A Swainson’s hawk, one of many species that has made itself at home in farming country, sets off to hunt for rodents in a hay field.

The North American Agricultural Revolution

Cropland is a wild place that has been partially tamed. By definition, it has been stripped of its native vegetation and seeded to one or another of the small, select group of plants that humans have brought under cultivation. (In the entire world, only about 70 species of plants are commonly grown as crops; by comparison, there are 5,000 wild plants on the Great Plains alone.) Agriculture was first introduced to the prairies at least two thousand years ago by immigrants from the hardwood forests of what is now the eastern United States. Sometime around the beginning of the current era, circa ad 1, these people began tending small plots of corn and sometimes beans, along the tree-lined river valleys of the tall-grass region, from western Missouri to Nebraska. (At one point, around ad 1000, they had extended their influence west to Colorado and north to the Red River lowlands of Manitoba.) By crafting an economy that depended not only on small-scale agriculture but on hunting and gathering as well, the people of this Plains Woodland tradition sustained themselves on the grasslands for a thousand years.

Sometime between ad 900 and ad 1400, however, disaster apparently struck (perhaps in the form of a severe regional drought) and the Plains Woodland culture appears to have died out. Yet even as this group was declining, another wave of farming-and-hunting people was already on the move. Known as the Plains Gardeners or Plains Village culture, these folks established themselves in small, scattered clusters of earth lodges along rivers and creeks from the upper Missouri River south to the Republican River and its tributaries, in present-day Nebraska and Kansas. There they lived by hunting, fishing, gathering, and small-scale agriculture, growing not only beans and corn, as the Plains Woodland people had, but also squash and pumpkins. Known from archaeological strata that date back to ad 1000, this tradition was maintained by peoples such as the Pawnees, Mandans, and Hidatsas for more than eight hundred years. It was only in the mid-1800s, after the introduction of smallpox, that the villages and gardens were abandoned.

Obviously, this pioneering agriculture was responsible for a small, localized loss of natural habitat. But as minuscule islands of human activity in a vast expanse of native grassland, the gardens did not pose a threat to the functioning of the ecosystem. The natural disturbance regime of the prairies was still operating at full force. Wildfires swept freely across the landscape, as random as lightning and wind, removing dried-out vegetation and making way for fresh green growth to push in. Bison, too, tracked across the plains and generally stirred things up, leaving behind a free-form mosaic of grazed and ungrazed plants, dusty wallows, and splotches of dung. By constantly creating new habitats, these natural forces kept the prairie in a continuous state of renewal, able to the meet the vital requirements of its many and varied inhabitants.

The introduction of intensive agriculture, however, brought these natural processes to a halt. Within less than a human lifetime, between the 1870s and the 1920s, wildfire was brought under control by the settlers, the bison were killed off, and a mind-boggling expanse of grassland was brought under cultivation. The most hospitable and fertile lands—those with the deepest soils, most generous rainfall, and gentlest slopes—naturally bore the brunt of this agricultural revolution. By the time the dust settled, virtually all of the tall-grass prairie, with its rich tangle of grasses and flowers, had disappeared, replaced primarily by an ever-expanding monoculture of corn. (Originally native to Central America, corn is a warm-season grass, like the native species it displaced, and is suited to the growing conditions in the sunny and well-watered eastern regions of the Great Plains.) In the mixed-grass zone, by contrast, the dominant crop is wheat, a cool-season grass that was first domesticated in the Middle East. Together with oilseeds and a number of specialty crops, such as lentils and chickpeas, wheat now dominates between 15 and 99.9 percent of the mixed grasslands, the exact percentage depending on local growing conditions. Only land that is utterly unsuited for crops has been left in native grass, a circumstance that is increasingly common in the south and far west. But even there, on the drought-prone ranges of the short grasslands, about 30 percent of the landscape is under tillage. (These estimates are drawn from Terrestrial Ecoregions of North America, a 1999 publication of the World Wildlife Fund.)

Of all the rodents, none has made the transition to farmland more successfully than the pretty little deer mouse. Largely nocturnal, it takes shelter in underground tunnels and subsists on a diet of seeds and large numbers of insects and insect larvae, mites, and ticks.

