Terrible Tree-Killer Ants
Ants are hard to like. They are leggy, nippy, creepy-crawlies who get in where they don't belong and can act like characters in our worst nightmares---think of columns of army ants stripping the tropical countryside! Yet, they are also capable of the most fascinating behavior, complex activities that seem little short of incredible for tiny animals that are hardly more than armor-plated reflexes.
One of the most mind-boggling things I've ever learned was that some ants keep aphids the way dairy farmers keep cows. The mind was very young at the time, and easily boggled, but it was an astounding fact.
If ants were thoughtful creatures, aphid ranching would be logical behavior. But ants don't reason. They haven't the brain power. For both protected aphid and well-fed ant, the cooperative arrangement is the result of trial and error, gene and enzyme, luck and evolution, all operating over millennia piled atop one another. Somehow, ants' simple programming tidily matched that of aphids, and a long-lasting partnership was born.
Recently, Austrian researchers have discovered ants engaged in another astounding partnership. The Austrian team was working with Peruvian colleagues in the Andean foothills at the headwaters of the Amazon River system, territory little studied by scientists. Among the plants they observed was a quick-growing flowering shrub, Tococa occidenralis. This plant sprouted up quickly where light permitted, and was one of the first to grow on the raw scars of landslides.
Generally the tococa plants were quickly overshadowed by stronger plants, so they were found only here and there in the vast study area. In a few places, however, the team found roughly circular patches up to 30 meters on a side (about the size of a baseball diamond) containing pure stands of tococa. Usually these patches were surrounded by strips of bare earth where no plants of any kind grew.
When the scientists examined plants from these patches under a microscope, they found colonies of tiny ants of a previously unknown species. The plants' hollow stems harbored full colonies; individual ants patrolled the leaves. The ants apparently fed only from specially developed glands on the tococa leaves.
It's long been known that some ants protect plants from which they can gain food. Gardeners who grow peonies can sometimes observe ants gathering nectar from the flower buds; the ants will attack gnawing caterpillars they find on their peony picnic grounds. And ants have been known to prune other plants that encroach on their preferred gardens. But, after a few simple experiments, the scientists realized they'd found something quite different. These little ants practiced mass vegetational murder.
The researchers moved plants of other local species into the bare track surrounding the tococa stand. Within an hour, every new plant was under assault by the resident ants. They bit the most sensitive tissues, such as growing tips, then sprayed the wounds with poison from glands on their abdomen. The toughest plant lasted five days before dying; the weakest, a papaya, was dead in an hour "and was left after 3 h as a soft clump of necrotic plant material," to quote from the Austrian scientists' report in the journal Biodiversity and Conservation.
The ants killed everything from tiny sprouts to thirty-foot trees. They allow only shoots from their chosen plants to grow, and soon tococa plants are all that's growing. Fortunately for the rest of the Peruvian jungle, the ants are lazy. They don't wander farther than about four meters (a dozen feet or so) from the edge of their plantation.
Even more fortunately, they can't think things through. They don't realize that young trees growing just beyond their range will eventually grow up and close the canopy overhead. Shaded out, the ant-tended plantations die. The ants move on, looking for other tococa plants, but the poisoned soil remains barren for years.
