In tropical forests, certain types of trees serve as homes for ants, providing hollow stems or leaf pouches where the insects can live and raise young. In return, the ants keep hungry herbivores at bay and occasionally kill surrounding vegetation, creating a clearing around the trees. Tropical biologists have now discovered that sometimes these ants branch out farther, invading other types of trees beyond the clearing. But are they out to destroy--or are they just trying to make new friends?
Nineteenth century naturalists fiercely debated the relationship between ants and their trees. In 1873, botanist Richard Spruce likened the ants to fleas on a dog--a nuisance. But others contended that some plants, acacia in particular, provide hollow thorns and food rewards to keep ants that could attack and drive off herbivores. A few studies over the past 4 decades have supported this hypothesis. It seems the ants also bite and poison vegetation surrounding their "antplants," reducing the competition for space, water, and sunlight.
In 1995, scientists observed a new, peculiar ant behavior. While exploring the jungles of southeastern Peru, two ethnobotanists came across a number of ant-made clearings. The researchers were surprised by what the natives showed them next: Trees on the outside of the clearing were scarred and swollen with networks of cavities filled with worker ants, queens, brood, and mealy bugs. "These galls made up a large percentage of the swollen trunk volume," says team member David Edwards of the University of Leeds in the U.K. Sometimes the internal excavations were so extensive that the tree had collapsed, the team reports in the November issue of The American Naturalist. Many locals blame the scars on forest spirits, as the clearings are considered by some to be "devil's gardens."
The researchers think the ants are attacking these other trees because there aren't enough antplants to house ever-expanding colonies. At this point, it seems clear that, although the ants benefit from these auxiliary nest sites, the tree gets nothing in return. The researchers didn't observe the ants warding off herbivores, for example, and the trees may even succumb as a result of the ants' invasion. But "the jury is still out" as to whether the relationship might become more harmonious over time "until we see the net effect of the galling," says Mike Kaspari, an ecologist at the University of Oklahoma in Norman.
Whatever the outcome, the discovery "gives a new level of insight into how these [relationships] evolve," says John Longino, an entomologist at Evergreen State College in Olympia, Washington. "It's amazing what those tiny little ants can do to those big trees."