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The robotic rovers Spirit and ...
The robotic rovers Spirit and Opportunity made headlines as they scraped their way across the Martian landscape, searching for signs of life. But while our eyes have been turned toward the skies, teeming beneath us and largely unexplored lies what Science magazine recently called the true "final frontier." A growing array of scientists is exploring life in soils and sediments, uncovering a living world literally alien to our own senses--and yet one whose integrity turns out to be crucial to life above ground.
Yvonne Baskin takes the reader from the polar desert of Antarctica to the coastal rain forests of Canada, from the rangelands of Yellowstone National Park to the vanishing wetlands of the Mississippi River basin, from Dutch pastures to English sounds, and beyond. She introduces exotic creatures--from bacteria and fungi to microscopic nematode worms, springtails, and mud shrimp--and shows us what scientists are learning about their contribution to sustaining a green and healthy world above ground. She also explores the alarming ways in which air pollution, trawl fishing, timber cutting, introductions of invasive species, wetland destruction, and the like threaten this underground diversity and how their loss, in turn, affects our ownwell being.
"Baskin takes us on a tour around the globe and beneath the soil surface to describe the unique and fascinating creatures that make their living underground in some of the earth's most interesting habitats...Every one of the eight intriguing chapters will be appreciated by anyone with an interest in the natural world, especially the unseen world right under our feet."
Opening the Black Box
Two golf cart–sized rovers named Opportunity and Spirit bounced to a landing on opposite sides of Mars in early 2004. From 200 million miles away, NASA scientists sent these robotic vehicles rolling about the rubble-strewn surface, poking their sophisticated instrument-tipped arms at rock outcrops, dunes, and dusty plains. Their mission: to search for geologic evidence that Mars was once a warmer, wetter, and perhaps even habitable planet.
The prospect of life on Mars has captivated dreamers and visionaries for ages. Barely a century ago, astronomers and fantasy writers could peer into the night sky and imagine the red planet's mottled surface laced with canals or seething with warlike aliens set to invade Earth. In the 1960s, the first images beamed back to us by Mariner spacecraft quashed any lingering visions of canals or ruined cities. If we were ever to find signs of Martian life, it was clear we would have to search beneath the surface of an arid, bitterly cold planet with air too thin to breathe. A Viking lander did just that in 1976: it scooped up material from the planet's surface, analyzed it chemically, and found no clear evidence of life. That disappointment, however, did not quench our curiosity. Perhaps there was once a golden age on Mars, a warmer time when the planet nodded toward the Sun, polar ice melted, rivers flowed, seas surged, and life took hold.
It was almost three decades later when ecstatic space agency scientists announced that Opportunity had found evidence that Mars once hosted water—not just soggy plains but a shallow equatorial sea or swamp of briny, acidic water that had left ripple patterns and salt deposits in surface rocks. For months I followed the news and watched online as the rovers beamed back startlingly clear images of the Martian surface.
Captivated as I was, one seemingly trivial point kept jarring me. News reports and often the scientists themselves persisted in calling the loose stuff that the rovers were probing "soil."
Soil? Among space buffs, that use of the word had become common enough that Merriam-Webster's Collegiate Dictionary defines soil in one sense as "the superficial unconsolidated and usually weathered part of the mantle of a planet." But by the time Spirit and Opportunity were sent roving across Mars, I had spent more than a year learning about the mysteries of the earthly stuff we call soil, and using that word to describe the Martian surface sounded, well, oddly alien. Consider this description from Daniel Richter of Duke University who, like many ecologists, considers soil to be not simply the loose surface material of a planet but "the central processing unit of the earth's environment":
Soil is the biologically excited layer of the earth's crust. It is an organized mixture of organic and mineral matter. Soil is created by and responsive to organisms, climate, geologic processes, and the chemistry of the aboveground atmosphere. Soil is the rooting zone for terrestrial plants and the filtration medium that influences the quality and quantity of Earth's waters. Soil supports the nearly unexplored communities of microorganisms that decompose organic matter and recirculate many of the biosphere's chemical elements.
In this light, Mars enthusiasts are jumping the gun when they call the dust of that planet "soil." Theirs is an understandably hopeful act—a hope that the Martian surface contains, if not current life, at least a legacy of life. So far, however, such hopes linger unfulfilled. True soil, as ecologists see it, remains at least as rare in the universe as life itself. Indeed, life—abundant and long-flourishing life—must precede soil. It is life that substantially organizes and transforms the weathered parent material of the planet into soil. The only soil discovered so far is often called "earth" after the only planet on which it's found.
Ironically, the money and vision expended on probing the secrets of Mars—$820 million for the latest two rovers alone—vastly exceed what has been spent exploring the earth beneath our feet. Yet it is the soils of our gardens, fields, pastures, and forests, as well as the sediments beneath streams, lakes, marshes, and seas, that harbor the most diverse and abundant web of life known in the universe. What's more, it is life underground that makes possible the green and fruitful surface world that allows us to create flourishing civilizations with the means and the curiosity to probe the universe.
