Prairie
Page 4
Over the last two hundred years, human beings have hit the prairies with the force of a major geological crisis, triggering not only extinctions and extirpations—of plains wolves, plains grizzlies, plains elk, plains bighorn sheep, free-ranging plains bison—but also dramatic shifts in the vegetation. Taken as a whole, the Great Plains grasslands now rank as one of the most extensively altered ecosystems on Earth. There is scarcely a patch of ground where we have not left our footprints. The southernmost Short Grasslands, for instance, are as sun-baked and arid a country as you could ever expect to see, yet even there an estimated 29 percent of the ecoregion has been brought under cultivation. The surviving native prairie in the region is now devoted to livestock or converted to ranchettes, on the advancing front of urbanization. In the mixed grasslands, by contrast, the percentage of land under cultivation rises from 15 percent (in districts with scant precipitation) to over 99 percent (where conditions are most conducive to crop production). And in the tall grasslands, with their relatively generous climate and deep, black earth, as much as 99.9 percent of the native grasses have been plowed under to make way for agriculture. Largely as a result of this destruction of natural habitat, at least 464 prairie species have declined to such rarity that their long-term survival is in question, and more names are added to the list with every passing year. (This tabulation includes only species that have been officially designated as at risk of extinction, either locally or nationally.) Of this total, a majority are unique organisms found exclusively, or almost exclusively, on the Great Plains grasslands.
The Foothills Grasslands of Waterton National Park, Alberta, bask in the autumn sun. This ecoregion not only benefits from the trailing edge of storms borne over the mountains but also enjoys relatively mild and bright winters, thanks to the influence of chinook winds.
These trends are deeply troubling, and we could easily get lost in the dark. To find our way forward we will have to be sure-footed, willing and able to move quickly from sorrow to hope, from past to present, from celebrating wildness to accepting and honoring our own accident-prone presence. We will need to see both the splendor of the life that has faded away and the abundance that still extends across the whole wide world of the prairie in every direction. For however diminished, the Great Plains are blooming and buzzing and wriggling and squirming with wildlife wherever we look. In the Northern Mixed Grasslands ecoregion, for example—where as much as three-quarters of the natural habitat has been lost to the plow—there are currently no fewer than 13 species of amphibians, 18 reptiles, 72 mammals, at least 160 butterflies, 222 birds, and 1,595 species of grasses, sedges, and wildflowers. This gives the region a total “species richness index,” on the books of the World Wildlife Fund, of 2,095, much higher than many areas that are typically thought of as biodiversity hotspots. (By comparison, the rain forests of northern California have a richness index of only 1,710, while the Everglades come in at 1,855.)
On the Southern Short Grasslands, by comparison, where significant areas of natural grasslands remain intact as grazing land, the picture is brighter yet, with 17 species of amphibians, 61 reptiles, 86 mammals, 230 butterflies, 245 birds, and an astonishing 2,359 species of grasses and other nonwoody plants, for a richness index of 3,011. Although this book can’t introduce you to all those species—you’ll need the appropriate local field guides for that—it will explain how this abundance of life is sustained and renewed, season after season. Far from being a sacrifice on the altar of progress that we can dismiss from our thoughts, the prairies are still very much alive and worth caring about.
About This Book
In the riotous interactions of nature, everything happens at once—sun, wind, rain, growth, birth, death—and change ripples organically through the ecosystem. For the purposes of discussion, however, it has been necessary to isolate aspects of this holistic system and discuss them one by one, each in a separate chapter. Although the subject matter is tightly interrelated, each section has been designed to stand on its own, so the chapters can be read individually and in any order. Chapter 3, “The Geography of Grass,” for instance, provides a detailed look at the prairie grasses and their dynamic relationship with the extremes of a midcontinental climate. Chapter 4, “Secrets of the Soil,” ventures into the dirt—a life zone all its own—and introduces a few of the strange little creatures that live beneath the ground. In Chapter 5, “Home on the Range,” we come back out into the sunshine to ride through cattle country and find out how life is lived on the surviving expanses of native prairie. Chapter 6, “Water of Life,” by contrast, takes us knee-deep into the nearest prairie river or pond to look into the lives of ducks, shorebirds, fish, and other aquatic organisms. Chapter 7, “Prairie Woodlands,” examines the unexpectedly important role of trees in grassland ecology and asks what difference it makes that woody growth is now invading the prairies. Chapter 8, “The Nature of Farming,” studies the potential and challenges of croplands as wildlife habitat. And finally, Chapter 9, “Long-Range Forecast,” reconsiders the conservation status of the Great Plains—is this really the most endangered ecosystem on the continent?— and discusses a range of options for protecting and restoring its wildness.
