Prairie: A Natural History

THE GRASSLANDS OF the Great Plains tug at our senses, challenging us to overcome our limitations as mere humans. On the one hand, the landscape calls us to enlarge our horizons and soar across its spacious expanse. But on the other, the prairie seduces us with details, quietly drawing our attention to the little things that matter in this big, round world. The flick of a ground squirrel diving into its burrow, the wheeling shadow of a hawk, the ripple and run of its wingspan across the bending grass. If we are willing, the prairies will even lead us beyond these small sightings to truly minuscule events, beckoning us into an alien but all-important world that lies almost entirely beyond the reach of our ordinary awareness. For the typical life-form on the grasslands is not a bison or a gopher or even a blade of grass. It is some weird little creepy-crawly, far too small to be seen by the unaided eye, that lives and dies just beneath our feet in the deep prairie soil.

Recent estimates suggest that the total weight, or biomass, of the invisible organisms that live in prairie soils is greater than the mass of all the visible, aboveground animals put together. If all the microflora and microfauna from grassland soils could be piled onto one arm of some Great Cosmic Scale, with all the insects, reptiles, mammals, and birds from the upper world heaped onto the other pan, the balance would tip sharply toward the soil organisms. Together with the plant roots around which they live, these little creatures are the powerhouse of the prairie, responsible for anywhere between 60 and 90 percent of all the biological activity in the Great Plains grasslands.

The soil has been described as the “poor person’s tropical rainforest,” one of the richest and most diverse ecosystems anywhere on Earth. The most numerous organisms in this busy world are also the smallest—thousands upon thousands of different kinds of single-celled organisms, including algae, fungi, bacteria, and protozoans. Although a few of these creatures may be familiar from Biology 101—the ooze-along, blob-shaped amoebae, for example, and the whip-tailed flagellates—many (if not most) of the soil microbes are still completely unknown, never having been studied, named, or even seen by anyone. A single teaspoon of dirt typically holds around 5 billion of these fantastically varied little animals and plants, all of them fiercely engaged in the business of life and death. In their billions and their trillions, they expend as much energy as if several thousand people were living on every acre, or hectare, of the prairies.

Adding to the hubbub is a lively assemblage of somewhat larger (though still mostly microscopic) and more complex animals. The bestiary includes plump, lumbering, eight-legged little critters called tardigrades, or waterbears, that waddle through the film of water around soil granules, searching for food. When they find plant roots or fungi, the “bears” drill into the cells with their sharp snouts, or stylets, and suck out the nourishing fluids. Another, more important group of plant eaters, the springtails spend their lives bumbling blindly through the dirt, munching on bacteria, fungi, dying roots, and the occasional dead waterbear or worm. Jumpy and alert, springtails are equipped to leap into action at the first tremor of danger. As their name suggests, they have spring-loaded tails that when all is well they carry tucked up under their abdomens. But when danger is detected, the tail fills with fluid and suddenly snaps down, sending the springtail catapulting to what, with luck, is safer ground. (The light-colored specks of “dust” that sometimes somersault away from compost when the pile is turned are actually springtails taking flight from the disturbance.)

About one square yard of prairie can support anywhere from 5,000 to 15,000 springtails in the top foot of soil. (That’s about a square meter dug to the depth of 30 centimeters, or enough to make a couple of wheelbarrow loads.) But these statistics pale compared with the totals for another major group of soil dwellers. The soil is crawling with microscopic mites, those scurrying, scuttling, eight-legged cousins of spiders and ticks. The same small patch of prairie soil that is hopping with springtails may simultaneously be home to up to 60,000 mites, representing dozens, or even hundreds, of species. (Although 48,000 kinds of mites have so far been identified around the world, the final catalog of species is expected to reach half a million or more.) While many mites make their living peaceably by feeding on microbes and plant roots, others are voracious predators—the tigers of this invisible world. Spinibdella cronini, for example, is a largish red mite (about the size of the period at the end of this sentence) that is common in prairie soils and that preys on soft-bodied mites and springtails. Its modus operandi is to seize its prey by surprise, tie it down with silk, and then suck the life out of it.