Although money for exploring soil life remains relatively sparse, the pace of exploration and sense of excitement are growing among scientists who look down instead of up. Like space scientists, soil ecologists, too, are harnessing new technologies to reveal cryptic realms as little understood as the rusty skin of Mars—and far more vital to our existence. Unlike space exploration, however, the drive to understand life underground is fueled by a sense of urgency. Human activities are increasingly degrading and impoverishing soils and soil life, and this loss, in turn, threatens to diminish the earth's capacity to sustain us.
Soils have been called "the poor man's rainforest" because a spade of rich garden soil may harbor more species than the entire Amazon nurtures aboveground. Two-thirds of the earth's biological diversity—biodiversity for short—lives in its terrestrial soils and underwater sediments, a micromenagerie that includes uncataloged millions of microbes, mainly bacteria and fungi; single-celled protozoa; and tiny animals such as nematodes, copepods, springtails, mites, beetles, snails, shrimp, termites, pillbugs, and earthworms. Some are accessible to anyone curious enough to poke through rotting leaves, backyard dirt, or the muddy bottom of a tidal marsh, but most are too small to see without a microscope or magnifying glass. So little effort has been devoted to life underground—and so few scientists specialize in identifying these organisms—that at best only 5 percent of the species in most key groups of soil animals have so far been identified, and in marine sediments, less than 0.1 percent of species may be known.
Taken together, however, these inconspicuous creatures dominate life on earth, not just in diversity but also in sheer numbers and even body mass. Harvard University ecologist Edward O. Wilson points out that 93 percent of the "dry weight of animal tissue" in a patch of Amazonian rain forest in Brazil belongs to invertebrates living everywhere from soil to treetops, from mites and springtails to ants and termites. And that doesn't count the microbes. Despite their submicron stature, the bacteria in an acre of soil can outweigh a cow or two grazing above them. Indeed, bacteria may contain more than half of the "living protoplasm" on earth, most of it to be found either in terrestrial soils or in the mud of the oceans that cover three-fourths of the planet.
Underworld creatures are not only numerous and weighty in aggregate, but ancient and exceedingly durable. Toughest among them are the "extremophiles," bacteria and ancient microbes known as archaea that can live a mile or more deep in the earth, or in boiling hot springs or polar ice, enduring extremes of heat, cold, pressure, and pH that were considered unfailingly lethal to any form of life only a few decades ago. Some tiny soil animals can time-travel for decades or more in dormant states, impervious to extreme heat, cold, desiccation, and otherwise lethal radiation. Although most soil organisms are small and short-lived, some of the oldest and largest creatures ever identified are sprawling underground masses of the root-rot fungus Armillaria that far outclass blue whales in size. A 220,000-pound specimen that stretches across 37 acres of Michigan woodland was reported in 1992, setting off a race of sorts to find the biggest "humongous fungus." By 2003, a 2,200-acre Armillaria in Oregon had captured the record. Finally, although we think of plants as denizens of our aboveground world, many plants spend more than half the energy they capture from the sun to grow roots that nurture and interact with life underground. A prairie, for example, grows more grass biomass below the surface than above.
If scientists still know very little about who lives underground, they know even less about what each species in particular does for a living. Yet the creatures of mud and dirt are so important to our life that Wilson calls them "the little things that run the world." Together they form the foundation for the earth's food webs, break down organic matter, store and recycle nutrients vital to plant growth, generate soil, renew soil fertility, filter and purify water, degrade and detoxify pollutants, control plant pests and pathogens, yield up our most important antibiotics, and help determine the fate of carbon and greenhouse gases and thus, the state of the earth's atmosphere and climate. All of these ecological services arise from the spontaneous activities of billions of creatures going about the business of nourishing and reproducing themselves in a series of elaborate food webs below the surface.
Since the dawn of agriculture, humans have recognized the value of the soil itself, often invoking its fertility in ritual and sacrifice. Yet most societies have given little thought to, or have been simply unaware of, the multitude of creatures that live and work in the soil. The scientific study of soil developed in the 19th century, driven largely by the desire for greater crop production. Even soil scientists, however, have traditionally treated the soil as a "black box"—a system whose internal workings remain hidden or mysterious—measuring physical and chemical attributes such as pH and organic matter content, monitoring inputs of nitrogen and outputs of carbon dioxide, but making little effort to identify the dynamic workforce within. Yet we now know that these soil attributes and outputs reflect the legacy of billions of organisms eating, breathing, growing, interacting with one another, and, in the process, altering their environment—and ours.
