The Forest Unseen: A Year's Watch in Nature (13 page)

Not only do ants sow seeds in convenient places, they also help move them away from their parent plants and into potentially unoccupied spaces. Most ants move the seeds of spring ephemerals a few feet, rarely more than a stone’s throw from the parent. This is enough to avoid competition with mom, but such short dispersal distances are hard to reconcile with what we know about the history of the ephemerals. Many ephemerals have populations that cover the entire extent of the temperate forest in eastern North America, starting in Alabama and stretching all the way into Canada. Yet sixteen thousand years ago this temperate forest was squeezed into a few pockets on the Gulf of Mexico. The last ice age covered the rest of the east with ice or, in the more southern areas, with the kind of boreal forest now found only far north in Canada. Spring ephemerals have therefore moved from Florida to Canada in sixteen thousand years. But if post-ice-age ants behaved in the same way as modern ants, the ephemerals would have moved just ten or twenty kilometers since the retreat of the ice, not the two thousand kilometers they evidently have managed. Either today’s ants are shadows of the great sprinting ants of yesteryear, which is unlikely; or the fossil and geological evidence for the ice ages is a mirage, which is even less likely; or our understanding of seed dispersal is incomplete, and spring ephemerals have some unknown method of long-distance transport.

Until recently candidates for this “mystery disperser” all seemed rather weak. Freak windstorms carrying
Hepatica
seeds to Canada? Unlikely. Mud under the toenails of migrant birds, or seeds carried in bird bellies? Possible, but most migrants have passed through southern forests before the ephemerals have set seed. Toadshade trillium plants set seed so late that migrant birds have started their return journey and would carry seeds the wrong way. Rodents or other herbivores carrying seeds in their guts? Dismissed out of hand: they grind up seeds in their mouths, then destroy them during digestion.

Ecologists have dubbed the mismatch between the ephemerals’ rapid advance and their seemingly poor dispersal abilities “Reid’s paradox,”
after a nineteenth-century botanist who encountered a similar problem with the spread of oaks across postglacial Britain. Philosophers and theologians love paradoxes, regarding them as honorable signposts to important truths. Scientists take a dimmer view, having learned from experience that “paradox” is a polite way of saying that we are missing something obvious. The resolution of the paradox will likely show one of our “self-evident” assumptions to be embarrassingly false. Perhaps this is not so far removed from a philosophical paradox. The difference lies in the depth of the false assumption: relatively shallow and easily uprooted in science, deep and hard to dislodge in philosophy.

The false assumption underlying Reid’s paradox may not be buried at all but perhaps lies on the leaf litter in mandalas all across the continent. Deer droppings, which, like the feces of rodents, we had assumed contained no viable spring ephemeral seeds may turn out to be the paradox’s solution. This resolution meets the criterion for classic scientific paradox-solving: a simple experiment with a “why didn’t anyone think of that before?” answer. Step one, collect deer droppings from the woods; step two, search the droppings for seeds; step three, plant the seeds, watch them grow, and conclude that “ant-dispersed” seeds are misnamed. Perhaps “ant-jiggled” and “deer-lobbed” would be a better description, because deer, it turns out, can transport seed many kilometers. Ants manage mere centimeters. And what of the other herbivorous mammals that we also dismissed as potential carriers of seed? No one has stooped to pick up after them to find the answer. We have a lot of dung sifting ahead of us.

Whatever the sifters find, we can already conclude that we were premature to categorize many spring ephemerals as “ant-dispersed,” even giving their relationship a heavyweight label, myrmecochory. The reality of seed dispersal is more complex and seems to depend on scale. At a small scale ants are indeed the main dispersers. They excel at collecting the seeds and planting them in prime locations. Deer are much less careful gardeners. Therefore, from the perspective of an individual
seed, there is no better fate than to be discovered by an ant. However, at a larger scale mammals are vastly more important than ants. The occasional successful long-distance transport of a seed by deer can found a new population and vault the species into a previously unoccupied forest. From the perspective of the whole species, footloose deer are more important than fastidious, shuffling ants. Without deer, the ephemerals would be confined to a small strip of forest on the Gulf Coast. Instead, they have hitchhiked across the continent.

