The Forest Unseen_A Year's Watch in Nature

by David George Haskell

  Organic chemists confirm the experience of our taste buds. The world is a bitter place, full of deterrents, digestive disrupters, and poisons. Hawks know this also, using fresh greenery to line their nests and drive out fleas and lice. Consider also the New York Times. Insects grown in containers lined with old copies of the paper fail to reach maturity. The quality of the insects’ reading material is not the culprit, although insects raised on the London Times mature into adults. The New York Times is printed on paper containing the pulped wood of balsam fir. The fir tree produces a chemical that mimics the hormones of its insect herbivores, thus protecting itself by stunting and neutering its enemies. The London Times is made from trees that lack hormonal defenses, making the pulped flattened remnants of their bodies safe to use as bedding for laboratory insects.

  We can now turn our question around and ask not how plants manage to survive assaults by herbivores, but how herbivores cope with noxious plants. The puzzle is no longer that the world is green but that the greenery has holes made by creatures who do not die after dining. Detoxifying countermeasures are the foundation of the herbivores’ ability to eat poisoned plants, but the insects also try to dodge around the defenses by feeding on those parts of plants they are most able to digest. It is no coincidence that the green caterpillar in the mandala is feeding on young maple leaves. Maples, like many tree species, defend their leaves with bitter tannins. Tannins are effective deterrents only in high concentrations, so young leaves have not yet accumulated enough of these chemicals to make them noxious. If the same caterpillar were to hatch in August it would face a forest steeped in tannin. The springtime emergence of many herbivores allows them to sidestep the plants’ defenses.

  The biochemical swordplay between plants and their herbivores has created a tense stalemate in the mandala. Neither side has yet routed the other. The holes and incisions of the mandala’s leaves are the marks of this year’s round of cut and parry, the venerable duel from which emerges the mandala’s fundamental character.

  May 25th—Ripples

  Hungry ladies dance in the air, swoop at my arms and face, then land and probe. They have flown upwind, excited by my smelly mammalian promise. No doubt the nakedness of my skin further stimulates them; no dense mat of hair to obscure their dinner table. What an easy meal!

  One of the mosquitoes lands on the back of my hand, and I let her probe my skin. She is mousy brown, just a little furry, with scallop patterns along her abdomen. Slender curved legs hold her body parallel to my skin. A needle juts from under her head. She slowly moves this lance across my skin, seeming to test for a suitable spot. She stops, holds steady, then I feel a burn as her head drops between her forelegs and the needle slides in. The sting continues as she penetrates deeper, sliding in several millimeters. The sheath that held the needle has bent back between her legs, leaving a tiny length of thin tube exposed between her head and my skin. The needle looks like a single shaft, but it is a bundle of several tools. Two sharp stylets help cut into the skin, making way for salivary tubes and a strawlike food canal. The salivary tubes ooze chemicals that prevent blood from clotting. These same chemicals cause the allergic reaction that we call a mosquito bite.

  The needle is flexible, so it bends after it enters the skin and, like a worm seeking a soft patch of soil, it probes around inside my skin, sniffing out a blood vessel. Capillaries are too small, so the mosquito searches for a larger vessel, a venule or arteriole, the state highways of our blood system. Veins and arteries, the interstates, are too tough-coated to be of interest. When the needle finds the object of its search, the sharp end punctures the vessel’s wall. The flow of blood over the needle stimulates nerve endings that signal pumps in the insect’s head to start pulling blood. If the mosquito fails to locate a suitable vessel she will either pull out and try again, or feed on the small pool of blood that the needle created as it tore through capillaries in the skin. This pool-feeding method is much slower, so most mosquitoes that fail to hit a good-sized vessel prefer to pull out and try again, seeking a rich seam elsewhere under the skin.

  The mosquito on my hand has evidently pierced a productive vessel. In just a few seconds her light brown underbelly balloons into a shining ruby. The brown scallops on her back that mark each segment of the abdomen move apart, seeming to dislocate her tidy body. She rotates as she feeds, perhaps pushing the needle around a curve in the blood vessel. When her belly is stretched into a half globe, she abruptly lifts her head and, in a blink, flies off. I am left with a slight burn on my hand and two milligrams less blood.