But before we look to the future, Chapter 2, “Digging into the Past,” will take us back to the very beginnings of time and the great adventure of existence.
Today’s prairies are a picturesque mosaic of natural and human-altered environments, like this glowing agricultural landscape along the Kansas River.
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DIGGING INTO
THE PAST
Time is God’s way of keeping everything from happening at once.
ANONYMOUS
THERE IS AN unseen dimension to the far-and-away spread of the prairies, and that dimension is time. At first glance, one might mistake this for a place that time and change have somehow overlooked. These level plains and soft, rolling hills seem to have settled here quietly, their surface unmarred by signs of geological strife. But appearances can be deceiving. The great grasslands of central North America have been shaped over the past three or four billion years by the same forces that raised the Rockies and excavated the Grand Canyon. Their surface has been seared by the sun, scoured by ice, blasted by blowing sand, and buried in deep drifts of gravel. As a result of immense energies beneath the surface of the Earth, the plains have been raised up, forced down, drowned by oceans, and blanketed in ash. They have experienced every shudder and wrench as continents have collided and torn away from each other, only to collide and tear away again.
The traces left on the surface of the prairies by this planetary bump and grind are surprisingly minimal. Yet if you know what to look for and where to look for it, the subtleties of the prairie landscape become eloquent. An oil well bears witness to ancient tropical seas. A vast level plain provides an unexpected reminder of the protracted violence of mountain building. A hummocky wheat field speaks of the lumbering passage of glaciers. To an observer with a little basic geological knowledge, even the most unspectacular prairie landscape suggests a long and spectacularly interesting history.
Under the Waves
Trilobite
To go back into the prairie’s history means to go down. The record and residue of times past lie beneath our feet, so wherever we go on the prairies, we are traveling across vanished worlds. Straight beneath you, for example, at a depth of between 2,000 and 4,000 miles (3,000 and 6,500 kilometers), lies the Earth’s core—the yolk of the planetary egg—which coalesced out of a whorl of star dust some 4.5 billion years ago. This partly solid, partly fluid center is encased in an equally ancient layer of rock called the mantle. And surrounding the mantle is a covering of waxlike malleable material known as the asthenosphere, which is kept at a lethargic boil by the heat of its own radioactive decay. As the source of the molten magma that periodically shoots up through volcanic fissures and rifts in the ocean floor, the asthenosphere is the main powerhouse of geological turmoil.
T
he roiling-and-toiling asthenosphere occupies a zone between about 45 and 150 miles (70 and 250 kilometers) below the surface. Between it and us lies a relatively thin and fragile shell of rock, known as the lithosphere. The outermost membrane of this rocky shell is the Earth’s crust, a layer that is thinner, proportionately speaking, than the skin of an apple. On the prairies, the crust extends to an average depth of 25 to 30 miles (40 to 45 kilometers). Yet this comparatively short vertical distance takes us back in time some 3.8 billion years, to an era when the flying debris of creation had begun to subside and the Earth’s crust was finally able to stabilize. In this remote and inhospitable age, we find the first traces of life—microscopic stains, a few microns long, made by filaments of cyanobacteria, or blue-green algae.
