Newfound Gap Road twists for 26 breathtaking miles across the mountainous heart of Great Smoky Mountains National Park, climbing from the northern gateway at Gatlinburg, Tennessee, across a 5,ooo-foot divide to Newfound Gap, where it intersects the Appalachian Trail, and then on to join the Blue Ridge Parkway in North Carolina. This is the most visited national park in the United States, and most of the 10 million people who flock here each year will drive up at least as far as Newfound Gap to enjoy the ridgetop views and perhaps venture out a ways on the Appalachian Trail.
It is a sunny Saturday morning in early August when I arrive at Newfound Gap with park service biologist Keith Langdon. The parking lot is already beginning to fill, and people are piling out of cars with daypacks and cameras. This is not going to be an ideal day for scenic photographs, however. Despite several days of cleansing rains, bright haze shrouds the green slopes nearby and obscures the ancient mountains beyond.
The Cherokee Indians called these mountains the "place of blue smoke" because of the forest-generated haze that frequently clings to the hollows. Today's opaque white skies, however, are created not by trees but by power plant and automobile exhaust generated throughout the eastern half of the United States. The pall of airborne sulfates, nitrates, and ground-level ozone is at its worst on summer days like this.1 The National Parks Conservation Association has consistently included the Great Smoky Mountains on its list of America's 10 most endangered national parks, and in 2002 declared it the most polluted national park in the nation.2 Even more worrisome than the loss of mountain views are the unseen effects of the park's deteriorating air.
As we climb out of Langdon's car and begin collecting our hiking gear, two young men nearby are setting up a card table under an awning. Their sign reads "Western Carolina University Hiker Health Study." Langdon encourages me to go over and blow into a device called a spirometer that will test my lung capacity before and after our planned hike. Up here on the ridges of the park, I later learn, we are breathing ozone concentrations that average as much as twice what we would experience in urban Knoxville or Atlanta.3
Human visitors aren't the only potential victims of air pollution, and that is part of the reason for our hike today. Some 90 plant species in the park show leaf damage characteristic of ozone injury, and ozone is suspected of slowing the growth of sensitive plants such as tuliptree and black cherry. The park also suffers some of the highest acid precipitation levels in the nation, with sulfur and nitrogen pollutants falling in the form of dry particles as well as rain and shrouding mountaintop spruce-fir forests in fog as acidic as vinegar. Already, some high elevation streams suffer excessive nitrate levels and some forest soils are saturated with nitrogen, a condition that causes leaching of vital soil nutrients such as calcium and makes potentially toxic aluminum more available to trees.4
Unfortunately, air pollution is not the only threat to plant and animal life in the park. Invasive species such as feral pigs wallow and root, destroying plants and soil communities; imported aphid-like insects known as balsam woolly adelgids and hemlock woolly adelgids literally suck the life out of Fraser firs and hemlock trees. There are also pressures from growing numbers of visitors, millions drawn here not just by the rugged mountains but also by the maze of theme parks, bungee jumps, petting zoos, factory outlet malls, golf courses, wedding chapels, and resort home developments proliferating along the park boundaries and isolating it from nearby natural areas.
Langdon and other park personnel can spot woolly masses of adelgids attacking pollution-damaged trees, black bears hit by cars, and ferns and rare orchids plowed up by foraging pigs. But it's hard to monitor the fate of lesser known, highly diverse groups in the park that fall under their stewardship. Most of that life, particularly soil life such as mushrooms, snails and slugs, springtails, and slime molds, has only been sporadically collected or cataloged, and the diversity of most small creatures in the park remains completely unknown, much as it does in the rest of the world. Indeed, there is no patch of soil or sediment on earth where we know the identity of every creature. Yet we know that many of the mushrooms in the park spring from soil fungi that form nurturing partnerships with tree roots, and other creatures from springtails to slime molds are part of the great web of decomposers that maintain the fertility of these soils. How would Langdon and his colleagues know if these creatures were in trouble or understand how their loss might ripple through the community?