The opulent, fragrant blossoms of the gumbo, or tufted, evening primrose are typically found on dry, eroded slopes, usually where shales laid down in the Cretaceous Period are near the surface. These insect-friendly flowers open in early morning and close at night.

Pity the poor springtails. For “giant” mites are not the only menace in their shadowy world. They must also be prepared to flee from pseudoscorpions, jaunty little monsters that stalk through the soil with their venomous pincers armed and ready for action. Danger can also come swimming through the darkness in the form of carnivorous worms, innocuous-looking creatures (mostly smaller than the letter i) that either swallow their prey whole or attach themselves to the victim’s body and scrape at it until the internal organs can be drawn out through the puncture. These silent predators are members of an enormous tribe of roundworms, or nematodes, that are the most numerous multicellular animals on the grasslands. Hundreds of species have been found at sites across the Great Plains, including those that feed on bacteria, protozoans, fungi, algae, mites, other nematodes, and the roots of higher plants, or some combination of the above. A heap of soil that contains thousands of spring-tails and mites typically also holds between 3 million and 5 million nematodes. Across the broad expanse of the grasslands, their numbers spin out of control, quickly mounting toward the astronomical.

> CHEMICAL WARFARE

Nematodes are not only the most numerous mul-ticellular animals on the grasslands. They are also the most important herbivores. While aboveground plant eaters nibble on stems and leaves, herbivorous nematodes eat their way through the roots, attacking silently from beneath. One study conducted on a conservation reserve determined that the unassuming little worms worked their way through more plant matter per unit area than either bison or prairie dogs. And on pasturelands, the standing population of nematodes has been known to consume twice as much vegetation as a herd of cattle.

Fortunately, some plants are able to protect themselves against the attack of the killer worms. A number of common prairie forbs, including pasture sage (or fringed sagewort) and nodding onion, produce natural nematicides that ooze out of the roots and create a worm-free zone for themselves and their neighbors. Such protection is presumably one of the benefits of a diverse plant community in which the roots of different species are intertwined.

But not all plants are so community minded. Some use chemical warfare not to fight against pests but to resist the encroachment of other plants. For example, a dainty-looking short-grass species called prairie three-awn produces root exudates that kill nitrogen-fixing algae and bacteria. (These are soil organisms that capture nitrogen gas from the air and convert it into a water-soluble form in the soil, ready for plants to use.) Because prairie three-awn can get by without much nitrogen, this action creates ideal conditions for its own growth and keeps out competing species that need nitrogen-rich soil.

All soil is full of life, whether under prairie, forest, tundra, or desert. Yet compared with other biomes, the temperate grasslands are remarkable for the extent to which they concentrate their resources in the underworld. The reason is simple: it’s safer down there. The prairie climate is grueling— too hot, too cold, too wet, too dry, too wild and variable—but go a little way beneath the surface and the environment becomes surprisingly equable. During the growing season, for instance, when the thermometer can easily spike from 50˚F to 105˚F (10˚c to 40˚c), the temperature 20 inches (half a meter) down in the soil remains pleasantly cool and virtually constant.

The claret cup, or mound hedgehog-cactus, is a dryland specialist that prefers well-drained gravelly or sandy soil. A native of the southwestern deserts, it can also be found in suitable sites on the southern plains.

In a region where moisture is often the limiting factor for life, the soil also provides a buffer against drought. Long after the surface of the ground has been wicked dry by the sun and wind, the soil retains hidden reserves of moisture. Not only does water pool in the spaces, or pores, between the soil granules, but it is also glued to the surface of the particles by powerful chemical bonds. Thus, even apparently dry dirt often retains a thin film of water that is tightly bound to the soil grains. To tap this precious resource, grasses put down dense networks of thin, fibrous, probing, branching, fast-growing roots, all seeking moisture and nutrients. On average, grasses produce three or four times more roots by weight than they do leaves and stems, giving them a root-to-shoot ratio that is ten times as high as that of a forest.

