Adventures Among Ants - Mark W. Moffett [56]
Based on the number of driver ants pouring out of the nest, however, I was sure that she had been killed and her colony vanquished. Then I recalled my dad, who had worked on an ambulance in his youth, complaining that people made up to be dead in the movies were often shown with wounds that, though horrific, wouldn’t be fatal. The same was likely true of attacked ant colonies. In tropical American forests with high densities of army ants, every spot on the ground is raided on average at least once a day, so all the colonies have to be able to survive repeated attack.19 Despite the devastation, the fact is that army ants largely cull rather than eliminate their prey. This culling may make the army ant’s role in the ant community similar to that of a grazing mammal that crops grass just enough so that it grows back.
Do army ants practice sustainable harvesting, then? Not likely. I’ve never seen an army ant worker hold back on making a kill. After all, army ants don’t control territories, so it’s likely another army ant colony would reap the benefits of an earlier group’s restraint. I imagine their assaults simply cease when they reach a point of diminishing returns.
Whether the carpenter ant colony was defunct or wounded, the local population of ants would have plummeted, giving other ant species a chance to colonize the area during the following weeks. Over this same time, the driver ants’ other prey—the crickets, spiders, scorpions, nightcrawlers, snails, and varied kin whose numbers had been diminished through death or exodus—would crawl, hop, or slither back, repopulating the ransacked ground.
TRACKING PATCHES AND PREY
Before my departure to Nigeria, I had spent days rummaging through the Biosciences Library at the University of California, Berkeley, reading all I could about army ants, particularly African driver ants. I found that because of the time it takes a prey population to recover, it has been widely assumed that army ants avoid overhunting a site, perhaps warned off by the chemical trace of past raids. This idea was corroborated by the behavior of the most-studied army ant, the New World Eciton burchellii. This species has a highly mobile life organized around a huge, synchronized brood produced in a predictable pattern. Colonies migrate almost every day for a couple of weeks while feeding their hungry larvae, then settle in one spot for another two or three weeks until the clutch of tens of thousands of adult ants emerges. During this stationary period, the ants minimize the overlap between their forays to some extent by raiding in a starburst pattern around the nest, with each day’s raid separated from the tracks of previous days like the spokes of a Ferris wheel.20
Most army ants don’t lead such regimented lives. Their queens produce brood continuously, or at least in a less regular way, and their migrations and raids are less predictable and probably, in the case of the migrations, less frequent. Nevertheless, I expected that most army ants would avoid recently raided areas. I was therefore surprised, on my third night at the field station, to see a raid going on in the area near our dining room where I had documented my first raid by this colony only two nights before. As they had that last time, they dined on watermelon we had discarded, bulldozing their way into the fruit, an unusual treat for them. Eciton burchellii has been observed to raid night after night under electric lights that attract clouds of insects, demonstrating that this species can be motivated to deviate from its starburst raid pattern. Perhaps under certain circumstances it doesn’t pay to give up on a dependable food source.
Patch is the scientific term used to connote a local food supply that takes a long