Rocks from this primordial era lie right out in the open on the Precambrian Shield, but they seldom break through to the surface of the Great Plains. Instead, these ancient formations generally lie a few miles beneath our feet, providing the foundation, or “basement rock,” on which the prairies have been built. Our region lies on what geologists call the North American craton, or the stable core of the continent. This is a large fragment of the Earth’s crust that sheared away from an unnamed supercontinent toward the end of the Precambrian Era. By the time this happened—some 600 million or 700 million years ago—the Earth (and the prairie region along with it) had already endured more than 3 billion years of mountain building, erosion, glaciation, deglaciation, and general geological Sturm und Drang. But things must have been starting to settle down, because when the supercontinent tore itself apart, it produced a North American continent-in-the-making that has persisted until the present.
The Earth has sometimes been likened to a layer cake, in which ancient sediments are overlain by deposits from successive geological events, creating an ascending timeline from past to present.
This infant continent was not exactly the land mass that we know today. The entire western Cordillera was missing, with the result that the west coast of the craton ran south through present-day British Columbia and the Pacific states (much closer to the prairies than it is today). At first, the cratonic land mass lay exposed—a low, eroding plain, as barren as the face of Mars. But, as the geological strife continued, sea levels began to rise and the land was gradually overrun by the ocean. In time, the entire continent (with the periodic exception of a chain of tropical islands that ran diagonally across the plains, from Lake Superior toward Arizona) had disappeared beneath the waves.
For roughly the next 55 million years (from about 545 million to 490 million years ago), much of the North American craton lay under a shallow sea. Wherever the land remained exposed, it was eroded by water and wind, which ground the gritty Precambrian rocks into rounded grains of quartz sand. This sand was then swept to the coasts and out into the sea, where it settled to the bottom in beds tens to thousands of yards thick. Eventually, these lustrous sediments were overlain by layers of fine-grained mud. And whether sandy or silty, this ocean floor was literally crawling with life, particularly three-lobed, many-legged, bottom-feeding arthropods known as trilobites. After an agonizingly slow start with the cyanobacteria, evolution was finally hitting its stride, producing a menagerie of weird and wonderful undersea life. As generation upon generation of these animals lived and died, their remains settled onto the ocean floor, where they were buried under thick layers of sediments. Today these fossil-rich deposits—now compressed into solid sandstone and shale—are buried some 3 miles (5 kilometers) beneath the wheat fields of the northern plains and at lesser depths in other parts of the prairies. But in a few places—like the Judith and Little Rocky mountains and the northern Black Hills—they have been pushed up to the surface, exposing their maritime history to plain view.
> GEOLOGICAL TIMESCALE
When the Cambrian sea finally withdrew and dry land emerged again, the forces of erosion immediately began to tear away at the newly formed rocks. But soon, geologically speaking—after a break of little more than 20 million years—the water rose and slowly spread over the land. This time, even the transcontinental island chain was bathed in the warm, clear seas. Now primitive snails munched on algae and were themselves preyed upon by giant squid-like nautiloids, with shells up to a couple of yards in length. Hundreds of new species of shelled animals evolved, including crinoids, or “sea lilies” (distantly related to modern sea urchins), and exotic reef-forming corals. There was so much life in these oceans that when they finally withdrew some 440 million years ago, they left behind thick deposits of shell fragments and calcium-rich debris, which eventually solidified into fossil-rich limestones. These Late Ordovician deposits include the elegant Tyndall stone that is quarried in Manitoba and graces so many buildings in the Prairie provinces.
Enchanted Rock, near Austin, Texas, was formed as a massive upwelling of molten rock during the Precambrian Era. The granite eventually solidified and now lies exposed by the eroding action of wind and water.
Nautiloids
Crinoid/sea lily
And so things continued for about the next 100 million years, as shallow oceans advanced across the North America craton, only to withdraw and then flood back in. If the run and roll of the grasslands sometimes remind us of the sea, surely this is a result of the landscape’s long marine history. With every advance and retreat of the ocean, the land was burdened with fresh deposits of sand, silt, and crushed shells, which built up, year by year, in nearly horizontal, banded layers. Although some of these contrasting sea floors have since been exposed by erosion (where rivers have cut deeply down through the sediments), for the most part they lie thousands of feet beneath the grasslands.
