• Choose a category
• All
• Classic-Fiction

By Root 508 0

a geopolitical map of Europe—even on the ground, despite the high defense costs. In America’s Southwest, the harvester ants Pogonomyrmex barbatus and P. rugosus control seed-rich swaths of desert flatlands in separate territories with the tenacity of gold miners guarding claims. Yet they ignore nondominant harvester ant species, which survive on the inferior seeds the dominant ants overlook. In the tropics, though, mosaics on terra firma tend to be hard to make out: most colonies there are small and packed together, with on average five nests per square meter.38 One reason may be the army ants, which are the dominant tropical ground-dwellers. Because of their concentrated, shifting raids, army ants don’t partake in mosaics. Rather, they move across the litter and soil like a hurricane, plowing through other colonies, giving less dominant ants an opportunity to move into the land they have cleared.39

What makes mosaics obvious in the canopy are the trees, whose crowns form discrete units without parallel on the ground. While different dominant ants at times control different portions of one crown, commonly each tree is a nation-state unto itself, owned by a colony. The territorial checkerboard of arboreal ant colonies is ever shifting, established by the ants’ history in much the way international borders define territorial stakes for us. Ranges expand and contract depending on conflicts with other colonies and the growth and death of trees. Competition may be somewhat reduced by the fact that dominants are often partial to particular plants, such as the weaver ant to citrus and the Azteca ants to Cecropia trees, or by the tendency of some ants to restrict themselves to certain layers of the canopy. Otherwise, though, the whole canopy is open for confrontations. Remove a dominant colony, and an adjacent colony of the same species may take over, an annexation simplified by the fact that its workers can exploit the previous tenant’s trails.40 Or a different dominant species might seize the space, starting a cascade of community shifts.41

These shifts occur because each dominant ant species alters its territory as an ecosystem engineer. This means that, as an outcome of their social skill set and incessant activity, the workers modify or create the environments in which they live and maintain that milieu thereafter, much as humans do in their societies.42 But each ant species is different and fosters the survival of different treetop residents, among them the insects it may tend for honeydew, while driving off its competitors and culling its prey.43 Many ants are similar to people who intentionally encourage the reproduction of some living things, like those we use for constructing our homes (such as trees), while unintentionally encouraging others, like the molds and vermin that consume our refuse. In such ways, the patchwork of ant species must profoundly enhance the diversity of a rainforest.

Terry Erwin estimates that a hectare of rainforest canopy contains a thousand trillion individuals belonging to a hundred thousand species, most of them invertebrates.44 Ant mosaics could be a factor in the forces that cause some of these species to evolve in the first place. On islands such as the Galápagos, populations evolve independently from others of their kind, diverging into separate species. Similarly, both tree crowns and ant territories can function much like islands for small, fast-breeding insects.45 But rather than being isolated like the Galápagos in an ocean of water, crowns and territories exist within a quiltlike sea of other crowns and territories, each combination of which is acceptable to a different degree to each canopy-dwelling insect.46 A beetle species, for example, may thrive where its food tree is occupied by a carpenter ant, but be killed where weaver ants live on the same tree species. Populations of that beetle will therefore come and go across the landscape as their ideal island habitats, defined by both ants and trees, change over the centuries. And so it must be for countless other canopy residents.