The sheer volume of roots produced by prairie grasses is mind-boggling. In the mid-1930s, for example, one determined agrologist undertook to excavate and measure all the roots produced by a small patch of tall-grass prairie in Nebraska. From half a square yard, or meter, of ground, he extracted about 150 miles (250 kilometers) of fibrous roots—enough, laid end to end, to reach from Lincoln, Nebraska, to Sioux City, Iowa. The tally included 11 miles (17 kilometers) of green needlegrass roots, 13 miles (21 kilometers) of big bluestem, 23 miles (37 kilometers) of little bluestem, and 109 miles (176 kilometers) of Kentucky bluegrass. And that was without factoring in the silvery fuzz of absorbent root hairs that coat root fibers from base to tip. In another heroic experiment, a single rye plant (the domesticated cousin of several native prairie grasses) was found to have produced 7,000 miles (11,000 kilometers) of roots and root hairs in one four-month growing season.

In nature, these root systems obviously do not strike out across the continent. Instead, they squiggle and writhe into every nook and cranny of the soil, holding the dirt together with their slender, grasping fibers. Since most prairie grasses also produce lateral rootstocks, or rhizomes, that send down roots along their length, each plant—and each clump of root-bound earth—is connected to the next and the next. As plant intertwines with plant, and roots interweave with roots, the soil becomes tightly tied together in a thick, fibrous mat—the famous prairie sod, which the settlers used for building their first homes.

For thousands of years before the settlers arrived, that same deep fabric of roots had served as the foundation of the soil ecosystem. The enormous biomass of underground plant matter, both living and dead, provides an almost inexhaustible supply of food for soil organisms. Huge populations of herbivores graze on the roots themselves, including hordes of fungi, nematodes, and springtails. Still other microorganisms feed on the rich soup of nutrients that leak from the roots into the dirt. (Since these resources are concentrated in the rhizosphere—the soil that surrounds the root—much of the life in the soil is concentrated there as well.) In the dark underworld of the soil, everything gets eaten. The dead, the dying—even minute particles of excrement—are all on the menu for some miniature soil creature.

As a result of this endless round of digestion, organic material in the soil (principally dead roots) is first broken down and then gradually built back up into complex and relatively stable molecules known as humus. Dark brown or black, these humic substances not only give prairie soil its rich palette of tones but also contribute to its ability to support a dense tapestry of crops and native grasslands. To thrive, plants need to take in nutrients such as potassium and nitrogen (the K and N found in synthetic plant foods), which they soak up through their roots. Because these chemicals are water-soluble, they are readily leached down into the earth, beyond the depth at which the plants can reach after them. But if the nutrients are bound up in humus, they are effectively stored in the upper 6 inches to 1 foot (15 to 30 centimeters) of the soil. As the soil organisms eat away at the hard-to-digest humus, the stored nutrients are gradually supplied to the plants, much as if they were being freed from a time-release capsule.

Thus, prairie soils are fertile largely because—from the end of the Ice Age until the land was plowed—the native grasses have consistently produced more root fibers than the soil organisms have been able to consume. The surplus resources have collected in thick layers of dark, crumbly, humus-rich earth. (By contrast, desert soils contain little or no humus and forest soils have only a thin, upper horizon, or layer, of black dirt.) To an earlier generation of soil scientists, these remarkable grassland soils were known as “prairyerths,” simply and profoundly because that is what they were.

> HEALTHY INFECTIONS

Many of the microorganisms in the soil can infect plants and sap their strength, even to the point of making them visibly “sick.” But there are also remarkable cases in which infection produces health, as plants and microbes join forces for their mutual benefit.

For instance, almost all prairie plants depend on mycorrhizae, or fungal cells that infect root tissues and feed on the plant’s hard-won reserve of sugars. As much as 30 percent of the plant host’s resources may be spent on feeding its fungal guest. In return, the fungus produces a network of ultrafine threads that extend out from the roots and penetrate into the tiniest interstices of the soil. A pinch of soil that contains an inch or so of roots and root hairs might also hold up to 130 feet (40 meters) of fungal filaments. This extensive network feeds water and nutrients back to the plant, amply reimbursing it for its services.

