Unseen City

by Nathanael Johnson

  Josephine plucked a bud from the camellia bush and peeled back the petals. She’d been quietly absorbed for half an hour—longer than she usually sits still for just about anything, except cartoons. But eventually she got up and wandered off. My own mind meandered. Then I saw it: a dark cluster on a camellia bud. Aphids. And there, sure enough, were three ants standing guard, patrolling their little herd. Perhaps the ants were ignoring my cookies because they were intent upon the aphids. I leapt up and found Beth and Josephine in the living room. “I found what the ants are eating,” I announced. “Come see.”

  The aphid-covered bulb was at about Josephine’s eye level. “What’s that—all that black stuff?” She wrinkled her nose. The aphids were so tightly packed they looked like a solid patch of fungus.

  “Lots of little bugs,” I said. “Do you see the ants climbing on top of them? They herd the aphids like cows and milk them.”

  “Milk them?” Beth said.

  “Yeah, the aphids express a sugary liquid called honeydew from the anus.”

  “And the ants eat that when they could have pecan sandies?” Beth joked.

  I reached into the camellia bush and plucked the twig supporting the blossom. At the first tremor, the guardian ants began running frantically around the cluster of leaves, waving their antennae. They were looking, I suppose, for whatever might have landed to attack their herd. I carried the twig inside and dropped it in an empty strawberry jelly jar.

  Aphid Herding

  Insect livestock are the most reliable source of sugar for many ants. Ants most frequently herd insects from the order Hemiptera, of which aphids are one example. They are herbivores that suck the juices from plants and excrete the excess sugars in the honeydew. Some aphids produce their body weight in honeydew every hour, and if ants aren’t harvesting it, the sugar can pile up to such an extent that it makes it worthwhile for people to gather it. According to Wilson and Hölldobler, “The manna ‘given’ to the Israelites in the Old Testament account was almost certainly the excrement of the scale insect Trabutina mannipara, which feeds on tamarisk. The Arabs still gather the material, which they call man.” In Australia, honeydew is called sugar lerp, and one person can collect three pounds of the stuff in a day.

  If you turn up your nose at eating excrement, you should know that you may have already done it: A lot of honey comes not from the nectar of flowers, but from this honeydew, which bees collect from leaves. Essentially, this honey is insect poop that’s been processed inside another insect and then vomited up.

  WHAT THE HELL AM I LOOKING AT?

  There were two ants and several dozen aphids on the camellia bud I’d dropped into my jelly-jar terrarium. One ant ventured out to explore the jar while the other stayed behind. Then they switched roles. I imagined that they were coordinating their efforts so that their flock always had a guard.

  I was eager to identify the ants, and optimistic that I would be able to do so. There’s a trove of information online: AntWeb.org, AntWiki.org, and the Encyclopedia of Life (eol.org) all have detailed pictures of just about every known ant. After perusing the well-magnified and perfectly lit creatures pictured by my various guides, I would turn back to the tiny insects dashing around under my magnifying glass and rub my eyes. I had no clue.

  One problem was that there are thousands upon thousands of known ant species, and even more unknown species. Trying to find a match was like trying to find an identical grain of sand on a beach. I suspected I had something I hadn’t heard of before, something that might shun cookies.

  One of the best tools for identifying any species is a dichotomous key, which provides a series of either-or choices (for example, a dichotomous key for trees starts by asking, “Are you are looking at needles or leaves?”) to lead you down a narrowing path, like the game Twenty Questions, until you reach the correct identification. But there are more than 350 dichotomous keys listed on AntWiki alone. These have titles like: “Key to Myrmica inezae species group.” I needed a dichotomous key to the dichotomous keys!

  Another problem was that my ants were so small. From the human perspective, an ant is pretty much just an ant. As soon as you begin to look at ants through a magnifying glass, however, a world of wonderful diversity snaps into focus. With the benefit of magnification, Wilson and Hölldobler write, ants “differ among themselves as much as do elephants, tigers, and mice. In size alone the variation is spectacular. An entire colony of the smallest ants . . . could live comfortably inside the head capsule of a soldier of the largest species, the giant Bornean carpenter ant, Camponotus gigas.”

  I’d ordered a hand lens online, a high-quality jeweler’s loupe with tenfold magnification. As luck would have it, it arrived by mail later that afternoon. Using the lens took some getting used to, and I found it was impossible to catch the speedy ants in its narrow field of focus.

  Eventually I realized that I’d never be able to identify my ants on the hoof. I plucked one out of the jar—stricken by guilt for a moment for leaving the other ant alone—and dropped it into a pool of rubbing alcohol. It stilled immediately. I fished it out with a pair of tweezers, repositioned my light, and bent over it with my hand lens.

  There’s something miraculous about a good magnifying glass. I squinted into a blurry mishmash of color, then pinpointed the correct focal distance and I found myself in a different world. When I’d been watching the insects in the grass with Josephine, I’d fancied it was something like a real-life version of Tolkien’s Mirkwood—a forest filled with monsters. I’d loomed over the forest, separated by my bulk. But by bending light, the hand lens was able to transport me into that world: I was no longer peering down from above, but eyeball to antenna with one of the monsters. The ant was a great sodden beast splayed on an endless plain of woody cells—my bamboo desktop as I’d never seen it before.