  These milligrams are a trifle for me, but they have doubled the mosquito’s body weight, making her flight ponderous. The first thing she’ll do after finishing her meal is rest on a tree trunk and offload by urination some of the water she has swallowed. Human blood is much saltier than a mosquito body, so she’ll also pump salts into the urine, preventing my blood from disrupting her physiological equilibrium. Within an hour she will have dumped about half the water and salt from her meal. What remains, the blood cells, will be digested, and my proteins will find themselves turned into yolk in a batch of mosquito eggs. The mosquito will also take some of my nutrients for herself, but the vast majority will be used for egg production. The millions of mosquito bites inflicted on us every year are therefore preliminaries to mosquito motherhood. Our blood is their ticket to fecundity. Male mosquitoes and females who are not breeding feed like bees or butterflies, sipping nectar from flowers or drinking sugars from rotting fruit. Blood is a proteinaceous boost for mothers only.

  The mosquito’s colors and fuzz mark her as a member of the Culex genus. This means that she will lay a small raft of eggs on the surface of a pond, ditch, or stagnant pool. Culex often breed in the fetid water that surrounds human habitations, giving them their common name, “house mosquito.” Females fly a mile or more from these breeding areas, searching for a suitable blood donor. My blood may end up in eggs laid in the pond half a mile behind me or in a blocked gutter or sewer in the town a mile away. Here the eggs will hatch into aquatic larvae that live suspended just under the water’s surface. Their rear end is an air tube that adheres to the water’s surface film, providing both an anchor and a breathing hole. Their heads hang down into the water and filter bacteria and dead plant matter from the cloudy water. During their life cycle, mosquitoes therefore exploit three of the richest food sources available to animals: the bounty of wetlands, the concentrated sugars in nectar, and the viscous feast of vertebrate blood. Each meal propels them into their next life stage, creating a nearly unstoppable momentum.

  If I had not visited the mandala the Culex would likely have found another blood donor for her meal. Despite their fondness for human habitations, Culex usually feed on bird blood. This is to the birds’ detriment because Culex transmits disease, most notably avian malaria and, of late, West Nile virus. Avian malaria lives in the blood of about one-third of the birds flying above the mandala. Most of the infected birds appear to be able to live out their lives without being substantially weakened by the parasite. Birds infected with the invading West Nile virus have higher mortality, perhaps because America’s birds have no natural resistance to this virus from Africa.

  When Culex mosquitoes cannot find a crow or chickadee, they feed on humans. This flexibility of dining arrangements brings bird parasites into contact with human blood. Some, like avian malaria, die in the alien surroundings. But others, including West Nile virus, will sometimes take hold and infect the human. This jump from bird to human blood requires that a mosquito first feed on an infected bird and pick up the virus, which then multiplies in the mosquito’s salivary glands. If the mosquito then feeds on a human, the drop of saliva may now carry an unwelcome guest, and the West Nile virus may jump from crow to human.

  Perhaps I should not have been so sanguine about my exsanguination; my curiosity may have allowed another life-form to take over my body, perhaps even kill me. I am, however, hardly dancing on the precipice. In the whole of North Ame
rica only four thousand people were infected with West Nile virus last year, fifty-six of them in Tennessee. About fifteen percent of these cases are fatal, making the virus fearsome if you get it but, compared to the other risks we face each day, a very minor threat. The newsworthiness of the virus comes not from the magnitude of the threat that it actually poses to us but from its novelty, its indiscriminate choice of targets, and our inability to predict whether it will bloom into a larger threat. The virus is also a gift to pesticide manufacturers, scientists feeding from the government’s largesse, and news editors desperate for sensational copy. Fear and profitability have launched the virus to stardom.