An even more remarkable collaboration occurs between the roots of certain plants—including alders and many legumes (vetches, milkvetch, vetchlings, clovers, and the like)—and a select group of nitrogen-fixing bacteria. In this case, the plant often takes the initiative by producing flavorful chemicals that draw the appropriate species of microbes to the site. Once contact has been made, the bacteria induce the plant to open a tubular passageway into the root so that the invaders can move unhindered into the underlying tissues. Not only does the plant feed the bacteria (again at great expense), it also shelters them in sturdy, pea-sized chambers, or nodules, on its roots.

The payoff for this effort is immense. The bacteria in the nodules extract nitrogen gas from air pockets in the soil, metabolize it, and convert it into ammonia. In this form, the nitrogen becomes available to plants, which use it to manufacture proteins. Although a certain amount of nitrogen is fixed by organisms that live freely in the soil, the bulk of nitrogen fixation is performed by bacteria housed in nodules. Without this delicate symbiosis between microbes and plants, life as we know it could not exist.

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Purple prairie clover

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Golden bean with root nodules

Technically speaking, prairie soils are characterized by a deep surface layer, or A horizon, of topsoil; a lighter-colored B horizon of subsoil; and a band of calcium-rich salts somewhere within about 6 feet (2 meters) of the surface. But within the broad bounds of this definition, there is room for almost infinite variability. Soil is a unique and intimate blending of living and nonliving elements, and although grasses have doubtless been the major influence on the development of prairie soil, they have not acted alone. The physical setting of the Great Plains has exerted a powerful force of its own.

Just as the sea can be described as a body of water, so the soil could be defined as a body of weathered rock that, like the sea, is permeated by life. The nature of the soil ecosystem depends, at least to some extent, on the nonliving substrate, or “parent material,” from which it developed. Grassland soils still bear the mark of the glaciers that retreated thousands of years ago. For instance, the region immediately south and west of the Missouri River (from Montana and the Dakotas across to northwestern Nebraska) escaped the direct impact of the ice. As a result, the grasslands there took root on an ancient, unglaciated landscape that had originally been shaped by the shallow seas of the dinosaurian age. Because of the chemical composition of those ancient deposits, the soils that formed on them are not particularly rich in some of the minerals that growing plants need.

By contrast, soils to the north and east of the Missouri (from the Canadian prairies to northeastern Nebraska) have been built on mineral-rich gravels and ground-up rock left behind by the glaciers. And large parts of the central plains (from southeastern Wyoming, south into Texas and east to Kansas and Missouri) are blanketed in thick drifts of silt, or loess, that were deposited by fierce windstorms around the end of the Ice Age. Whether ice-borne or wind-borne, this glacial debris has provided the soil with a generous resource of plant nutrients that enhance its fertility.

The active sand dunes south of Monahans, Texas, respond to every ripple of the prevailing winds. Formed from the debris of ancient mountains, the sands were deposited here about 25,000 years ago. Today, they provide habitat for remarkable plants like sand bluestem, sand sage and shinnery oak and for animals like the dunes sagebrush lizard.

In a number of places across the plains, the Ice Age also left behind a legacy of sand, a material that is much less favorable than silt for soil development. Sand is composed of relatively large particles, with large pores in between, a structure that permits water to run through it freely. As a result, organic material is flushed away before the soil organisms have a chance to set to work and very little humus is produced. In the millennia since the glaciers’ last retreat, sand pr airies like the Nebraska Sand Hills and the Great Sand Hills of Saskatchewan have produced only a fragile layer of topsoil that is susceptible to every passing footfall or gust of wind.

Yet the inability of sand to hold onto water also offers an unexpected advantage to certain plants. Because moisture is not tightly bound to the sand granules, it is readily available to the roots and often supports surprisingly lush stands of vegetation. The Nebraska Sand Hills, for example, are home to a distinctive community of grasses and forbs, including specialists such as sand bluestem, sand dropseed, sand reedgrass, and the amazing blowout grass, which pushes its rhizomes out to a distance of 40 feet (12 meters), searching for bare sand in which to establish new plantlets. Similarly, the lemon scurfpea, or lance-leafed psoralea—a delicate-looking plant that is actually tough as nails— relies on a network of long, shallow roots that hold the soil in place and allow it to flourish even on the shifting sands of windswept banks.