Conserving soil communities is surely a worthwhile goal in its own right, and part of the mission of the park. But knowing what lives in soils and sediments, where it lives, and what it does will also provide a stronger foundation for maintaining the health of the lands and waters we rely on for food production, timber, drinking water, and other goods and services. Soil life can also be viewed as a valuable and largely untapped resource to be explored for new antibiotics and pharmaceuticals, industrial enzymes, novel genes useful to genetic engineers, and versatile microbes that can be harnessed to counter pests and pathogens or devour and detoxify hazardous wastes. Even more vital is the role soil creatures play in sustaining the ecological processes that keep nutrients cycling, soil renewed, plants growing, water fit to drink, and soilborne pests and diseases in check.
With this in mind, the Great Smoky Mountains in 1998 became the first protected area in the world to launch an ambitious effort to inventory literally every species in the park. Interested scientists and educators formed a nonprofit organization called Discover Life in America, Inc. to direct a 10- to 15-year effort called an All Taxa Biodiversity Inventory (ATBI) under a cooperative agreement with the park.5 (Taxa are groups of related organisms.) The effort is funded by private donations and grants. Some 200 participating scientists and hundreds more lay volunteers join in organized sampling forays that involve literally beating the bushes for spiders; searching deep caves for millipedes, daddy longlegs, and rare amphipods; picking fleas, lice, and other parasites off netted birds; trapping bats, shrews, and voles; dissecting biting flies to search their innards for symbiotic viruses, bacteria, protozoa, and parasites; pawing through forest leaf litter for tiny fungus beetles; bagging up thousands of soil samples to be screened for unseen life forms such as nematodes and springtails; and searching under rotten logs for signs of leeches reputed to feed on earthworms. Taxonomists team up with volunteers several times a year for concentrated "bio-blitzes"—a Millipede March, Protozoan Pursuit, Snail Search, Bat Blitz, Lepidoptera Quest, Beetle Blitz, Fern Foray, Fungi Foray, or in today's case, a High Country Quest.
This morning I'm tagging along with Langdon, summer intern Donelle, and volunteer Dick—an engineer and amateur photographer from Knoxville—for a 5-mile round-trip on the Appalachian Trail to sift soil, shake trees, and search through leaf litter. We are one of a dozen teams collecting samples from high elevation habitats in the park this weekend. Another is an international team of taxonomists who are searching the high country for slime molds, odd creatures that inhabit tree bark, litter, and soil and make their living eating bacteria.
The students from the Hiker Health Study aren't finished setting up their equipment, so we gather up sample bags, vials, nets, and other gear and start for the trailhead without testing our lungs. Where the pavement ends, we pass a portable ozone monitoring station mounted on a metal railing, silently documenting what we and the creatures we seek will be breathing today.
Walking east, we quickly come to a "beech gap," one of the most rare and endangered plant communities in the region. Half of the beech trees in the park, Langdon tells us, are dead or dying thanks to an alien scale insect that carries a deadly fungus that infects the bark. Likewise, half of the flowering dogwood has succumbed to another alien fungus.
We walk on through narrow avenues of rhododendrons, the white-flowering ones in full bloom and the dark pink Catawbas just fading. Beneath them, amid the damp, mossy rocks, are ferns and numerous tiny mushrooms. Some of these mushrooms are the fruiting bodies of soil fungi that form mycorrhizae (pronounced micah-rizah)—literally "fungus roots." These fungi enter into symbiotic partnerships with the roots of most of the world's plants, drawing on a plant's sugar stocks for nourishment and in turn, sending out threadlike hyphae to serve as root extensions, absorbing water, phosphorus, and other nutrients the plant needs and offering some protection from pests and pathogens in the soil. The mycorrhizal fungi we are looking at here are called ectomycorrhizal fungi because they grow around plant roots like a sheath. They also produce mushrooms.
"I've been to Costa Rica a couple of times and I was shocked that I would go all day hiking in the tropical rain forest and I'd find maybe one mushroom," Langdon recounts as we move out onto a forested knife ridge, its sharp slopes obscured by lush greenery. After 19 years in the Smokies, he radiates an unabashed pride in all its wonders, from mushrooms to salamanders. "Here, you can stand in one place and look around and see 20 different kinds of mushrooms all growing out of the duff," the spongy layer of decaying leaf litter and organic debris on the forest floor. Most tropical trees partner with another type of fungi—arbuscular mycorrhizal fungi, whose hyphae actually penetrate and grow into the root cells—that do not form mushrooms.
