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The tropical forest canopy, with its multiple levels of foliage and branches, can have ten times more habitable real estate than the ground, a much higher ratio than in the temperate zones. With all that elbow room, it may be no surprise that a study in the Amazon basin found eighty-two ant species in a single tree, almost twice the number of ants in the entire British Isles.6 Though that sounds like a lot, compared to other insects in tropical canopies, ants have an almost negligible diversity. As Terry Erwin points out, a single tree in Peru can contain thousands of species of beetles alone. Still, arboreal ants more than make up for their relatively few species with an astonishing bounty of individuals. Workers, in particular, often make up 20 to 40 percent of the organisms in trees, microbes aside. Measured by weight rather than numbers, all of the ant species in combination account for 10 to 50 percent of the mass of arthropods living in tropical trees. Ants also weigh more than all the vertebrates in the same area, from frogs and lizards to parrots, monkeys, and leopards. With so many ants and so little else, canopy ants sustain their populations through heavy reliance on plant matter, as we saw for weaver ants.7 The same is likely to be true for tropical ants living on and in the ground, where they also roam in overabundance.8

Pervading the tropics of three continents with just two species, weaver ants are a particularly good model of success without diversity. In this regard, weavers and humans have a similar history. Our ancestors adapted better than Neanderthals and earlier branches of our evolutionary tree, which stopped producing offshoots in the wake of Homo sapiens’ aggressive dominance of the Earth—with six billion members now and counting. Weaver ants seem to have followed a similar course, controlling the environment to such a degree that they are often able to push out or mow down the competition. Along with South America’s Camponotus femoratus, they are among the most militant ants on Earth, capable of eliminating all adversaries except the most fierce. This they accomplish by being numerically and behaviorally, and therefore ecologically, dominant, using their force of numbers and tactical skills to suppress or conquer territorial competitors and thereby control the environment.

Is numerousness essential to weaver ants’ success in fighting, or is it their belligerence that allows them to expand their population? The two conditions seem to go hand in hand, making it difficult to distinguish cause and effect. Although marauder and army ants at times use strength of numbers and battle skills to overpower the competition, the goal of most violence in these mass-foraging ants is the practical one of securing food supplies. In contrast, weaver ant societies, much like Peru’s ant-garden ants, fight other colonies to control the surfaces on which they live.

Weaver ants tearing apart a driver ant captured in Ghana.

This difference in goals has parallels in human groups. Most early hunter-gatherers moved often in pursuit of foods that offered immediate large payoffs, such as big game. After the Pleistocene, human population pressure caused these slowly replenishing foodstuffs to become depleted and eventually forced people to settle down in areas chosen for the availability of fast-breeding foods such as grain and small game, which required more time and labor but could be harvested sustainably. This shift in turn necessitated vigorous defense of these territories against would-be usurpers.9 In the insect version of this “broad-spectrum revolution” (as anthropologists refer to this shift in human diet), each densely packed ant garden or weaver settlement, with its foraging centered on a broad range of such quickly renewable resources as insect prey and nectaries, has come to approximate a warfare state. Among animals, all-out war against their fellows occurs only among the largest societies of humans and ants.10