The nature of sandy soils is largely determined by their geological origins. But most soils on the plains are even more strongly influenced by a second physical force—the climate. In particular, soil development is especially susceptible to regional differences in temperature and precipitation. In general, the more moisture that is available to grasses, the more roots they produce and the deeper and darker the topsoil that accumulates. Because precipitation increases along a west-to-east gradient across the Great Plains, soils follow a similar trend, increasing in fertility across the region. The thin, light-colored soils of the western steppe grade into the deeper, richer soils of the mixed grasslands to the north and east and culminate in the thick, black earth that developed under the tall-grass prairies.

Originally hauled to the plains by the glaciers thousands of years ago, this boulder is now being cracked apart by frost wedging and transformed into soil.

Inevitably, in a world of intricate interactions, this simple pattern is complicated by a conflicting variable, namely, a north-to-south gradient in temperature. Where average soil temperatures are low, as they are on the northern plains, the soil organisms are sluggish and the rate of decomposition is slow, so humus accumulates in the topmost horizon. In the warmer south, by contrast, the more active microbes eat around the clock, causing rapid decomposition and slower rates of humus production. A glance at a generalized soil map confirms this effect. The rich black “mollisols” of the northernmost plains gradually give ground farther south to “alfisols” and “aridisols”—thinner, less fertile soils. At the same time, it is also possible to detect an overall improvement in soil quality from west to east, under the influence of increasing precipitation.

> THE TAXONOMY OF SOIL

Over the last several decades, North American soil scientists have devoted themselves to the complexities of soil classification with such enthusiasm that they have produced two completely separate systems. The USDA taxonomy, used in the United States, divides soils into eleven broad categories, or orders. Most grasslands soils are classified as mollisols (literally, “soft soils”), a designation that is further subdivided by climatic conditions. Soils may be udic, ustic, xeric, or aridic as they progress from humid to parched; they step up from cryic to thermic and hyperthermic along the vector from frigid to warm. Other terms describe characteristics such as the texture and chemistry of the soil. Thus, a cool, dry soil from northern Montana is classified as an aridic cyroll (a dry, cold mollisol), while a warm, moist soil from Texas keys out as a thermic udoll (a warmer, wetter mollisol). Meanwhile, the rich farmlands of west central Kansas emerge from the classification process as fine, smectitic, mesic typic argiustolls, or silty, warm, well-watered mollisols with a heavy subsoil laced with slippery clays. And so it goes, in an ardent outpouring of precisely defined vocabulary.

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Profile of a grassland soil, showing topsoil, A; subsoil, B; and parent material, C.

In Canada, scientists have retained an older system that is based more closely on observable soil properties, including the color and depth of the topsoil and the subsoil, or the A and B horizons. According to this system, prairie soils are not mollisols but chernozems, a term that comes from the Russian for “black earth.” But since chernozemic soils actually vary noticeably in color, this order is further subdivided into four major groups: Brown, Dark Brown, Black, and Dark Gray soils. Yet even this apparently straightforward system shows signs of linguistic strain as scientists bend to the task of distinguishing one patch of prairie soil from the next. Is this a Calcareous Brown soil (light colored and high in calcium) or a Gleyed Solonetzic Dark Gray soil (somewhat darker, saline, and poorly drained)? Soil science is clearly not a profession for the tongue-tied.