Langdon's mention of Costa Rica reminds me that the concept for an ATBI started as the brainchild of University of Pennsylvania tropical ecologist Dan Janzen, who beginning in the late 1980s set about laying the groundwork for the world's first all-taxa inventory in the rain forests of Costa Rica's Guanacaste Conservation Area.6 Costa Rican officials, however, opted in 1996 to survey a limited number of key groups such as plants and fungi at five conservation areas instead of inventorying every species at one area. Not long afterward, Janzen and other ecologists began talking with the U.S. National Park Service about the possibility of launching an ATBI in the Great Smoky Mountains.7 Langdon and his colleagues arranged a meeting for December 1997 in Gatlinburg to try to gauge interest in the idea. More than 100 ecologists, taxonomists, educators, and park administrators showed up to begin planning the survey.8
Biologists expect that this half-million-acre park in the Smokies harbors only one-quarter the plant and animal diversity of a tropical rain forest. Yet that diversity represents the richest array of plant and animal life in temperate North America. For millions of years, the Smokies have flourished as an ice-free refuge for northern species driven south during repeated glaciations, and the mountains still offer a wide array of climate and life zones. Driving up to Newfound Gap this morning, we had traveled through five botanical life zones, from lowland cove hardwood to high-elevation spruce-fir forests. The park's status as a hotspot for temperate biodiversity has earned it United Nations recognition as a World Heritage Site and an International Biosphere Reserve.
What's more, current comparisons of the richness of tropical and temperate diversity may be misleading. As I heard repeatedly from soil ecologists, the notion that life reaches peak diversity in the tropics comes only from observations aboveground. There's little evidence that life underground, the most diverse life on the planet, follows the same rules. The diversity of termites does increase as you move from temperate regions toward the tropics, but the diversity of nematodes, earthworms, protozoa, and fungi apparently does not.9 Neither does the diversity of slime molds, according to Steve Stephenson of the University of Arkansas, who leads the slime mold "taxonomic working group" or TWIG now assembled in the park. "Some of us think that the total number of slime molds here in the Smokies will rival that of any place on Planet Earth," he had told me. Why they are distributed across the earth as they are, Stephenson can't answer. But he is quick to respond when I ask what their virtues are: "The slime molds are one component of the earth ecosystem—obviously, to most people, a rather insignificant component. But they feed upon bacteria and release nutrients back into the biosphere. If we didn't have them, bacteria would tie up those nutrients, and I suspect the system would be very different."
When the ATBI began, biologists came up with an ad hoc estimate that there could be 100,000 species in the park, not counting microbes, although only about 9,800 species had been reported—and only a tiny fraction of those known species were soil and sediment creatures.10 By the end of 2004, according to Discover Life in America's running scoreboard, participating scientists had identified more than 3,300 previously overlooked species in the park. Among these are more than 500 species new to science—species never before collected or named anywhere on the earth—and many of those live on or in the soil. They include 92 bacteria, 3 fungi, 67 algae, 9 lichens, 20 slime molds, 8 tardigrades, 36 springtails, 4 earthworms, and 3 snails.11 Thousands more specimens yet to be identified float in vials already stored on shelves and in freezers in the cluttered basement of a cabin at the park's natural resources office or in the labs of participating specialists around the world. Identifying species, while fundamental, is not the ultimate goal of the inventory. Park personnel want to know what community or habitat a species favors, the size of its populations, what season it appears or hibernates or blooms, what it does for a living, and how it interacts with things it eats, feeds, pollinates, partners with, or parasitizes.
"One of our goals, of course, is to really strengthen our monitoring program," Chuck Parker, a U.S. Geological Survey biologist stationed in the park, had told me. "We're convinced this requires having a much more thorough understanding of what's here before we make decisions about what it is that should be monitored." The idea is to find "canaries" or sentinel species that are highly sensitive and respond early to changes in the environment and that can also be censused regularly with minimal effort and cost. These sentinels could alert park staff to insidious changes they might not otherwise recognize until irreversible damage has been done.
