So even though they spent the last half million years or so on opposite sides of the Atlantic, bison and cattle are fundamentally alike. Removing wild American bison and replacing them with tame Eurasian cattle—though a stunning act of hubris—was ecologically relatively neutral, the substitution of one large, ruminant herbivore for another on a landscape that had sustained large herds of grazers since the retreat of the glaciers. (The same cannot be said for the desert and semidesert grasslands west of the mountains, which had not evolved under heavy grazing pressure from bison and which have suffered serious damage since livestock were introduced.) The ease of this transition is demonstrated by the suite of animals that once lived in, under, and around bison pats. As nearly as anyone can tell, this group, which includes several dozen species of dung beetles and several specialized mites among many others, took to their new diet of cow pies without hesitation. And the species that dine on the dung dwellers—a diverse group that includes blackbirds, small rodents, and (in the tall-grass prairie) the beautiful little ornate box turtle—also seem to have made the switch without any noticeable problems.
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> HOOT, BOOM, STRUT, DANCE!
Four species of grouse are native to the Great Plains: the sharp-tailed and greater sage grouse, and the greater and lesser prairie chickens. Despite the differences in their common names, they are so closely related that, where their ranges overlap, they occasionally interbreed and produce fertile hybrids.
Historically, the areas of overlap were significant. Greater sage grouse, which, as their name suggests, prefer the dry, sage-dominated grasslands of the far west, nonetheless formerly occurred as far east as Nebraska. Prairie chickens, by contrast, were most common in the lush tall- and mixed-grass prairies of the eastern plains, but their range extended west to Alberta and Texas. (Of the two species of prairie chickens, the greater is found from the central plains north, while the lesser has a more southerly distribution.) The sharp-tail held the middle ground, occupying a variety of short-grass, mixed-grass, and grass-shrub habitats from central Canada to Oklahoma.
Although sharp-tails are still common throughout much of their range, their southernmost populations have vanished in the last hundred years, the combined result of overhunting and habitat loss. Sage grouse and prairie chickens persist as scattered and declining populations on fragmented habitat, and their long-term survival is in question. But who could bear to lose them, if only because of their antic, exotic, ecstatic performances? In the cold dawn of early spring mornings, the male grouse gather (each species in its own group) to compete with each other for status and access to females. First, they establish themselves on an open piece of prairie (stubble or closely grazed grass), where each bird stakes out a miniature territory for himself. Within this ritual domain, he faces off against his neighbors in a twirling, gobbling dance, puffing up pouches on his neck or chest to maximum effect. The object of these strange proceedings is to claim a territory at the center of the lek and thereby impress the hens as a top-quality prospect. The dominant male in the lek fathers most of the chicks and makes his contribution to the survival of the species.
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How Grazing Shapes Grasslands
Ornate box turtle
This is not to say that the introduction of cattle to the Great Plains has been completely benign. It has not. Cattle and bison, though similar, are not identical. A case in point: bison like to throw themselves on the ground and flail around in the dirt, a self-care routine that is thought to coat the skin with dust and offer protection from biting insects. In the process, they wear away shallow bowls, or “wallows,” in the earth. By rubbing out the grasses from these hollows, bison create openings for other kinds of plants, often moisture-loving species such as prairie dropseed, together with various rushes and sedges. Because they increase the diversity of plants that are available for shelter and food, bison wallows also augment the diversity of insects, birds, and mammals that the local landscape can support. And if the depressions fill with water (as they may do in the spring), they provide seasonal habitat for aquatic insects and water-loving shorebirds. Or at least they used to.
Because cattle do not wallow, this dynamic has been lost. Meanwhile, cattle have pushed the ecosystem in new directions. Less well adapted than bison to extremes of temperature and drought, cows spend more of their time in the shelter of trees and around sloughs and watercourses. This behavior severely affects both water quality and the condition of riparian, or bankside, forests. What’s more, although cattle and bison are both grazers with a special fondness for grass, there are subtle but significant differences in their diets. Bison prefer a steady diet of grasses, with just a garnish of other plants (forbs and shrubs make up only 5 to 10 percent of their diet). Cattle, by contrast, choose grass as their staple food but they also enjoy a side salad of mixed greens (forbs and shrubs provide between 20 and 40 percent of their feed). Although no one knows how the vegetation of the rangelands has changed since the bison were displaced—which grasses and forbs have become more common and which more scarce—Bossy the Cow’s exotic dietary preferences must have had an impact.