The newfound importance of deer calls into question the function of the elaiosome. We have assumed that the oily appendage was designed by natural selection to attract ants, thereby helping to place the seed in convenient potting soil. This explanation is still likely true, in part. Ants are, after all, the best sowers of seed, and natural selection will exalt any characteristic that helps pass on genes to the next generation. But selection also favors those characteristics that will carry genes to the four winds. Evolution commands not just “multiply” but “go forth and multiply.” Any mother that does not launch some of her children on long voyages will lose out in the long run. This is particularly true in species whose history has been marked by the recolonization of vast areas of habitat. Nearly every
Hepatica
flower in North America is the descendant of a successful long-distance disperser. We should expect to find itchy-feet genes, characteristics that make it more likely that a seed will be deposited far from its parents. The elaiosome, therefore, may be partly designed for this purpose, titillating supple deer lips, encouraging them to pluck the hopeful fruit.

The paradoxical lives of the ephemerals have had new layers of complexity folded into them by the arrival of Europeans. We have chopped the forest into pieces, making it harder for ants to move seeds around. At the same time, deer populations have crashed, then boomed. The ant-deer balance had tipped. How will the ephemerals respond? Or, can they respond? Large numbers of deer can turn the blessing of dispersal into the curse of overbrowsing. Sustained heavy
munching will wipe out ephemerals, making moot our speculations about their response to changing natural selection.

Now, a third weight is added to the balance. Imported fire ants have invaded southern woodlands and are moving north. They thrive in disturbed areas, making them particularly common in forests that are already reeling from the effects of fragmentation. Fire ants gather elaiosome-bearing fruits but are poor dispersers, depositing seeds next to the mother plant, thus dooming seedlings to a childhood spent competing with a larger relative. These competitions usually end with the seedling’s death. Fire ants may also be predators, eating the whole fruit instead of just the elaiosome. This invasion of foreign ants has the potential to undermine the relationship between the elaiosome and all its native dispersers, making the oily gift a liability instead of the asset it has been for millennia. Ephemerals may be caught in a race between natural selection and extinction. Either they will adapt to new conditions, or their numbers will dwindle in the face of a new reality for which they were unprepared.

The spring ephemerals’ passage through the tumult of the ice age shows that they can readily adapt to changing ecological winds. But the ice age was a storm that came and went over thousands of years. Now the plants are faced with unpredictable changes that squall over them in just decades. The ecologist’s paradox has become the conservationist’s prayer. This mandala may be part of the answer to that prayer, a relatively unfragmented, uninvaded piece of forest where the old ecological rulebook has yet to be entirely torn up and blown away. These ants, these flowers, these trees contain the genetic history and diversity from which the future will be written. The more wind-tattered pages we can hold on to, the more materials evolution’s scribe will have to draw upon as it reworks the saga.

T
he earth’s belly rumbles mightily. Intestines of stone shudder past one another, untwisting their tension, grinding into relaxation. The distress is centered sixty miles away, twelve miles below the skin. As the pent-up energy of stressed rock is released, some of the fury spreads outward in waves of shifting earth.

The compression waves arrive first, roaring. Like a herd of diesel trains, the sound rips over the land, jarring us into confusion from predawn sleep. The sound spills out of the earth, washes over us for a few seconds, then flies on. These compression waves rip through the earth at more than a kilometer every second. A pause follows, then surface waves hit, bucking and shaking the house. The waves combine horizontal and vertical motion, squeezing and shearing simultaneously. Like small boats tacking against ocean swell, houses are twisted and rolled in the geological storm. In a big swell, houses break apart, unable to withstand the wrenching stresses.

We are fortunate. The swell is modest and our house stays upright. The roar outside the house is replaced by chiming and clanging from within. Framed pictures hanging from the wall act as pendulums; when the earth lurches the house to one side, the heavy frames remain stationary, stilled by inertia. Then the wall returns, bang!, jumps away, snaps back, bang! Keys jingle, glasses knock together in a toast to the quake, plates slide and clang. Everything tied to the earth is moving, all else is still or slowed, but our eyes deceive us and tell us that the
contents of the house are dancing within stationary walls. The shakes continue for about fifteen seconds, then die out in trembling fits.