Our trail drops below the ridge with the north-facing slope rising steeply on our right. Langdon stops and proclaims this a good place to begin sampling. Dick and Donelle set to work, one whapping the branches of a cranberry bush with a stick and the other holding up a "beat sheet" to catch whatever bugs and beetles rain down. Then Donelle uses a rubber tubing and pipe apparatus known as a "pooter" to suck the catch into a jar.
Langdon clambers upslope through the ferns and shrubs a short distance and begins pushing aside the moss and duff, filling zip-top bags with moist, humus-rich black soil. The gallon bags will go back to park headquarters near Gatlinburg where the soil will be placed in funnels with lights suspended over them to "motivate" tiny organisms inside to crawl downward and fall into waiting flasks. Smaller bags of soil will be delivered to the slime mold team to be cultured in the lab for unseen amoeba-like cellular slime molds and other tiny slime molds known as protostelids. Like nematodes, cellular slime molds have one celebrity among them: a creature known as Dic-tyostelium discoideum, which has long been a "white rat" in biology labs. Scientists know its genes and enzymes well, but its life in the wild remains unexplored.
Langdon says we're going to sift another bag of soil right now for visible beasts. He tosses the bags down to me and works his way back to the trail to show me how to label them.
He reads off two strings of numbers, coordinates taken from a global positioning device. He adds the date and "Appalachian Trail, east of Newfound Gap, approximately 1 kilometer." "That's so even if someone transposes a number and this grid coordinate turns up in Botswana, we'll know about where it came from and be able to correct it," he adds.
Langdon takes a small-mesh sieve, pours a measured amount of soil and litter on it, and shakes it vigorously while I hold a pan underneath, catching the soil crumbs.
"Now we're going to look for everything that moves in here," he tells me, instructing me to sit with the tray of crumbs in my lap. "A lot of things won't move for the first couple minutes, so just be ready." I put the beige rubber tubing of a pooter in my mouth and hold the metal pipe end poised over the tray, waiting.
"This is something over here, really tiny, a spider," Langdon points. I move the pipe end over the crawling speck and suck. Success— it's in the jar, but so is a fair amount of dirt. I bag a tiny beetle, then several springtails. Over the next 15 minutes, I capture a few dozen more moving specks. Finally, nothing else on the tray seems to be stirring and Langdon holds a specimen vial while I tap the contents of my jar into it, ending with one reluctant springtail. I add alcohol and slip in a pencil-written label with the collection date and place and my name. Now I have an official sample in the inventory.
At Langdon's direction, we all begin a search for snails. "Snails are going to be at the base of trees," he says. "They'll be under the moss, under rocks, and it helps sometimes if you take your fingers and just claw gently through the leaf litter and see if anything comes out."
I ask what my search image will be, what color.
"They'll be horn-colored. But the thing is, there's so much acid falling at the upper elevations in these mountains per year that the mollusk specialists are telling us that there's not the snails there used to be, either in number or type. And I don't think we're going to find much here, frankly."
We do find a few, then gather our gear and move on up the trail. Within an hour, rain is sheeting down on us. I've left my raingear behind and soon I'm drenched. The others are better prepared. The trail becomes an ankle-deep stream as we hurry on to the backcountry shelter at Icewater Spring. The rain is warm and my soggy condition not uncomfortable, but I wonder what pollutants are being washed from the sky with this downpour.
If snails and trees already suffer from the acid levels, they are unlikely to be suffering alone. Many studies show that acidification of forest soils causes shifts and losses in plant and soil communities.12 Across central Europe, for example, where acid rain began causing noticeable damage to spruce and fir forests at least three decades ago, researchers have documented declines in the abundance and diversity of mycorrhizal fungi. Mushroom fanciers know that boletes, chanterelles, truffles, and many other prized forest delicacies are the fruiting bodies of mycorrhizae, and these have been growing scarcer since the 1970s in the forests of Europe.13 I can't help wondering about the health of the rich assortment of boletes and other fungi here that Langdon points to with such pride.
Late in the day we return to park headquarters at Sugarlands Visitor Center. Other teams are returning from their outings, too, and all of us are unloading samples and assorted collecting equipment onto the tables. Jeanie Hilten, administrative officer of Discover Life in America, appears with pizzas for everyone. Hilten has been out leading another team collecting salamanders, and later she and some volunteers will put today's soil samples into funnels to begin extracting tiny soil animals. Entomologist Ernie Bernard from the University of Tennessee at Knoxville will pick up the material in a few days, sort it, and ship some of the creatures off to other specialists. He himself will identify the springtails and the protura—an "oddball group" of six-legged, wingless creatures that are similar to springtails but have lost their antennae and so walk about on four legs, waving the front two in place of antennae.