Today, a growing cadre of scientists drawn from numerous disciplines and armed with new techniques is working to crack open the black box of soil life and soil processes and fill in that sketchy outline with deeper understanding. Soil ecologists in the 1950s pioneered research on soil biodiversity, food webs, and soil-plant interactions, but since the 1980s that effort has burgeoned dramatically in parallel with the development of ecosystem science. Researchers today view soils and sediments as complex ecosystems, and they recognize that the processes that take place underground vitally affect not only our food and timber supplies but also the quality and sustainability of our environment. Soils and aquatic sediments now draw the attention of multidisciplinary teams of, for example, ecologists, biogeochemists, microbiologists, zoologists, entomologists, agronomists, foresters, marine and freshwater biologists, geologists, and atmospheric scientists. These researchers want to know who is down there, what each contributes to the functioning of the soil, how they are organized into communities and food webs, why some communities are richer in species than others, and how our activities threaten soil life and processes.
Unlike Mars exploration, the increasing effort to understand life underground is not driven by curiosity or futuristic speculation alone. The diversity of life in soils and sediments is under increasing threat, just like plant and animal life aboveground, and as a result so is the integrity of the ecological processes that are influenced by underground life.
By some estimates, more than 40 percent of the earth's plant-covered lands, from dry rangelands to tropical rain forests, have been degraded over the past half-century by direct human uses such as grazing, timber cutting, and farming. Degraded land, by definition, has a diminished capacity to grow crops and forests and supply other goods and life support services to humanity. In that same half-century, erosion has lowered potential harvests on as much as 30 percent of the world's farmlands. Erosion not only sweeps away mineral soil but also reduces the abundance and diversity of soil creatures, which are concentrated in the top few inches of the soil. "A hectare [2.5 acres] of good quality soil contains an average of 1,000 kg [kilograms—2,200 pounds] of earthworms, 1,000 kg of arthropods, 150 kg [330 pounds] of protozoa, 150 kg of algae, 1,700 kg [3,740 pounds] of bacteria, and 2,700 kg [5,940 pounds] of fungi," according to Cornell University ecologist David Pimentel. As this life is lost, the soil's ability to hold water and nurture crops declines. Further, as soil and nutrients wash off the land and into rivers, lakes, and coastal waters, they damage water quality and smother and degrade sediment communities often already disrupted by pollution, dredging, and trawl fishing. Human-driven changes in climate, acid rain, excessive nitrogen deposition, the spread of nonnative species, and the continuing conversion of land to crops, cities, and other human uses all contribute to the loss of soil biodiversity and functioning.
Accelerating degradation of the earth's soils and sediments has not gone unnoticed by national and international organizations concerned with agricultural productivity, fisheries, food security, and poverty relief as well as biodiversity. Increasingly they recognize that defining, preserving, and restoring the health of soils and sediments are fundamental to addressing such problems as climate change, desertification, declining water quality, and the sustainability of agriculture, forestry, and fisheries worldwide. In turn, the health and quality of soils and sediments rely fundamentally on the work of the living communities within them.
One of the international efforts that grew out of this concern is the Soil and Sediment Biodiversity and Ecosystem Functioning project led by soil ecologist Diana Wall of Colorado State University and sponsored by a nongovernmental scientific organization known as the Scientific Committee on Problems of the Environment (SCOPE). Since 1996, a wide array of specialists from around the world has volunteered time to the project to pull together what is known about the biodiversity of the earth's soils and freshwater and marine sediments, its role in sustaining vital ecological processes, and threats to soil organisms and the services they provide. This book is an outgrowth of that project, and access to participating scientists has allowed me to explore how human activities threaten the integrity of soil and sediment communities, and in turn, the critical services they provide to human society.
The idea for this book grew out of a chance encounter in February 2001 when I happened upon Wall and John W. B. Stewart, a retired soil scientist and SCOPE editor in chief, outside a hotel conference room in San Francisco during a scientific meeting. I had already written one book based on findings from a SCOPE project and at the time was writing a second. Wall began telling me about the soil and sediment project and asked if I would like to get involved. How could I not be interested? Her enthusiasm for her science is infectious, and I'm an obsessive gardener, at least during the brief months when the soils in southwest Montana thaw. Furthermore, I had become fascinated by the link between biodiversity and ecological processes while working on my first SCOPE-sponsored book in the early 1990s. So little was known at that time about the ecological roles of specific soil creatures that SCOPE decided to launch a new effort—Wall's project—focused specifically on soil and sediments. The first question that occurred to me was would I be able to learn enough about soil life from the results of this second effort to fill a whole book? Wall assured me I would, and she followed up in the months ahead with stacks of journal articles and reports the project teams had produced. That material introduced me to a topic much larger and more significant than I had imagined.
Excerpted from Under Ground by Yvonne Baskin. Copyright © 2005 The Scientific Committee on Problems of the Environment (SCOPE). Excerpted by permission of ISLAND PRESS.
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Chapter 1. Introduction: Opening the Black Box
Chapter 2. Where Nematodes Are Lions
Chapter 3. Of Ferns, Bears, and Slime Molds
Chapter 4. The Power of Ecosystem Engineers
Chapter 5. Plowing the Seabed
Chapter 6. Microbes, Muck, and Dead Zones
Chapter 7. Fungi and the Fate of Forests
Chapter 8. Grazers, Grass, and Microbes
Chapter 9. Restoring Power to the Soil