  Until recently a much more deadly threat to humans hung over the mandala. Another species of malaria lurked in mosquito salivary glands, waiting not for a bird but for a human. In the first years of the twentieth century the average rate of mortality from malaria for people living in the southern United States was about one percent per year. In the swamplands of Mississippi the rate was three percent; in these Tennessee hills it was lower but still significant. Malaria’s terrible weight used to oppress people all across the eastern United States, but eradication programs eliminated it from the Northeast in the nineteenth century, decades before clearing the South. Malaria’s end in the South came about in the early twentieth century after a campaign that targeted many stages of the malarial life cycle. Huge quantities of quinine were distributed to treat infected people and prevent reinfection of mosquitoes. Screens on windows and doors were encouraged or required, breaking the link between mosquito saliva and human blood. Wetlands and ponds were drained to remove breeding sites for mosquitoes, or oiled to smother their larvae, or poisoned with insecticides. Although malaria’s hosts, mosquitoes and humans, still lived across the South, the distance between them was stretched sufficiently that the parasite dropped into extinction.

  Malaria is seemingly irrelevant to my modern experience in the mandala, but this is an illusion. The mandala has been spared the chainsaw because it lies in an area set aside by the University of the South. This university brought me here also. What brought the university to this hillside? Malaria, among other things. Like many of the older universities in the East, the school is located on a plateau, away from the swamps that breed malaria and yellow fever. The cool temperatures and relative freedom of the Tennessee hills from mosquitoes made them an ideal place to send the offspring of the southern gentry. The school year ran through the summer, allowing students to escape the heat and disease of the cities. School was closed and abandoned in the winter, when the mosquitoes of Atlanta, New Orleans, and Birmingham were in abeyance. This prime location helped cement the university to the mountaintop, ensuring its viability long after one of its primary benefactors, the malarial parasite, had faded from the land.

  The atoms that make up my blood were propelled to the mandala by these biological forces of history, and it is appropriate that a mosquito should carry away some of them to rearrange them into a raft of eggs. This physical connection to the rest of nature is often unseen. The mosquito bite, the breath, the mouthful are acts that create a community, that keep us welded into existence, but that mostly pass unacknowledged. A few people say grace at a meal, but no one does so with every inhalation or insect bite. This unconsciousness is partly self-defense. The connections through the millions of molecules we eat or breathe or lose to mosquitoes are too many, too multifariously complex for us to attempt comprehension.

  · · ·

  The whining reminders of interconnectedness persecute me as I sit at the mandala, so I pull up the hood of my sweatshirt and tuck my hands into the sleeves, trying to slow the barrage. I peer over the lip of my cocoon and study the evidence of another kind of atomic flow. A snail has been smashed on the rock beside me. A few translucent crumbs of the honey-colored shell lie on the surface of the rock. These are the remains of a bird’s calcium-hungry feast.

  The crushed snail in the mandala is one stream among many in the great springtime flow of calcium from the soil to the air. Breeding female birds scour the forest for snails, greedy for the sheets of calcium carbonate on the snails’ backs. Such hunger is well founded. Without a boost of dietary calcium the birds cannot make their chalky eggshells.

  Once a snail has been swallowed by a bird, the shell is first ground up in the bird’s gizzard, crushed by a knot of muscle and pieces of hard sand. The calcium then gradually dissolves into the mushy gut and is pumped across the walls of the intestines into the bloodstream. If the bird is laying eggs that day, the calcium may go straight to the reproductive organs. If not, it will go to special calcium storage areas in the core of the long bones of the bird’s wings and legs. This “medullary bone” is produced only in sexually active females. Over the course of a few weeks the medullary bone is built up in preparation for egg laying, then completely disassembled as the eggs are laid. Female birds take to heart Thoreau’s wish to “suck out all the marrow of life”—they suck dry their own bones to make new life each spring.