"Protura are amazingly original creatures," Bernard had told me earlier when he dropped off the funnels. "There's nothing like them at all." Springtails, too—formally, Collembola—display an originality captured in their common name. It refers to the spring-like appendage called a furca folded under their hind end that allows many to propel themselves forward like pole-vaulters. They are the most abundant six-legged creatures on earth, more abundant than ants and termites and extremely ancient. Springtails feed on decomposer fungi; apparently so do protura, although little is known about their occupation. Both springtails and protura share a recent distinction: molecular work has led many specialists to conclude that they are more closely related to crustaceans such as shrimp and copepods than to insects.14
Ernie Bernard seemed to relish recounting these idiosyncrasies, and I commented on it.
"Yes, one thing taxonomists want is they want their creatures to be weird," he confirmed enthusiastically. "You don't want to be working on just brown beetles!"
A taxonomist's affections can be quite focused, too. When Bernard comes across primitive arthropods known as pauropods— smaller and lesser known cousins of millipedes and centipedes—he sends them off to the world's only pauropod specialist, a retired high school teacher in Sweden named Ulf Scheller. Scheller, who has also traveled to the Smokies to sample, calls pauropods "delightful and charming creatures." He is talking about barely visible (up to 1/16-inch long) beasts that go about under rocks and rotting wood or in the rich surface layers of the soil, nibbling mold or sucking the juices from fungal hyphae or root hairs.15
Unfortunately, specialists with both the knowledge and the enthusiasm to identify living things, especially soil creatures, have been growing harder to find.16 Taxonomy and systematics have been un fashionable for decades and short of funds, computer database technology, and new recruits, despite the urgent need for information about the earth's biodiversity.17 Harvard biologist Edward O. Wilson, who has written eloquently about the growing extinction crisis and championed a revival of taxonomy, says, "To describe and classify all of the surviving species of the world deserves to be one of the great scientific goals of the new century."18
Interest in and funding for taxonomy are on the rise in a few places, however, including other U.S. national parks. Inspired by the project in the Smokies, Point Reyes National Seashore in California launched a 5-year ATBI in 2003 to survey the biodiversity of adjacent Tomales Bay. Since then, a half dozen other national parks as well as the state parks of Tennessee have started planning ATBIs. And there are numerous other types of surveys, inventories, and biodiversity assessments under way for key groups of organisms in protected areas around the world, although many such efforts ignore life in the soil.
Perhaps the most ambitious survey effort to date was launched in 2003 when the U.S. National Science Foundation teamed up with the ALL Species Foundation19 to launch a new global biodiversity tallying strategy called Planetary Biodiversity Inventories. Under this initiative, international teams will each focus on censusing a single group of organisms worldwide rather than all organisms in a single place. The first set of awards, totaling $14 million, is designed to assess the feasibility of completing such global surveys "within reasonable time frames."20 The four groups of organisms targeted in the first round include all the world's catfishes, plants in the genus Solanum (which includes potatoes and tomatoes), plant-feeding insects in the family Miridae, and the Eumycetozoa—slime molds.
"Most of the people in this room will be involved in some aspect of the world inventory," Steve Stephenson tells me as the slime mold TWIG members unpack their day's samples. He will direct the new global effort as well as continuing the ATBI in the Smokies and mapping the distribution of various slime mold species to the plant communities in which they occur throughout the park. The crowd in the training room today includes experts from England, Lithuania, India,
Costa Rica, and the Ukraine as well as the United States, and this is only a portion of the slime mold TWIG team. With the majority of the world's experts already working together here, the team was better prepared than most taxa groups to go after the global project, Stephenson says. In addition, the anticipated diversity of slime molds seems manageable—perhaps 1,200-1,300 species worldwide— although in individual numbers these creatures probably outnumber springtails. Slime molds fall into three groups: myxomycetes, cellular slime molds, and protostelids. "Myxos" are 10 times more diverse than either of the other two groups—875 species are known so far— and they have been found everywhere from hot deserts to the Arctic and the Antarctic Peninsula. Myxos live their individual lives as amoeba-like single cells known as plasmodia, foraging for bacteria to engulf and digest. Then at some yet-unknown signal, groups of myxos assemble and team up to form a slug, which sends up tiny mushroomlike fruiting bodies. That is truly enough weirdness to capture the allegiance of a global team of taxonomists.