Herbivory shapes grasslands. The first principle of range management is that grazing causes shifts in the relative abundance of the many and various species of grassland plants. In mixed-grass prairie, for example, blue grama, buffalo grass, and pasture sage tend to become more common on range that is grazed by cows; these plants fall into the class known as increasers. Conversely, midheight grasses such as needle-and-thread and western wheatgrass become more scarce and hence are called decreasers. (It has even been suggested that the short-grass prairie, with its mat of increasers like blue grama and buffalo grass, is really just mixed-grass prairie that has been subjected to heavy grazing.) Some plants—like the big, bold butterfly milkweed of the tall-grass prairies—vanish from the range the second they appear because the cows enjoy eating them. Out in the pasture, grizzled rangemen shove their Stetsons back off their brows and lean against their pickups to discuss the status of these “ice-cream plants” in their pastures.
Until fairly recently, range managers have worked on the comforting assumption that the disruptions caused by grazing could be corrected simply by removing the cattle and letting the pasture rest. This belief was based on what seemed at the time to be a sure basis in science: the cogent theories of a pioneering ecologist from the University of Nebraska, Frederic E. Clements. As Clements explained it in the 19-teens and 1920s, mature, natural grasslands were the endpoint, or climax, of a linear process of plant succession. This climax community, he said, represented a state of equilibrium in which the conflicting needs of the various plant species (their demands for nutrients, water, and light) were held in such fine balance that the grasslands, if left to themselves, would wave on unchanged forever. But any disturbance, such as heavy grazing, fire, or drought, upset this steady state and sent the system tumbling back into an immature and unstable stage. Cattle, for example, not only evicted the ice-cream plants and decreasers from the climax community but in so doing opened the prairie to early successional species such as broomweed and Russian thistle. These same “weedy,” or invasive, plants were always first on the scene when plowed lands were abandoned or pastures were degraded by overgrazing.
By rolling, or wallowing, in the dust, bison wear hollows in the earth, creating seedbeds for plants and catch basins for water.
Arthur Savage photo
Butterfly milkweed flourishes along roadsides in the Flint Hills of Kansas but is conspicuously absent from adjacent pastures, where cattle can get at it. Like other species of milkweed, this one is also a favorite food plant of butterflies, notably monarchs.
The good news, as Clements saw it, was that plant succession was a predictable and reversible process, in which pioneering species inevitably created conditions for their successors, which created conditions for their successors, and so on, until the climax community was ultimately restored. Thus, any damage caused to the rangela
nds by overgrazing could be repaired, in due course, through the healing processes of nature. If only this were so. Instead, it turns out that succession is more fickle than the good professor believed—more subject to the vagaries of climate and simple luck. Once the prairie has been disturbed, there is no great likelihood that it will grow back in, plant for plant, just as it was before it was disrupted. It is even possible that it will not come back at all. In the last hundred years, for example, large areas of the southwestern United States have been permanently converted from grassland into a prickly shrubland of mesquite and creosote bush, probably as a result of heavy grazing during the 1800s and 1900s.
Don’t Fence Me In
But the main ecological challenge on the rangelands—the real thorn in the side of the ecosystem, so to speak—is not the absence of bison or the presence of cattle. It is barbed-wire fencing. The “devil’s rope,” as it was once known, went into mass production in the mid-nineteenth century to meet the needs of settlers moving onto the treeless (post-and-rail-less) plains. By the early 1880s, factories in Illinois were spinning out enough barbed wire every year to make twenty-five complete twists around the equator. Not surprisingly, the Great Plains were quickly crisscrossed with fences, and by the close of the decade, the era of the open range had ended. With it went the free interaction between grazers and grasslands.
Unlike most of the wild flowers on the plains, the prairie sunflower is an annual rather than a perennial. It is often one of the first species to colonize a site following a disturbance.
Leafy spurge hawkmoth
Before fences, there had been movement. Every spring for millennia, the bison had poured out of their winter ranges on the foothills and parklands, massed together in vast herds, and flowed across the continent on a quest for greener pastures. Like the barren-ground caribou of the tundra, they had charted new routes each year, jogging east to track recent rainstorms, north to follow the spring, south to savor the green-up after a fire. But wherever their travels took them, they always looped west again in the fall and followed a halting, zigzagging course back toward their wintering grounds. There, in the dead of winter (when the tall grasses of the eastern plains had little value as food), the bison sustained themselves on a steady diet of sun-cured, protein-rich blue grama and buffalo grass. Thus, season by season, the bison tracked the resources of the plains and responded to the interactions of the climate, the soil, and the plants.
What happened wasn’t always pretty. Even with the freedom to move at will, tens of millions of bison couldn’t always find enough to eat. In fact, the fossil record suggests that the species disappeared from the plains entirely for the better part of 3,000 years, between about 4,500 and 7,500 years ago, when the climate was much drier and warmer than it is at present. More recently, during the ferocious winter in 1876, several million bison starved to death along the Brazos River of southern Texas, so many that their humped carcasses reminded one observer of a field of pumpkins.
