Monday, June 21, 2010

Army Ants

Years ago, I joined Lee Harper for a World Wildlife Fund trip into the Brazilian Amazon. Lee had studied army ants in rainforest near Manaus, setting out to learn how big a forest patch needed to be in order to retain its army ants and the obligate antbirds, which need to follow army ants in order to survive. Lee developed a novel method of transporting ants into variously-sized forest fragments.

Army ants don't build permanent nests. Instead their colonies roam unceasingly through the forest, preying upon insects and other small animals in their path. But they do go through two phases, a nomadic phase lasting about 2 weeks, when their food needs are greater, and a stationary phase lasting about three weeks, when the colony protects the queen and her eggs in a bivouac formed of living ants. During the nomadic phase, the bivouac's location changes nightly. During the stationary phase, they still do some raiding, but return afterwards to the same bivouac.

Army ant column in Guyana (Photo by Narca)

When the ants bivouac, they form a living nest of the interlocking legs and mandibles of as many as a million ants, protecting the queen and her new brood at the center. This living nest is complete with chambers, bridges and corridors.

Lee would capture the queen and place her on crossed sticks inside a very large metal garbage can. Then he'd wait for the colony to bivouac around her. Once the ants had formed their living nest, he would place the lid on the garbage can and duct-tape it shut. Then he could place the entire colony on a backpack frame and carry it into a forest fragment. If the forest fragment was large enough (say, 100 hectares), the ants would stay. Otherwise they marched in a straight line out of the forest patch, setting out in a random direction across the denuded land in search of a forest big enough to support them.

Lee quickly learned to be sure that the lid of the garbage can was well-secured!

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