Inventories can set the stage not only for conserving and harnessing the resources of soil life, but also for exploring questions about underground ecology. For instance, soil ecologists want to know: What makes some sites richer in soil species than others? Does a higher diversity of plants aboveground encourage greater diversity of soil creatures below? In turn, does the diversity of life underground affect the diversity of plants above? Would it matter to the firs and ferns and bears of the Smokies or to human societies if soil systems were reduced to skeleton crews? Who are the most important players underground, and what are we doing that threatens or compromises them?
Similar questions have been hot topics for more than a decade for ecologists working aboveground. For obvious reasons, however, experimenting with uncensused millions of largely invisible species in an opaque medium makes fieldwork with soil organisms quite daunting. Even sampling relatively well-known groups such as springtails or nem-atodes can be overwhelming, except, as we saw, in the Antarctic Dry Valleys, where diversity is reduced to single digits. Rather than deal ing with soil creatures as individual species, scientists usually lump them into broad functional groups according to their occupations —grazing on fungi, shredding leaves, munching on plants, or preying on other soil animals, for instance; or converting ammonia to nitrite, or nitrite to nitrate, both key steps in the recycling of nitrogen; or churning and aerating the soil, the work of "bioturbators" or "ecosystem engineers" such as earthworms on land and polychaetes in sediments. Assigning soil creatures to functional groups gives us only a rough cut, however, since we don't yet know how most of them go about making a living and to what larger functions their activities contribute.21 Nevertheless, ecologists are picking up the pace of research on life underground.
One of the questions that has puzzled soil ecologists is how such a rich array of species can coexist in the soil. One piece of the answer lies in the tremendous diversity of habitats available at various scales, from tiny soil pores to clumps or aggregates of soil particles to larger patches created by the engineering work of ants or earthworms or the roots of plants, on up to landscape-level variations created by different soil and vegetation types and even human activities such as farming. Likewise, food resources such as plant litter, root secretions, dung, carcasses, and prey items are patchily distributed throughout the soil, and the timing of their availability is highly variable. Competition, a major factor limiting how many species can pack into aboveground communities, is probably not as important belowground because potential competitors are often physically isolated by their limited mobility and the complexity of soil habitats. Nematodes and protozoa both feed on bacteria, yet they probably compete very little, for instance, because protozoa can pursue their prey into tiny soil pores where larger nematodes cannot go. Besides, as we've seen, only a fraction of the organisms in a community are active at any given time. Microbes and soil animals alike spend much of their lives in dormant states.22
I realized at some point while researching this book that describing the soil as "teeming" with life and repeating the superlatives— "one cup of soil may hold as many bacteria as there are people on Earth"23—while true enough, had given me a misleading vision of that unseen world. I pictured an urban scene of elbow-to-elbow crowds jostling for existence. One of the people who helped alter that image was microbiologist George Kowalchuk at the Netherlands Institute of Ecology, Center for Terrestrial Ecology in Heteren.
"If you just think of it as one big soil system teeming with life where everything's competing with each other, then obviously you wouldn't have all these species," he explained. "You'd have a couple that would out-compete the rest and that would be it. You think of the soil as really densely colonized, but it's probably just little islands of colonization with large gaps between. Things like competition might not actually be relevant because you have to interact with your neighbors to compete with them. And if the next bacterial cell is, from your own perspective, kilometers away, then it's not going to compete with you for food or space. The soil is a heterogeneous place where you have little patches of different activity. You have these little mi-crobial colonies, little towns, and each is probably rarely in contact with the next one because their inhabitants move slowly or not at all."
