other species that army ants support.

Carl Rettenmeyer, who died in April last year, and his wife Marian started collecting these associate species in 1952.

The research team, who continued the work after Carl's death in 2009, have now collated data from 1200 coloniesof E. burchellii ants, adding samples from 345 colonies studied by other scientists.

What they found is remarkable: 557 separate species have been recorded associating with this single type of army ant.

Of those, more than 300 are known to depend on the ants in some way for their survival.

"And I think this is only the tip of the iceberg," Dr Berghoff told the BBC.

Close-up of a E. burchellii bivouac
Close-up of a E. burchellii bivouac

At least 29 species of bird, including "typical" antbirds, ground antbirds and woodcreepers feed on arthropods flushed out by the ants, such as millipedes, cockroaches and stick insects, rather than the ants themselves.

At least 239 butterfly species have been seen or collected at ant swarms, feeding on bird droppings laid by birds that have been attracted by the ants, while springtails run with the army ant columns at night.

Tiny wasps buzz around the ant swarms, seeking out small spiders that flee the raiding parties. These wasps are parasitoids of the spiders, laying their eggs within them.

Thousands of blowflies, flesh flies, tachinid flies and scuttle flies accompany each swarm raid too, belonging to numerous different species.

Many appear to target dead insects or those newly killed by the ants, laying their eggs within the insect carcasses.

SMARTY ANTS
Ants

Such species flock to each army ant raiding party, but many others frequent refuse deposits left by the ants, or the bivouacs they live in.

Army ants create refuse deposits, or garbage dumps, which contain all the hard bits of prey items, such as insects' legs, that are inedible to them and their larvae.

However, these deposits are teeming with life.

Beetle, mite, wasp and springtail species all depend on them as a habitat and food.

Tens or hundreds of similar small creatures also survive with the bivouacs of E. burchellii.

Many of these associate species are difficult to capture and study.

Because of that, say the researchers, it is likely that many, many more species that depend on E. burchellii will remain to be identified.

"If really well studied, I would think the number of E. burchellii associates could double," says Dr Berghoff.

SOURCES

"It is a bit of a paradox that E. burchellii, bringing death to so many species, has this other role for such a high number of associates," she adds.



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