The oceans that left these deposits behind were hospitable to life—shallow, warm, well lit, and typically tropical. During the Silurian and Devonian periods in particular (between about 440 million and 355 million years ago), these waters provided ideal conditions for reef-forming sponges and corals. In what would one day become the Canadian Prairie provinces, the reef builders of the Devonian had a heyday, constructing barrier reefs and ringlike walls that rose to heights of 300 feet (100 meters). Wherever the sea was constricted by these limestone palisades, the water gradually became super-salty. If the circulation of the sea was inhibited, water lost to evaporation could not readily be replaced, and the concentration of salts steadily increased. In time, the salts precipitated out of the sea water in these areas, leaving thick beds of potash and other minerals, notably under present-day Saskatchewan. The potash deposits in Texas were formed by a similar process but some millions of years later, during the Permian Period.
The last truly continentwide inundation withdrew from the North American craton about 300 million years ago. The next time the sea attempted to overrun the land—as it would continue to do for millions of years to come— it found itself lapping around the shores of a rocky upland that had started to rise in the eastern half of North America. Apparently, the asthenosphere had heated up and begun to force masses of molten rock up through rifts in the ocean floor. This event had sent the continental plates on a slow and perilous collision course. First, Europe smashed against North America from the northeast. Then a massive supercontinent called Gondwana (the combined land masses of South America, Africa, India, Antarctica, and Australia) crunched into North America from the south, causing the land to buckle and forcing the Appalachian Mountains to lift along the east coast. The forces involved in these mighty adjustments were even felt in the middle of the craton, where a range of mountains 3,000 feet (1,000 meters) high rose out of the plains of present-day Oklahoma and Colorado. Known as the Ancient Rockies, these mountains have since been eroded to their roots by the action of water and wind.
Apart from the appearance of these new highlands, the west coast of the craton was comparatively untouched by these titanic collisions. Through all the commotion, the sea continued to wash up over the land, even splashing around the base of the Ancient Rockies and turning them into a cluster of south-sea islands. With
every advance and retreat, the sea again left behind layers of sediment and the fossilized remains of a strange coterie of underwater life. In addition to the crinoids, corals, and other unusual beasts that had occupied Devonian waters, there were now small filter-feeders, called archimedes, that had perfect corkscrew skeletons and others, called productids, that held themselves up off the sea bottom by perching on stiltlike spines. (A wonderful jumble of 250-million-year-old sea life has been preserved in the Guadalupe Mountains of western New Mexico and Texas, which were once a complex of reefs in the western ocean.) Bony fishes swam through these waters, sometimes hotly pursued by large, saw-toothed sharks. The game of evolution was being played with feverish exuberance.
Archimedes
Productid
Meanwhile, back on dry land, the surface of the continent was continuing to buckle and twist. As the Appalachians were thrust upwards, land in the center of the craton was forced to rise along with them. A broad plain formed along the edge of the eastern highlands, sloping gently toward the western sea. When the waters receded, this coastal plain extended all the way west to present-day Alberta and Texas. And even when the sea rose up and flowed across the land, the eastern margin of the plain (roughly from present-day Manitoba south to Kansas and Missouri) was now high enough to escape all but the most severe flooding.
A new frontier for life was emerging not only in North America but on the other continents as well. Land plants, which had put in their first appearance some millions of years before, had never made much of a showing. But as stable new habitats became available, the evolutionary tree began to bud and sprout with explosive energy, producing more and larger species of land plants than ever before. In time, the soggy, boggy landscapes left by the retreating oceans were filled with riotous jungles of giant sphenopsids, or scouring rushes, tree-sized ferns, and leafy conifers. These tremendous swamps, which flourished between about 355 million and 300 million years ago, disappeared soon afterward, probably as a result of a cooling and drying trend in the climate. Buried where they fell—in modern-day Iowa, Missouri, and Kansas, among other places—the swamp plants eventually turned into coal, the characteristic rock of the Pennsylvanian, or Upper Carboniferous, Subperiod.