"The vast majority of microbial cells in the soil are dormant, waiting for their opportunity," Kowalchuk continued. "The typical soil microbe is playing a waiting game. If the wait is too long, they'll eventually die, but they can wait really a long time, perhaps years. Then when a root comes along or a drop of water or whatever, the cell blooms into a colony. And wherever you have a bacterial bloom, you'll certainly have a bloom of predators coming to eat those. Then once the plant or source of riches is gone, the colony probably dies out quite quickly and the few remaining microbes go back to waiting."
Breaking dormancy is often a gamble, as microbiologist David Hopkins of the University of Stirling in the United Kingdom explained to me. "Bacteria have a huge metabolic repertoire, but they don't keep all of the enzymes synthesized all the time," Hopkins said. "That's a waste of resources, particularly if they're in a dormant state. An organism may be sitting there, or a little colony, and they're all half-starved, they've all gone into physiological shutdown. And then immediately next to them an earthworm cast is deposited or a great big cowpat lands on top of them, and carbon starts diffusing into the soil. The organisms that respond and switch on at that first signal will be all tooled up and ready to go when the rest of the carbon arrives. But they are basically speculating, because they expend more carbon in preparing their metabolism than they actually get from that first little burst of carbon they're responding to."
Some of the same factors that allow soils in general to support high levels of biodiversity—that is, a wide diversity of habitats and food resources—can also create hotspots that are richer in species than others. Plants and soil animals such as earthworms both influence the physical character and the resources in the soil around them, and a fundamental link exists between plants, which produce carbon compounds through photosynthesis, and the soil community that decomposes those organic compounds. That in part is what has led soil ecologists to ask whether the number of plant species aboveground affects species numbers in the soil and vice versa.
The few studies conducted so far have shown no consistent link based on sheer numbers of species above or below the ground.24 The relationship between plant species richness and the soil community may vary from one group of soil organisms to another, and even within groups—say, between nematodes that feed on specific plants and nematodes that graze on microbes. A project led by Colorado State University researchers, for instance, found no consistent trend in nematode diversity or soil properties between field plots planted with one versus two different species of prairie grasses. The results suggested that the traits of individual plants are more important to denizens of the soil than the diversity of plant species.25 Another team from the Netherlands Institute of Ecology conducted a 3-year field experiment with mixtures of up to 16 plant species and found that both the number of plant species and the identity of the plants in the mix affected the diversity of nematodes in the soil food web. But the number of species apparently mattered only because it supplied an array of plant types that vary and complement one another in the quality of food resources they provide to the soil community. Nematode diversity varied more from one kind of plant to another than between different levels of plant species richness.26
Many other studies confirm that the identity of the plant species and the makeup of the plant community aboveground can influence both the diversity and abundance of certain soil organisms for better or worse.27 This should not be surprising: plants vary in traits that critically affect the soil community, such as the nutritional quality and amount of litter they produce, the timing and amount of their root ex-udates, the depth of their roots, and how much they shade and cool the soil.
Plants also may "choose" which components of the soil community to support. Kowalchuk and his colleagues found clear differences in the makeup of the microbial communities in the rooting zone of two plants, hound's tongue and spear thistle, growing in the same experimental field. Molecular techniques also revealed that the diversity of microbes around the root zone or rhizosphere of each plant species was actually lower than microbial diversity in the bulk soil nearby. That suggests that each type of plant encourages a different subset of the local microbes within its sphere of influence, yet exerts little or no effect on microbial communities beyond the root zone.28
Kowalchuk expects that molecular techniques will allow researchers to further fine-tune the broad questions they have been asking about the links between diversity above- and belowground. For instance, some microbes such as root symbionts and disease agents that interact closely with plants should be sensitive to changes in the plant community. Other microbes "couldn't care less what type of plants are there," he believes. "They're only interested in what the total nitrogen load or phosphorus load is, or the total pH, things like this." Already he has found that ammonia-oxidizing bacteria—key workers on the assembly line of the nitrogen cycle—are oblivious to the diversity or identity of plants above them.29 In contrast, the diversity of mycorrhizal fungi that live in intimate contact with roots sometimes declines along with the diversity of plants.