The inertia of suspended objects is put to use in measuring the strength of earthquakes. A pen hung from the weighted bob of a pendulum will scribe the earth’s movements on graph paper held below its point. When the quake strikes, the pen remains still, but the paper and the pendulum’s frame both move, causing the pen to record the extent of the movement. Some seismographs have pendulums three stories tall, recording every little quiver in the ground below.

The calibrated scratchings of dangling pens give us the Richter scale. This morning’s earthquake scored 4.9 on the scale, about the same magnitude as a small nuclear weapon or one thousand times the strength of a powerful quarry blast. Because the Richter scale is logarithmic, the actual amount of energy contained in an earthquake increases exponentially with the numbers on the scale. An earthquake scoring 3 on the scale is minor, a 6 will do some damage, 9 is devastating, and a Richter 12 quake is so powerful that it would crack the earth in two, or so we are told.

I hurry to the mandala at first light, anxious to witness the geological consequences of the earthquake. Mountains are dynamic beings, so I hope to see some rolled rocks or cracked cliffs. But things are just as I left them. The mandala appears entirely unmoved. If change has occurred, it is beyond my perception. The sandstone boulders sit like old monks, beyond stillness in the depth of their contemplation.

I have run up against a break, a discontinuity in the nature of reality. The biological drama that plays around and over the mandala’s stones keeps time in seconds, months, or centuries and measures physical scale in grams or tons. Geological reality ticks in millions of years and weighs in billions of tons. It seems that I am extraordinarily unlikely to see geology in action at the mandala, even after an earthquake. The tempo and scale of geology are incommensurable with biological experience.

We bury our incomprehension in the usual way, with words. The
mandala’s rocks are about three hundred million years old, built from the huge river of sand that flowed from a still older mountain chain to the east. The earth’s crust has, grain by grain, disassembled and rebuilt itself over and over to the rhythm of millions of millennia. These are preternatural ideas, beyond the nature of our experience or imagination.

The earth’s slow movements seem to exist in another realm, separated from life by a wide chasm of time and physical scale. This is challenge enough for our minds. But the most unfathomable truth about the chasm is that there is a thread across, a thin connection from life’s moment-by-moment to the impossible longevity of stone. The thread is woven by life’s persistent fecundity. Tiny strands of heredity join mother to child and combine to stretch back billions of years. The strands spool year by year, sometimes branching into new lines, sometimes ending forever. So far, diversification within the thread has kept pace with extinction, and the mortal biological fleas on the immortal stony gods have bought a contingent immortality of their own. But every strand in the rope is a race between procreation and death. Life’s generative force has been strong enough to win this race year by year for millennia, but final victory is never guaranteed.

The mandala sits at just one point along this thread. The rest of the chasm is bridged by the ancestors and descendants of the species here. None of these living creatures will truly experience the vastness of geological time. It is therefore easy to forget or ignore this vastness, assuming that our physical setting is fixed, “set in stone.” I sit at the mandala under a bluff that is now the western edge of the Cumberland Plateau. The land here is made from sandstone, with limestone farther down the slope. Water from this mountainside runs into the Elk River, then on to the Gulf of Mexico. These realities form the seemingly firm walls of the mandala’s world. But walls turn out to be veils. Behind the veils, across the chasm, the world is in motion. The mandala sits on an old river delta that, in turn, sits on an ancient
sea floor. All this was uplifted and eroded. Oceans, rivers, and mountains changed places in a dance of terrifying magnitude. The mandala was shaken by an infinitesimally small finger twitch of the dance last night, a reminder of the overwhelming otherness of the physical earth.

A
gumball-sized
Mesodon
snail slides its gray body across the leaf litter, then climbs a twig. Halfway up the twig it lurches sideways and falls to the ground, felled by the wetness that slicks every surface in the mandala. Two days of storms have pushed water into every crack and pore. Saplings are weighed down by the burden of droplets, and the remaining ephemeral wildflowers are squashed down, defeated by the incessant pounding of rain. A patch of mayapple just to the west of the mandala has been leveled, as if crushed by a huge roller. Although it is well past dawn, the sky is dark and casts a dim light that deepens the wetness. The damp air oozes around the mandala, merging sky and forest. The leaf litter seems to have no upper surface; the rotting leaves simply bleed upward and turn into dark wet air.