  The first thing you are supposed to do when trying to identify an ant is to look to see if it has one petiole or two. If you draw an ant, you’ll likely start with three dots: the head, the thorax (the base for its six legs), and the abdomen (also known as the gaster). The petiole is a little lump (or a double lump) at the ant’s waist, between the thorax and abdomen, and it was quite obvious in the photos I’d found online. When I looked through my hand lens, however, the petiole was nowhere to be found. In fact, I wasn’t even sure my ant was right-side up. I took a breath, pulled the light down closer, and slowly began to find my bearings.

  It took a good half hour of frustrating concentration and ungainly prodding with the tweezers, which appeared, in this magnified world, like rough-hewn steel girders. There’s a peculiarity of the mind, or at least of mine, that presents tangible resistance to seeing something new. I had no system of categorization or naming to make sense of what I was seeing, and so for the first few minutes I saw nothing but a chaotic jumble of legs and body parts. I would stare at the images on my computer screen, then bend over and search for something similar on my ant. Eventually I found one landmark, then another.

  I had thought this ant was black, but actually it was a translucent brown, with a darker, striped abdomen. It was beautiful, actually. I followed the curve of the head, found the antennae mounted about where you’d expect a nose, and to each side a black, multifaceted eye that was perfectly symmetrical, as if shaped by a jeweler from jet. I felt a shiver of delight. The eye of an ant! I’d seen thousands of ants, but until that moment, I hadn’t realized I’d never seen one of their eyes. I felt richer for having seen it.

  By nudging the ant’s body to form a curve, I was able to expose the place where the petiole was supposed to be—and yes! There it was, nestled up against the abdomen, and there was very clearly just one. I’d found a simple dichotomous key for California house ants that walked me through a process of elimination starting with the question “One petiole or two?” A few steps later, the guide indicated that I was looking at Linepithema humile. I felt a chill of disappointment. This was not some interesting new species, it was the most common thing possible: an
Argentine ant.

  The Argentine Empire

  Though common, the Argentine ant is also strange. Actually, the thing that makes it common is what makes it so interesting. There is just one species of the Linepithema genus in North America, and for a long time, there was just one colony—one massive imperial family. In most ant species, each colony is independent. Each lives for as long as the queen lives—fifteen, twenty-five years sometimes—then dies. When the queen reaches the end of her life the workers continue collecting food for a while, but the nurseries empty, and eventually there are no workers to replace the fallen. A colony without a queen is like a headless chicken: still running, but going absolutely nowhere.

  Argentine ants, in contrast, produce multiple queens that generally stick around the nest. So instead of dying when the queen dies, an Argentine ant colony can grow in sprawling immortality. In a fit of creativity, scientists named the biggest one the Very Large Colony. In the summer of 1998, Neil Tsutsui and Andy Suarez, both grad students at UC San Diego, decided to see how big this Very Large Colony was. They collected some ants around San Diego, put them in jars, and drove north. They knew that ants from a different colony would fight with the ants they were carrying, but all the local ants they put in a jar with the ants they’d brought with them got along happily with the out-of-towners. Tsutsui and Suarez found that there was just one family of Argentine ants from beyond the Mexican border up through California to Ukiah, in Mendocino County. It was a single supercolony six hundred miles long.

  That turned out to be just the beginning. On the West Coast, the ants made their way up to Vancouver Island, and scientists began asking, Where else? When they plopped ants from California and Japan in the same jar, the ants again recognized each other as colony mates. Same with ants from the Pacific Islands, Australia, New Zealand, South Africa, and Mediterranean Europe. The sun never sets on the Very Large Colony; it exists anywhere with the Mediterranean climate that Argentine ants love.

  Most other species of ant are not as aggressively expansive as the Argentines. Colonies frequently squabble over borders, but manage to live side by side. Even ants that enslave other species don’t entirely wipe out their adversaries. Argentine ants, on the other hand, simply overrun other colonies. They aren’t built for war—they don’t have the fancy weaponry that some other species have—but they are able to overwhelm their competitors by virtue of their numbers and efficiency. They take up all the territory and gobble up all the food. Some ants eat mostly meat, some ants need mostly sugar, but Argentine ants eat everything. As a result, the native ants have been wiped out wherever the Very Large Colony dominates. It wasn’t just chance that my backyard ants were L. humile. In fact, it would have been surprising if I’d found anything else.

  Perhaps the greatest advantage held by Argentine ants is that they thrive in the habitats created by modern humans. L. humile comes from the banks of the Rio Paraná, which frequently floods, tossing up massive piles of driftwood and mud, then washing them away just as quickly. The Argentine ants work with this constant change. They might form a nest under a rotting log until that location gets too wet, and then move the larvae uphill, under some leaves. Then they’ll shift the nest yet again to be closer to a delicious rotting fish—all in one day. Urban and suburban lawns are the California versions of the Rio Paraná: landscapes that are regularly flooded by sprinklers, raked clean of leaves and sticks, and shorn of shaggy growth by mowers. Any species that wants to survive in this shifting environment has to be adaptable, and ready to abandon a home at a moment’s notice.

