Adventures Among Ants - Mark W. Moffett [89]
In 1993, I spent an afternoon at Guanacaste National Park, Costa Rica, watching the thin, wasp-like workers of Pseudomyrmex ants that resided in the hollow thorns of their ant plant, an acacia tree. The action was ongoing as ants killed or drove off caterpillars and other insects. But what especially irritated the workers was a vine touching one of the acacia twigs. They examined its looping tendril, then spent an hour pulling and shredding its tissues, at which point the vine fell away. Why so much attention to what for this predatory species must be inedible vegetable matter? The answer is simple: a vine can become an access point for invasion by neighboring colonies. This specialized form of clearing, which was so thorough that the ground around the base of their tree had been denuded of all other plants, served the acacia as well, if only coincidentally.34
A Pseudomyrmex worker in Guanacaste, Costa Rica, tearing the tendril of a vine that has touched her nesting acacia tree. If it were permitted to grow, the vine might overwhelm the tree.
Jorge, a Matsigenka Indian, standing in a spirit garden in Manu National Park, Peru. The undergrowth has been cleared away by Myrmelachista schumanni ants, whose aggressive attacks have warped the bases of the surviving trees.
It’s common for ants to clear the area around their ant plants, and those that do it the best are found in Peru’s forest glades, called spirit gardens by the local Matsigenka Indians, who believe that spirits clear the underbrush. I visited a half-acre spirit garden with an Indian named Jorge and Doug Yu, at the time a graduate student at Harvard studying the coevolution of ants and plants. Sweat bees, wasps, and killer bees, hungry for the salt in our perspiration, landed in such droves that we had to shout over the buzz and hold our arms out stiffly like Frankenstein’s monster to keep from being stung. The trunks of the few big trees were swollen like potbellies with malformed bark, which the Matsigenka ascribe to fires set by the spirits. Slicing a trunk with his knife, Jorge showed us the true cause of the deformity: tunnels eaten out by the minute Myrmelachista schumanni ant, which were killing the trees. Doug pointed out the ant plants nearby, small trees, easy to miss in the clearing, that shelter the brood and queens of this ant in their stems. The trees were doing well under the unobstructed, illuminated conditions provided by their ants. Sparing only these hosts, the workers will spray formic acid at any other plant, whether scrawny herb or mammoth tree; poisoned in this way, a sapling loses its leaves in five days, while the large trees barely get by or slowly expire. A spirit garden can contain three million ants and, judging from the slow rate of expansion of the garden space, last for eight centuries.35
CANOPY MOSAICS
The patchwork distribution of residents within the trees is called a mosaic. This distribution becomes most ecologically interesting when ants of different species live near—but not with—one another, separated typically by territorial friction, like that seen between weavers and the “exploding” ants.36 Ant mosaics are not universal. They are often indistinct in New World tropical forests, which can be more uniformly dominated by colonies of the same ant-garden ant species and some of the aggressive ants that control ant plants.37 And they are rare at extreme latitudes, where winters preclude occupation of the canopy year-round and most of the foraging in the treetops is done by ground-nesting ants.
Still, any environment where multiple ant species control exclusive territories can produce a mosaic that resembles