Prairie

by Candace Savage

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  > RIVER OF CRANES

  Congruence: The state of being in agreement, harmony or correspondence, from the Latin “con,” or with, and “grues,” or crane.

  Every spring, between late February and early April, about 375,000 sandhill cranes—roughly 80 percent of all the sandhills in the world—descend on the Platte and North Platte rivers in central Nebraska. There, at the halfway point of their annual journey from the Llano Estacado of New Mexico and Texas to their nesting grounds in Canada, Alaska, and Siberia, they settle in for several weeks to rest and refuel. By daylight, the birds disperse to forage in wet meadows and cornfields along the river, where they put on weight for the breeding season to come. At night, they return to the river to roost among its shallow, braided channels. Packed in shoulder to shoulder, they rest at half-alert, stirring the darkness with the tremolo of their high-pitched, gurgling voices.

  With a fossil record from Nebraska that dates back more than 6 million years, sandhill cranes are the oldest of the world’s living birds. The Platte River, by contrast, is a mere youngster that traces its origins to the end of the most recent glaciation. From this it follows that the congruence between the cranes and the river arose sometime in the last 10,000 years. Recently, however, this harmony has been threatened by the construction of more than 200 dams and diversions on the Platte system. With peak spring flows now well below what they were even during the 1930s drought, the river is no longer able to maintain the flood-fed wetlands and the open, unvegetated sandbars that the cranes require. Forced out of historic roost sites by the encroachment of woody plants, the cranes are now heavily reliant on a few key stretches of the river system. Despite a conflict-ridden effort to “mitigate” the impacts of water management, current research suggest that cranes are showing signs of physiological stress. In particular, the birds’ ability to store energy for their onward journey (their daily rate of fat deposition) has declined by as much as 50 percent since the late 1970s. Meanwhile, the total count of spring migrants has dropped by 125,000 birds, or 25 percent, since the fat years of the early 1980s. The consequences for the endangered whooping crane and piping plover, which also rely on the Platte River, have not been thoroughly investigated.

  Sandhill cranes

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  In North Dakota, the population of the wild turkey has risen from zero in 1950 to an estimated 20,000 in 2010. Following an initial introduction program, the birds have established themselves along the Missouri River and in the Pembina Hills, among other places.

  > NEBRASKA NATIONAL FOREST

  When Euro-Americans began to think about settling permanently on the bald-headed prairies, many people found the idea bewildering. How, they wondered, could a civilization that had risen in the dappled woodlands of Europe and eastern North America put down roots in a land that was unable to support a proper forest? The lack of trees on the prairies was widely seen as a mark of deficiency: no lumber, no fuel, no rain. No nothing.

  From the 1890s onward, countering this aspersion became the personal mission of a pioneering Nebraska botanist (the Johnny Appleseed of the plains) by the name of Charles E. Bessey. As Bessey saw it, the Great Plains grasslands represented the ruins of a prehistoric forest that had been brought low by bison and grass fires. If only the trees could be restored, he thought, the climate would improve— precipitation would increase—and life on the plains would be easy.

  With this utopian vision in mind, he and his colleagues set out to plant a mixed-wood (largely pine) forest on a tract of windswept sand and grass near Halsey, Nebraska. Although Bessey died in 1915, his eccentric dream lived on, and in the 1930s his efforts were extended by the tree-planting brigades of the Civilian Conservation Corps. Today, the Bessey Division of the Nebraska National Forest sprawls over 31 sections (80 square kilometers) of land and provides habitat for a wealth of woodland species that would not otherwise be present in the Sand Hills, including cardinals and other songbirds, wild turkeys, and white-tailed deer.

  Unfortunately for Bessey’s project, however, his namesake forest cannot sustain itself and has to be augmented through planting and other human interventions. In the end, it seems that the grasslands were not a failed forest after all but a natural and fitting response to a midcontinental climate.

