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Getting bedeviled by ants

By S. Ananthanarayanan

A species of ant, which lives in a certain plant in the Amazon valley has been found to ensure the prosperity of its dwelling place by poisoning off all other plants in the vicinity of its host, says S.Ananthanarayanan.

The Amazonian rainforest contains some areas that are entirely of trees of the same species. This is unlike the general structure of natural vegetation, which consists of a variety of species, or ‘biodiversity', for mutual support. These Amazonian exceptions, known as ‘ Devil's Gardens' , are the result of a species of ant, which inhabits these patches, having poisoned off all other plant varieties. Research to show that it is indeed the ants that are responsible has been reported in this week's issue of the journal, Nature.

Biodiversity

The stability and permanence of natural forests is a result of the mutual support that all plant varieties derive from each other. If one variety were to increase in number and dominate the available nutrients, the same species would also become easy prey to parasites. Parasites would then control the growth of the runaway species and the balance would be restored. The parasites of varieties in small numbers would likewise be less active and allow these species to multiply. As different species use different components of nutrients in the earth, low numbers of a species would also find abundant food. Above a low threshold, the survival of any species is thus guaranteed.

The balance gets disturbed when particular species are bred through cultivation. A pestilence that strikes a strain of wheat, for instance, can rapidly wipe out hundreds of square kilometers of plantation. And agriculturalists then have to fall back on the ‘wild' strains to breed fresh cultivation stock

Jealous tenant

The behaviour of the Amazonian ant, myrmelachista schumanni, which nests in stems of the plant duroia hirsuta , has evolved to promote its host plant by eliminating other plant species. The ant achieves this by injecting all but its host plant with formic acid, in its sting. The acid rapidly causes the plant leaves to die and the plant itself then cannot survive. The result is a luxuriant copse, adevil's garden , of only one kind of plant, in the middle of the Amazonian forest.

Formic acid, which gets its name from formica , the Latin word for ‘ant', is the pain causing component of many insect stings, including the sting of wasps and bees. It is also the chief irritant in the leaves of thestinging nettle . Chemists have known for centuries that anthills gave of an acidic vapour and formic acid was first prepared by distilling ants' bodies! It is now a common organic reagent and is useful as an anti-bacterial, used to preserve livestock feed.

Is this symbiosis?

This Amazonian peculiarity is difficult to explain in the light of how plants normally evolve. The mainstay of plants' survival is the mutual support that plant species provide each other, particularly against parasites. The Amazonian ant and its host, it would seem, have ‘painted themselves into a corner', in evolution. Other plants would tend to evolve to defend themselves againstD hirsute (the Devil's garden plant) and would also not be there to defend it against its parasites, should they attack. But it would seem that devil's gardens have not been so widespread as to create plants that can withstand injections of formic acid! And it would appear that devil's gardens are not frequent, again, maybe because they get wiped out before they can go far. But the ant and plant teamwork has kept the plant thriving wherever it manages to grow, despite its being such a bad neighbour!

[The writer can be contacted at simplescience@gmail.com]


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