The transformation of these broad expanses of wild prairie into farmland has been an abrupt and radical shift. Where the cycle of life was once driven by natural processes, much now depends on the activities of humans. The seasonal regimen of field work—the annual round of tillage, seeding, spraying, and harvesting—creates a new rhythm of disturbance that native species must cope with as best they can. At the same time, the substitution of a handful of cultivated plants for the riot of native grasses, forbs, and shrubs has reduced both the range of food and the diversity of habitats available to wildlife, with predictable consequences. It is no great surprise that cultivated fields support fewer species of native prairie animals and plants than do native grasslands. For instance, according to one study, a small (27-acre, or 11-hectare) remnant of unbroken land in Saskatchewan provided habitat for twice as many species of butterflies as were found on the entire surrounding acreage of cropland. Similarly, research done in Nebraska found thirteen species of birds nesting on untilled land but just two in a nearby cornfield. In other words, 85 percent of the native grassland bird life was missing.

These young barn swallows have almost outgrown their nest under the rafters of an abandoned grain bin.
Arthur Savage photo

But for species that can adapt to the changed conditions, these human-dominated “agroecosystems” are a new and wide-open frontier. In addition to the crops themselves (and what are they but birdseed, after all?), farming has introduced a number of wildlife-friendly features to the landscape of the Great Plains. Shelterbelts now provide habitat for tree-dwelling species such as squirrels and woodpeckers; fences and telephone wires crisscross the country with a network of elevated perches. Skunks den under abandoned buildings and porcupines winter in rusted-out cars, finding shelter in places where it did not exist before. These examples could be multiplied many times over. Although farmland is not native prairie and does not support the full array of species that were here before, it very clearly has a vitality all its own. Simply by going about their business, prairie farmers have provided living space for wild animals and plants. In return, they enjoy a daily access to the natural world that makes them the envy of many city dwellers.

Unwanted Guests

The presence of wildlife in an agricultural ecosystem is not always welcome. Farming often appears to be a struggle against nature, just one darn losing battle after another. Take the fight against weeds, for example. Weeds are wild plants that seed and sustain themselves mainly, and sometimes entirely, on cultivated land. Because they are able to fit their life cycle into the routines of agricultural work, they can compete with crops for sunlight, water, nutrients, and space and greatly reduce yields. Although a few weedy species, like quack-grass, are perennials, which sprout up from fragments of root, most are annuals, which rely on seeds to reproduce. They tend to be fast growing—able to mature between farm operations (for example, before planting or after the harvest is off)—and to produce large crops of big, well-nourished seeds. Fecundity is their main defense against frequent disturbance. If every plant produces scores of viable offspring, at least a few of them are likely to avoid being weeded out and survive to produce their own superabundant generation of seedlings.

Perhaps because the original prairie grasslands were largely composed of perennials, with few annual plants, it is uncommon for native species to have weedy characteristics. Instead, our common weeds are mostly exotic species that were accidentally introduced during the transition to agriculture. Though the bane of the farmers’ existence, their nutrient-rich seeds now provide a reliable, year-round food resource for small mammals and birds, from mourning doves and horned larks in the summer to snow buntings in the winter. Weeds also provide nourishment for a wide variety of insects, especially larval forms. For example, the leaves of Canada thistle (a plant that, despite its name, was introduced into Manitoba from Europe in the 1880s and has since spread across the plains) have become a staple food of painted lady caterpillars, so much so that the adult forms are now sometimes known as thistle butterflies.

Insects that consume weeds may be welcome; not so those that eat crops. Controlling herbivorous insects is the other major challenge facing agriculture. Again, most species that become pests have been introduced from abroad, including such “bad guys” as the Hessian fly, the European corn borer, the English grain aphid, and—a relative newcomer—the Russian wheat aphid. Many native insects, by contrast, have no chance of becoming economic threats, because they are adapted to feed on species or families of plants that are of no commercial importance. To cite one typical example, the female Harris’ checkerspot butterfly lays her eggs exclusively on a wild plant called the flat-topped aster, the only food her caterpillars will accept. Nothing else (not even one of the other seventy-odd species of wild asters) will meet their requirements. So unless flat-topped asters someday become a marketable crop, this is one insect—among many—that will never trouble farmers. Similarly, the larvae of the great spangled fritillary and a number of related butterflies feed only on violet leaves and are unlikely to do anything more worrying than nibble on pansies.