If the climate has mediated the development of prairie soils over several thousand years, it also sets the pace of life in the here and now. Although the weather in the underworld is relatively stable compared with the chaos that reigns above ground, the soil ecosystem is not immune to climatic disasters. Drought is a particular challenge. Most soil organisms are aquatic species that have become adapted to live in the film of water surrounding the soil grains. If this habitat dries up (whether for a few weeks in the heat of midsummer or during a prolonged drought), the microbes are thrown into a crisis. Although many individuals die, many more survive by encasing themselves in protective armor, transforming themselves into cysts, and becoming dormant. Water-bears, for example, shrivel up into little kernels called “tuns” that are capable of withstanding unimaginable cold (approaching absolute zero), unspeakable heat (well above boiling), and brutal doses of Xrays (a thousand times the lethal dose for humans). When a piece of moss in a museum collection was moistened after 120 years, the long-dormant waterbears roused from their sleep and immediately started grazing.

Although waterbears may set the outer limits of biological endurance, most soil microorganisms have an extraordinary ability to “play dead” for months or years. A nematode egg can lie in the soil for up to a decade, waiting for its preferred plant host to appear. The minute that conditions improve—for example, when a dry crumb of soil is moistened by long-awaited rain—the soil organisms come back to life and start to eat and reproduce at a hectic rate. Life occurs in manic pulses—wet, on; dry, off—typically with peaks in the spring and autumn, and lulls in the hot summer months.

But there is one major group of soil organisms that has not adapted comfortably to the erratic moisture regime of the Great Plains. Earthworms, those most celebrated of all soil dwellers, breathe through their moistened skin, and for them, desiccation can be fatal. While most species can survive moderate drying by burrowing deep into the subsoil, coiling up in a knot, and entering dormancy, few can survive severe or prolonged water shortage. In the whole of the Great Plains, earthworms are naturally abundant only in the relatively well-watered soils of the east and southeast, including all or part of Nebraska, Iowa, Kansas, Oklahoma, Texas, and Missouri. In this region, the soil is home to worms of two dozen different species—gray, pink, brown, massive or delicate. Some are twice the length of a man’s outstretched hand; others would fit neatly on a fingertip.

Among the most common earthworms in these soils are the so-called peregrines— hardy European and Asian species that have been transported to the prairies (and around the world) in the roots of potted plants. Ideally suited for life in agricultural soils, they have expanded their range tremendously in the last few hundred years, often at the expense of indigenous species. Of the six families of earthworms native to North America, four are known to occur naturally in tall-grass prairie soils. The populations currently seem to be strongest in relatively untouched regions such as the Flint Hills of Kansas, though even there the peregrines continue to intrude.

Nobody really knows what difference it makes when foreign earthworms appear on the scene, but it must have a profound effect on soil ecology. Earthworms are big eaters, and where they are abundant, they serve (in Aristotle’s undying phrase) as “the intestines of the Earth,” capable of producing their own weight in nutrient-rich castings every twenty-four hours. But not all earthworms are created equal, and experiments hint that native species may be specially attuned to meeting the nutritional requirements of native plants. When big bluestem was grown in pots with either the native earthworm Diplocardia smithii or the introduced pasture worm Aporrectodea turgida, the grasses produced significantly more roots in the presence of the homegrown species. Perhaps D. smithii squirmed in closer to the roots and delivered the nutrients where they were needed most. Or maybe the native worm produced hormonelike “growth factors” (as earthworms are known to do) that were particularly effective in stimulating the tissues of its longtime evolutionary companion.

In one way or another, the introduction of alien life-forms into tall-grass prairie soils must certainly be having complex repercussions. And the disturbance is presumably even greater in the northern and western plains, where earthworms appear to have been rare, or entirely absent, before settlement began. Across the entire northern span of the continent—to the southern limit of the most recent glacial advance—there are essentially no native species of earthworms. Either such creatures never existed or they all perished under the ice, never to be seen in modern times. (One telling exception is a small, pallid species called Aporrectodea bowcrowensis that was recently discovered in Alberta’s Porcupine Hills, in an area that was not glaciated.) Native species of earthworms are also virtually absent from the short- and mixed-grass prairies of the western and central plains, a region that is chronically short of rain. The only earthworms to occur there are the immigrants that arrived with settlement. Although little is known about their progress on this new frontier, they appear to be expanding into both grasslands and croplands, boldly going where no worm has gone in at least 10,000 years.