  Humans alter the natural landscape by building and gardening, and then Argentine ants move in and alter it still more: They raise herds of aphids and scale insects, which thrive on our rosebushes and fruit trees. They evict native ants, and by extension, native plants like the California bush poppy, which relies on harvester ants to distribute its seeds.

  Argentine ants seem inexorable, but they are not on their way to global domination. It’s more likely that we are now watching the decline and fall of their empire. The Very Large Colony is essentially one happy family, but other colonies are following it around the globe, and when these unfamiliar ants meet, they fight. Four colonies of Argentine ants are battling it out in Southern California now, keeping each other in check. And there are multiple colonies all along the Gulf Coast, showing up as far north as New Hampshire on the East Coast (which makes sense, because the bulk of the shipping traffic from Argentina goes to the southeastern United States). It’s only a matter of time until more families of Argentine ants begin competing with the Very Large Colony, or until the colony breaks into warring factions. And then there are challengers among other species of immigrating ants, such as the red imported fire ant, the Asian needle ant, and the crazy ant, each of which appears to be wiping out its predecessors.

  Perhaps the most significant threat to the Very Large Colony is its own success. It has created an international group of ants that recognize each other because they are so genetically similar. That lack of diversity is its Achilles’ heel; it makes the entire empire susceptible to the same pathogens. Argentine ants abruptly vanished from a few blocks in the California cities of Riverside, San Diego, and Davis. Perhaps they were wiped out by insecticide treatment, but Philip Ward, a UC Davis ant scientist, favors another hypothesis: “Some pathogen has spread locally, causing population declines, a sort of colony collapse disorder for Argentine ants,” he suggested. In New Zealand, Argentine ants were spreading like a tsunami throughout the 1990s, but by 2011 researchers found that the species had undergone a “catastrophic collapse” and native species were resurgent. Change is the only constant in nature.

  In Defense of Invasion

  The story we generally tell about invasive species is one of domination and extinction. A new species is always an invading army of ruthless totalitarians, replacing diversity and color with uniformity, leaving the world poorer. But this dire vision doesn’t hold up to scrutiny.

  Invaders get a lot of attention at first, when they look like they might be an existential threat. But journalists and scientists pay much less attention to what happens once they settle in to an environment. Environmental journalist Fred Pearce noticed that he would write stories about the terror of some new invasive, but then the furor would pass and no one would follow up to see what eventually happened. One of those stories was about a “killer algae,” Caulerpa taxifolia, that was smothering the shores of the French Riviera and spreading through the Mediterranean. But when Pearce went back to check, he learned that soon after his piece was published the algae began to disappear. “They are now virtually gone,” he wrote. “Nobody reported that. Not even scientists.”

  Another time, Pearce wrote about the zebra mussel, which overtook the Great Lakes, outcompeting native species for plankton and clogging pipes. People were fascinated by stories of the invasion. But the media wasn’t interested in reporting that zebra mussels were only thriving because the lakes were so polluted. Zebra mussels eat the polluting algae, and in the long run have probably done more good than harm. Pearce writes, “Light penetrating through the water has revived plants on the lake bed, and many fish—like the previously endangered lake sturgeon and smallmouth bass—have done very well by eating zebra mussels.”

  “A lot of studies happen when a new species arrives, but people don’t often go back to do follow-ups,” said Eleanor Spicer Rice, a scientist who documented the way Asian needle ants displaced their Argentine ant predecessors in Raleigh, North Carolina. One study provides a snapshot of the disruption, but we often simply don’t know what happens as the flux settles down.

  When scientists have gone back to follow up, they have never found that the new arrivals created a single-species wasteland. The native species might not be present in the same abundance, might not be in the same places, but they are there. For instance, a study in 2000 suggested that the number of Blainville’s horned lizards had declined by more than 50 percent in areas dominated by Argentin
e ants. These California lizards eat ants, but for some reason they don’t thrive on a diet of Argentine ants. But then another study in 2013 found that the lizards were doing all right, and still occupying 75 percent of their 1989 range. Basically, the researchers found them everywhere except the places where people had built houses and shopping centers.

  When I first heard about the Argentine ant invasion it sounded like a catastrophe, with them spreading implacably around the world. And they do spread, wiping out other ants and the lizards that eat them, but only within a narrowly defined habitat that allows them to thrive. That habitat just happens to be the one created by human development. Instead of saying that Argentine ants are spreading out of control, it would be more accurate to say they are simply a symptom of our presence.

  Immigrant species often do reduce native populations, sometimes significantly. But the ecologist Mark Davis has pointed out that they rarely cause extinctions, and when they do it’s of populations in isolated habitats like lakes or islands. All this mixing may yield more biodiversity by producing more combinations, hybridization, and new species. Centaurea plants introduced into California from Spain have evolved so much in their new habitat that they can no longer reproduce with the old-world species. Is this, asks conservation biologist Chris Thomas, now a California native plant?