The flip side of this research is finding out whether changes in the soil community affect the plant community. Clearly, organisms such as earthworms and termites that alter the structure of the soil and organisms closely associated with plant roots such as disease agents, root-feeders, nitrogen-fixing bacteria, and mycorrhizal fungi can influence the makeup of the plant community.30 And another telling experiment at the Netherlands Institute of Ecology, which we'll return to in a later chapter, recently showed that the makeup of the soil animal community, including nematodes, mites, and beetle larvae, strongly affects the composition of natural plant communities.31 Recent studies by Heikki Setala of the University of Helsinki and his colleagues have shown that some species of soil animals in the decomposer food web— which affects plants indirectly by influencing nutrient availability— can strongly enhance nutrient uptake and plant growth.32
Finally, ecologists have been debating since the mid-1990s whether the sheer number of species in an ecosystem plays a fundamental role in how the ecosystem operates. The debate is far from academic. Just as in the Smokies, human pressures everywhere are eliminating distinct populations of plants and animals and threatening to drive vast numbers of species extinct. What else will we lose when they go? Will the ecosystems we rely on for basic life support services falter as species disappear?
The vast majority of experiments testing these questions have focused on a single taxa and a single critical process: that is, the impact of plant species richness on productivity. Productivity means the total mass of greenery, roots, and other plant tissue produced on a site; it's the process that fuels food webs above and below the ground, including those that supply us with food, timber, and other essential goods. The results so far demonstrate fairly clearly that more plant species are usually better for maintaining lush growth on a site through good times and bad, but the reasons are hotly contested. It is generally agreed that the identity and talents of the plant species in a community strongly affect productivity levels. Ecologists do not agree, however, on whether the actual number of plant species matters much—except that the more species there are, the more likely it is that the community will contain a few dominant and highly productive ones.33
In the soil itself, researchers are increasing their efforts to test the relationship between the diversity of soil creatures—or soil functional groups—and ecosystem functioning.34 Just as in the aboveground studies, results so far indicate that the talents of individual species and the array of different functional types in a soil community have more influence over ecological processes such as decomposition than does the number of species actually present.35 Vast numbers of species in the soil perform most jobs, a phenomenon known as "functional redundancy." To get an idea of how many different types of soil animals throughout the world contribute to the decomposition process, for instance, an international team of scientists led by Diana Wall filled i ,000 mesh bags with alfalfa grass and put them out on the ground in 18 countries, from the Namibian desert, Polish fields, and Tas-manian forests to the grasslands of the midwestern United States. After a few months, the participants in this Global Litter Invertebrate Decomposition Experiment collected all the creatures that had crawled up from the soil to shred, tear, grind, or otherwise begin breaking down the litter in the bags and sent them to Australia for identification. By late 2004, the GLIDE team had collected nearly 62,000 soil animals from 37 different taxonomic orders, including earthworms, ants, termites, millipedes, beetles, spiders, mites, snails, thrips, and woodlice.36
Different species may perform the same job in slightly different ways, of course, and some functional groups contain fewer players than others, which may make the work they perform more vulnerable to a loss of species.37 These relatively species-sparse groups include shredders of organic matter such as mites, millipedes, earthworms, and termites; soil-movers such as ants, termites, and earthworms; my-corrhizal fungi; specialized players in the nitrogen cycle such as nitrifying bacteria and denitrifying bacteria; and bacteria that specialize in processing methane, hydrogen, iron, and sulfur.38
Even in functional groups with large numbers of species, there is no reason to assume that each can serve as an exact substitute for another or that one can compensate fully for the loss of another. As one soil researcher noted, too often "redundancy may be more apparent than real."39 We see redundancy because we lump creatures into broadly defined functional groups of our own creation, unaware of the special conditions, hidden talents, strategies, or subtleties in the way each goes about a task. Species we lump into the same functional category, for example, often respond quite differently to disturbances, above- or belowground. Thus, creatures that seem redundant in today's context may prove to be vital backup players as conditions change. Think of it as insurance. This individuality in talents and tolerances also means that we need to go beyond functional groups and learn more about the species underground if we are to understand how soil systems will respond to the kinds of human pressures that threaten the Great Smoky Mountains National Park and too many other places we value: air pollution, acid rain, climate change, the introduction of exotic species, and changing human land use patterns.40 As both soil degradation and threats to biodiversity accelerate, learning "who's there and who matters most" will be vital to protecting not only the health of special places like the Smokies but also the integrity of our working lands and waters.
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