  The list of woodland birds that have extended their breeding range on the prairies since settlement began includes such once rare and now at least locally common species as tree swallows, mourning doves, downy woodpeckers, house wrens, chipping sparrows, black-billed magpies, American crows, American robins, merlins, Mississippi kites, and red-tailed hawks, among many others. The combined effect of these changes is suggested by a recent study of the birds in northeastern Colorado, which showed that almost 90 percent of the species found in the area today did not occur there at the turn of the twentieth century. The vast majority of the newcomers are highly adaptable, go-anywhere woodland species with broad distributions in other parts of the continent and no special affinities to the prairie region.

  Sexual Tensions

  Although most of us would not want to do without the robins bobbing on our lawns, the nuthatches hanging from our feeders, or the chipping sparrows chittering in the shrubbery, the abundance of woodland birds on the prairies—and the ecological changes that they represent—are also a subtle menace. What appears at first glance to have been an unequivocal enrichment of life turns out, on closer examination, to pose a covert threat to the specificity, the very prairieness, of the prairie region. For instance, in the old days, before woodland birds could leapfrog across the grasslands at will, the open plains (especially the sparsely treed western rangelands) acted as a barrier between eastern and western species. On one side of the great grassy divide, in the ecotone along the eastern hardwood forests, for example, one would find species such as blue jays, eastern bluebirds, and indigo buntings. On the other side, toward the mountain forests of the west, were closely related birds that occupied much the same ecological niche as their eastern counterparts yet were obviously different: Steller’s jays (with black crests rather than blue), mountain bluebirds (with blue breasts rather than red), and lazuli buntings (with rich splotches of cinnamon adorning their plumage). Presumably first separated thousands of years ago, at the time when the Ice Age forests were forced off the Great Plains and the reign of grasses began, these populations had been held apart long enough for each to develop its own genetic specializations. Thus, merely by preventing regular contact, the grasslands had driven the process of evolutionary change and sparked the creation of new species.

  No more. These days, with riparian woodlands extending into the prairie heartland along the Platte River, and woodlands dotted hither and yon, eastern and western species are meeting head on. Of a total of thirteen east-west pairs of woodland birds that converge on the Great Plains, seven have been studied in depth and all have been found to interbreed freely along their zone of contact. In some cases, the interchange appears to be stable, suggesting that the two gene pools have been in intermittent contact over the centuries and now coexist peacefully. For example, the yellow-shafted (eastern) and red-shafted (western) phases of the northern flicker meet and merge in the Missouri borderlands of Montana and North Dakota, where they produce orange-shafted offspring just like the ones that John James Audubon saw in the same region in 1843. By contrast, Baltimore and Bullock’s orioles have been jostling back and forth across the prairies for the last twenty-odd years, with Baltimores extending 120 miles (200 kilometers) west across the Canadian prairies and up the Platte River, and Bullock’s orioles pushing strongly eastward across southern Kansas. Along this fluid zone of interaction, the two species interbreed, producing birds with mix-and-match markings and raising the possibility that the two distinctive populations might eventually merge into one. If this were to happen, the diversity of the world’s bird life—the range of answers that life can offer to the questions posed by change—would be incrementally diminished.

  > IN THE
HYBRID ZONE

  This chart (adapted from a study by Paul Ehrlich and his associates at Stanford University) lists thirteen pairs of closely related species of birds that were isolated on opposite sides of the continent by the emergence of the Great Plains grasslands some 10,000 years ago. Long held apart by un-bridgeable expanses of prairie, these species have recently been brought back in touch by the expansion of deciduous woodlands. Although some of these long-lost cousins may have become sufficiently different from each other that they cannot interbreed, others are still similar enough to hybridize freely.