But if bison sometimes felt the sting of random misfortune, they could also dish it out. After a herd of bison had trundled across the prairie, the prairie generally looked as if a herd of bison had trundled across it. (People traveling in the wake of a herd often complained that they couldn’t find so much as a blade of grass for their horses.) Fortunately, grasses are uniquely equipped to put up with this kind of abuse. Unlike shrubs and forbs, which produce new growth from buds on their branches and stems, grasses shoot up from buds at (or even slightly under) ground level. This means that if a grass is eaten, it can immediately send up fresh leaves, whereas a shrub or a forb may be done for until the following spring. What’s more, grasses that have been defoliated often shift hormones and other resources from their roots to their new shoots, so that their rate of photosynthesis increases dramatically. The loss of leaves to a herbivore causes some prairie grasses to grow so exuberantly that they slightly overcompensate for the tissues that have been removed. It probably doesn’t hurt that the saliva of some grass-eating mammals contains growth stimulants that encourage the grasses to send out new shoots.
Crested wheatgrass
Bluebunch wheatgrass
Not all grasses are equally well equipped to cope with the stress of being chewed. As a rule, increasers, a.k.a. grazing-tolerant grasses, have a greater capacity to fight back than do decreasers. For example, crested wheatgrass—an introduced species from Asia that can be recognized by its handsome, fishbone seed heads—puts out replacement leaves two or three times faster than does its native relative, bluebunch wheatgrass. This competitive advantage helps explain why crested wheatgrass has been able to crowd out native plants and establish itself across the plains at their expense.
To get the true measure of the relationship between grasses and grass-eating mammals, it helps to see the natural dynamics that evolved between them. Today, this is only possible in the few places on Earth where migratory herbivores are still free to roam, such as the tropical grasslands of the Serengeti Plains in East Africa and the temperate grasslands of Yellowstone National Park in Wyoming. Despite the obvious differences between these two ecosystems, researchers have discovered a surprising similarity between them. In both places, grasses that are grazed, each in their season, by migratory herds produce far more forage than do grasses that are protected from grazing (43 percent more in Yellowstone, 70 percent more in the Serengeti, according to one report). This bonus far outstrips any “overcompensation” that occurs when animals are confined within fenced pastures. (All the same, the management of cattle in fenced pastures does have its advantages, which we will come to in a moment.)
The unexpected synergy between migratory herbivores and grasses is probably the result of a chain reaction that begins when the leaves are cropped. The grass plant responds to this insult by shedding organic molecules from its roots. These molecules provide food for soil microbes, which go into overdrive processing humus and releasing nutrients in a form that plants can use. Thanks to this input of fertilizer, the plants experience vigorous growth, and the range is superabundantly replenished. But this response only appears to happen if the herbivores are free to migrate and select grasses in the juicy prime of youth. If the stress of herbivory is delivered in a well-timed pulse, the plants may actually benefit from it.
Although sometimes found in dry hay fields, LeConte’s sparrow is typically associated with wet meadows, where it nests on or near the ground and scurries through the matted grass like a mouse. This individual had taken up residence on the Manitoba Tall Grass Prairie Preserve near Gardenton, Manitoba.
Did the bison and the prairie grasses once engage in a similar interaction? No one knows for sure. But the loss of this cyclical rhythm may be the price we pay for curbing the wild prairie with barbed-wire fencing.
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> ALIEN INVADERS
Since the mid-1800s, hundreds of plant species have been plucked from their native surroundings, carted halfway around the world, and introduced into the grasslands of North America. Although many of these newcomers have to be coaxed along, others have flourished in this home-away-from-home. For example, Eurasian grasses such as crested wheatgrass, smooth brome, Kentucky bluegrass, and tall fescue were initially planted on restricted acreages to provide forage for cattle or (during the drought of the 1930s) to protect the soil from erosion. Since then, however, these exotic species have advanced beyond all bounds, out-competing the native species and eventually crowding them out.
Happily, these intrusions are not always a dead loss. To a herbivore looking for fodder or a songbird building a nest, grass is a useful resource, whatever its evolutionary origins. So even though these plants arrived as intruders, many of them have become contributing members of the ecosystem. But there are notable exceptions. Leafy spurge, for example, is thought to have arrived from Europe in 1827, as unspecified cargo in the water ballast of a sailing vessel. Freed from European diseases and insect pests, the newcomer laid claim to the continent, infesting both tilled fields and overg
razed pastures. Even today, leafy spurge continues to advance, especially across the northern rangelands. Not only does it outcompete native plants for nutrients, water, and light, it also exudes toxins from its roots that prevent other plants from putting up shoots. What’s more, it is defended against herbivores by poisons in its leaves, which are repellent to cattle, horses, and many wild species. Among the few animals that can eat it are certain species of European flea beetles and caterpillars that have been introduced to limit the plant’s spread, but even they cannot eradicate it. Resistant to every known method of control, leafy spurge—and other plants of the same ilk—are a menace to grassland ecosystems.
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