Adventures Among Ants - Mark W. Moffett [129]
A belowground dump may get its start as an old garden chamber, perhaps after the resident fungus goes bad. But other species, such as Atta vollenweideri, position their dumps deeper in the earth, in chambers created for this function. These isolated caverns can be the size of a human coffin, and just as pleasant. How many cumulative years of labor must be required of the workers to empty such volumes of hard earth, which can be 6 meters—the equivalent in human terms of 3 kilometers—underground! In this, as the Bible advises, we should consider the ant’s ways and be wise: trained by the long march of evolutionary history, the leafcutters have come to invest far more in recycling, environmental safety, and public health than we do.
LEAFCUTTERS AND TREES: A LOVE-HATE RELATIONSHIP
I awoke to the gunshot sound of splitting timber, the creak of cable-thick vines stretched to the limit, the rumble and boom of a tree cleaving the canopy and striking earth, and the ensuing downpour of canopy plants, animals, and debris. I sprang out of my sleeping bag and unzipped my tent to the first glow of morning light in the forest, my heart beating hard as I tried to remember where I was. It came to me: Una Biological Reserve, in the Atlantic coastal forest of Brazil. I had heard such cataclysms many times before, but the sounds of a treefall always filled me with a sense of dread, even though I knew they carried for kilometers, and even though I knew such an event brought forth new life. A new space had opened in the rainforest for young pioneer tree species and for the leafcutter ant colonies that prefer their especially succulent foliage.
The evolution of leafcutters and their fungus is a story of an alliance between an animal and its food. While most research on leafcutters has focused on the damage they inflict on plants—and certainly, the ants cause harm to most of their victims—Atta develop a positive alliance with some of the trees they plunder, by providing, as they do for their fungus, conditions suited for the plants’ reproduction, though in a form of species interdependence that is far harder to spot.
Treefall gaps are ordinarily filled in by vegetative regrowth, especially from pioneer trees. When a leafcutter colony matures in the gap, workers maintain the clearing by dismembering the foliage growing back at the site and hauling away the smallest bits of debris on the ground.24 While the original treefall tears the canopy fabric from above, leafcutters clear from below, opening a space to human height above the nest. In addition to creating an exposed and well-illuminated environment, leafcutters—like other ants at their nests, but at a grander scale—loosen and aerate the soil, cycle earth from underground, and assist in production of humus through decomposition of trash, thereby adding to the soil’s nitrogen and potassium content.25 The mature trees around the perimeter of the nest extend their roots into the trash middens on the surface of the ground, where nutrient levels can be enhanced a hundredfold.26 When they’re around Atta species that bury their refuse, the trees tap into whatever chambers their roots can reach.27
Good light, rich soil, and a clean shot at the canopy—conditions at a leafcutter nest would be perfect for plant seedlings, too, except for the obvious fact that the ants strip away any vegetation growing on their mounds, killing smaller plants.28 But just as the demise of a tree makes room for pioneer plants, when an Atta colony dies, it offers opportunities for new plant life. Over its lifetime, the tree Miconia argentea, for example, loses massive quantities of leaves to the leafcutters, but the ants retrieve its seeds to the nest, where they remove any clinging fruit for garden fodder.29 Then they throw the seeds into