  Facial markings of the red-shafted flicker, left; yellow-shafted-flicker, center; and their hybrid, right

  Meanwhile, another group of prairie birds is already vanishing, and the influx of forest species has likely not been helping. The grassland specialists— ground-nesting birds like meadowlarks and dickcissels, pipits, bobolinks and short-eared owls—have suffered dramatic population losses during the past forty or fifty years, and probably for longer. (The available data only go back to 1966, when the annual Breeding Bird Survey, or bbs, was initiated.) During that period, the “grassland bird indicator,” a composite measure that reflects the populations of two dozen prairie-loving species, has declined by almost 40 percent. In part, this trend is the result of loss of habitat to agriculture and urbanization. But the birds are also declining even where significant fragments of native grasslands still exist, and the encroachment of woody plants is a likely culprit.

  The bright, melodious song of a male western meadowlark announces his presence to the world. Though still common and widespread, both western and eastern meadowlarks have suffered declines during recent decades.

  In a recent study, researchers working in the cattle country of northwestern Oklahoma compared the advancing tide of woody vegetation (as revealed by a thirty-year sequence of aerial photographs) with the downward trend in grassland birds (as revealed by thirty years of bbs data). They detected a strong, inverse relationship between the two variables. The more trees and shrubs, the fewer grassland birds. But does this relationship hinge on simple appropriation of space, or are other, more devious interactions also at play? Could it be, for example, that the spread of trees has tipped the ecological balance against grassland birds and in favor of their enemies or competitors?

  In their search for answers, researchers have pointed the finger of suspicion at one of the prairie’s own native-born residents. The brown-headed cowbird, or “bison-bird,” as it might better be called, is a small-to-medium-sized member of the blackbird tribe. (Females are drab; males brown headed and splendidly iridescent.) In the olden days, cowbirds lived with, and even on, the bison, following them wherever they wandered and feeding on the insects that flew up from their hooves. Although food was seldom wanting, this nomadic lifestyle did impose a considerable cost: the birds had no chance to settle down, build nests, and raise young. Instead— remarkably—they acquired the ability to drop their eggs in the nests of other songbirds and fly off, leaving their offspring for somebody else to incubate, feed, and bring up. Accordingly, cowbirds are classified as obligate brood parasites, because they are entirely dependent on other species to perform nursery duties.

  The obvious losers in this transaction are the unwitting hosts, which often put so much effort into feeding their greedy guests that they cannot meet the demands of their own young. In one instance, only 12 percent of meadowlark chicks were found to have fledged in parasitized nests, compared with 26 percent in broods with no cowbirds present. Yet up to a certain point, this interference does not pose any real threat, since the hosts have had thousands of years to adapt to nest parasitism. (Even 12 percent of a suitably large clutch may be all that a species needs to keep its numbers up.) But if the rate of parasitism increases, the balance will shift and the host species will slowly but surely begin to slip.

  This appears to be exactly what happens, in some instances, when woody plants expand into the grasslands. Each intrusion increases the length of the “shoreline,” or edge, along which grass meets woods, the very zone in which opportunists like the cowbird are most successful. The nearer a nest lies to a stand of trees, the greater the chance that a cowbird will find it and come calling. This may be either because cowbirds concentrate their search along grassland-woodland edges (where they have the best of two habitats) or because they perch on exposed branches, overlooking the prairies, as they search for places to drop their eggs. In either case, improved success for cowbirds will likely come at the expense of already beleaguered grassland specialists. Thus, the apparently thriving populations of dickcissels on small, brush-invaded prairies in southwestern Missouri are suffering from poor reproductive success, probably because of nest parasitism.

  If You Plant It, They Will Come

  Cowbirds are not the only danger that lurks in the shrubby interface between grasslands and woods. These productive ecotones also provide habitat for a variety of mostly small, nosy, nest-raiding mammals, including skunks, weasels, raccoons, foxes, opossums, and Franklin’s ground squirrels. Again, the greater the penetration by woody plants, the greater the threat to ground-nesting birds. (Increased predation by small mammals appears to be a major factor in the decline of some grassland specialists.) For the edge-loving mammals, by contrast, the altered face of the prairies, with its patchwork of grass and brush, offers an irresistible wealth of resources. The whole place seems to shout, “Come and get it!” The list of mammalian predators that have responded to this call and expanded their range includes raccoons and opossums (which have been heading north), eastern spotted skunks and gray foxes (slowly pushing west), cougars (heading east along major watercourses), and red foxes (striking out wherever they can in every direction).