  The sucked calcium travels through the blood to the shell gland. Here the calcium carbonate leaves the blood and is added in layers to the egg. The shell gland is the last stop through the tube that carries the egg from a bird’s ovaries to the outside world. The earlier stages of this journey wrapped the egg in albumen, then two layers of tough membrane. The outermost membrane is studded with tiny pimples that bristle with complex proteins and sugar molecules. These attract calcium carbonate crystals in the shell gland and act as centers from which the crystals can grow. Like sprawling cities, the crystals build on one another and eventually join, creating a mosaic across the surface of the egg. In a few places the crystals fail to meet, leaving an untiled hole in the mosaic that will become a breathing pore extending from this first layer of the eggshell all the way to the surface of the completed shell. The next layer of calcium carbonate grows on top of the first, creating a shell made from pillars of calcium crystals pressed closely together. Protein strands weave across these pillars, adding reinforcement to the shell. When the thickest layer of the shell is complete, the shell gland lays a pavement of flat crystals over the shell surface and then paints the pavement with a final protective layer of protein. Thus has the snail’s shell been uncoiled, rebuilt into an avian cocoon.

  As the young bird grows inside the egg it pulls calcium out of the shell, gradually etching away at the walls of its home, and turns the calcium into bone. These bones may fly to South America and be deposited in the soil of the rain forest, or the calcium may return to the sea in a migrant-killing autumn storm. Or, the bones may fly back to these forests next spring and, when the bird lays her eggs, the calcium may again be used in an eggshell whose remains may be grazed on by snails, returning the calcium to the mandala. These journeys will weave in and out of other lives, knitting together the multidimensional cloth of life. My blood may join the snail’s shell in a young bird that eats or is bitten by a passing mosquito, or we may meet later, in millennia, at the bottom of the ocean in a crab’s claw or the gut of a worm.

  Winds of human technology blow at this cloth, billowing it in unpredictable directions. Atoms of sulfur that were locked into fossil plants when they died in ancient swamps are now tossed into the atmosphere when we burn coal to fuel our culture. The sulfur turns to sulfuric acid, rains down on the mandala, and acidifies the soil. This acidic fossil rain tips the chemical balance against the snails, reducing their abundance. Mother birds have a harder time bingeing on calcium and so breed less successfully, or not at all. Perhaps fewer birds will mean less blood for mosquitoes, or fewer predatory beaks? Viruses like West Nile that thrive in wild birds may, in turn, be touched by the changed bird populations. This ripple in the cloth floats across the forest, perhaps finding a hem at which to end, perhaps floating on forever, drifting through the mosquitoes, viruses, humans, ever outward.

  June 2nd—Quest

  A tick perches at the tip of a viburnum branch, a few inches from my knee. I suppress the urge to flick
the pest away. Instead, I lean in to see the tick for its own sake, trying to look beyond my quick mental dismissal of it as a mere pest. The tick senses my approach and lifts the front four of its eight legs in a frenzied wave, grasping at the air. I wait, still, breath held, and the tick relaxes back into its original posture with just its front pair of legs raised in a prophetlike salute to the sky. My eye is so close that I see tiny scalloped ornamentations around the edge of the tick’s leathery oval body. The raised legs have translucent feet at their ends, each of which catches the sun and glows. In the center of the back is a white spot, identifying the animal as an adult female lone star tick. The chestnut color from the rest of the body seems to bleed into the star, giving it a golden sheen.

  Ugly, unadorned weaponry on the tick’s head counterbalances the strange beauty of the rest of her body. The head is tiny, unnaturally so, and through my hand lens I see two stubby pillars jutting forward, barely covering a Swiss Army knife of sharp, grotesque mouthparts. I want a closer look at this nastiness, so I reach up, hold the viburnum, and pull it toward my eye. The tick senses my hand and snaps toward it, semaphoring wildly with her forelegs. This sudden catapult startles me and I jerk my hand back, releasing the branch, sorely disappointing the tick.

  This foot-waving tick in the mandala is engaged in what zoologists call questing behavior. This gives the animals a measure of Arthurian nobility, tempering our disgust at their bloodsucking habits. The image of a quest is particularly apt because both the Knights of the Round Table and the Arachnids of the Leafy Forest seek the same end: a blood-filled Grail. In the case of the lone star tick, this Grail is a warm-blooded animal, either a bird or a mammal.