  It is not only predators that have benefited from the increased interspersion of wooded habitats. A whole range of other mammals—from tree-roosting red bats and tree-climbing white-footed mice to tree-gnawing porcupines— have also taken advantage of the shifting tree line. Fox squirrels, a bright-eyed, bushy-tailed tribe of arboreal rodents that was once restricted to open woods along the easternmost fringe of the plains, have pushed steadily north and west in recent years, scampering from river valley to farmyard to city street. By 1979, for example, they had established themselves in Regina, by way of the Souris River. Since then, they have expanded east up Thunder Creek and north across the Qu’Appelle River, to within calling distance of the boreal forest. Meanwhile, their cousins in Nebraska have been pushing west along the South Platte River to occupy the shelterbelts and riparian forests of eastern Colorado. If you plant a tree, you end up with a squirrel.

  The fox squirrel subsists on a diet of acorns, supplemented with green shoots, flowers, insects, and even the occasional frog. Acorns and nuts are buried as food for the winter, and any that are left grow up to renew the forest.

  But perhaps the most remarkable story of all is that of white-tailed deer. Now a familiar sight almost everywhere on the prairies, these alert, graceful creatures were originally native to the hardwood forests of eastern Canada and the U.S., where they made themselves at home in grassy meadows and open woods. As recently as the mid-1800s, this was the white-tails’ world and they seldom, if ever, ventured westward onto the Great Plains. Although a small population seems to have colonized the riparian forest of the Missouri River— prime habitat, one might think—they were rare even there and were seldom noticed by early exploration parties. Instead, the journals of prairie exploration are filled with references to the abundant herds of elk that, in those halcyon days, flourished in the riparian forests and hill country. (Although the prairie subspecies of elk was decimated in the 1880s, due to hunting and habitat loss, Rocky Mountain elk have since been introduced to the Black and Cypress hills and several other refuges. The native subspecies persists in parts of southern and central Manitoba and Saskatchewan.)

  The mule deer, above, can be recognized by the black markings on its tail and by its big, muley ears.

  It was only when
the explorers struck out across the grasslands that they really started to notice deer: a small herd of does at the mouth of a brush-filled gully, a lone buck silhouetted along the crest of a rocky ridge. But even here, these sightings were not of white-tails. These animals were mule deer, a typically western species that ranged—and still ranges—from the woody draws and breaks of the prairies through the Rockies and all the way to the Pacific. Most at home in rough, open country marked with tangles of scrubby brush, the mule deer is characterized by its large, alert ears and hair-trigger nerves. It is perpetually poised to bound, or “stott,” with its peculiar, stiff-legged gait, up and over obstacles and out of harm’s way.

  The white-tailed deer, by contrast, is adapted to a gentler landscape of meadows and leafy groves, where a frightened deer can melt into the shadows. At the first hint of danger, the flaglike tail shoots up, sending a conspicuous, waggling warning to friends and relatives. And away they all run, into the nearest woody cover. (Female white-tail and mule deer both typically live in same-sex groups, often with their mother, sisters, and aunts. Bucks tend to be solitary, except during the rut, and seldom associate with close relatives.) Although of little benefit on open grasslands, this defensive strategy has proven remarkably well suited to the mosaic of croplands, hay fields, and scattered groves that has sprung up on the prairies since settlement. Despite a turn-of-the-century crisis, in which both mule and white-tailed deer were almost exterminated by overhunting, white-tails have come back in abundance and extended their range across the Great Plains and beyond. They are now by far the most numerous wild ungulate in North America, with a continental population that likely stands at four or five times what it was when the Mayflower landed.