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Promoting Patchiness
On the open range, bison (and cattle, for that matter) had gone wherever they chose and moved on whenever they liked. If they traveled through an area quickly, snatching mouthfuls on the run, the grasses were left standing in tattered clumps. If they lingered, the prairie was reduced to a worn-out, close-cropped lawn, where whatever remained uneaten was trampled or splattered with dung. These impacts occurred haphazardly, tracking the uncertainties of rain, lightning, and fire, to say nothing of the whimsies of the bovine mind. Once a herd had passed through an area, it might return in six months or five years or never again. As a result, the Great Plains were an ever-shifting mosaic, in which a patch of tall, ungrazed vegetation might stand next to a mouthful that had been cropped to the quick. On a larger scale, broad tracts of land might lie untouched (too dry to attract attention), while the green-up after a sweeping prairie fire attracted large herds of bison. Thus, at their most vibrant, the grasslands were a kind of living crazy quilt, with patches of vegetation of varying sizes in varying states of recovery from grazing, fire, or drought. This is the true climax condition of the Great Plains grasslands, a system that achieves stability by responding constructively to continual challenges.
Bobolink
Grasshopper sparrow
Upland sandpiper
The patchiness created by grazing and other disturbances provides a diversity of habitats for a diversity of prairie residents. (Under present-day conditions, the rotation of cattle from one fenced pasture to the next can be used to simulate this richness and complexity.) Take birds, for example. Of the 260-odd species of birds that regularly breed on the Great Plains, most are species that moved onto the grasslands from other (mostly forested) parts of the continent at the end of the Ice Age. By and large, these immigrant species still cling to the traditions of their ancestral homelands and nest in trees and shrubs. But a small, select group of species—a couple of dozen in all—prefer the open plains, where they nest in, or under, the forbs and grasses.
Within this Lilliputian jungle, the prairie-adapted species seek out the special conditions that each needs for its nests. Some, like the bobolink—a flashy black-and-white blackbird that enlivens the mixed- and tall-grass prairies of the north-central and southern plains with its bubbling song—require dense and expansive stands of thigh-high flowers and grass. The female typically chooses a site that is shaded from the grilling sun (often at the base of a large forb, such as meadow’s rue) but where the canopy is open enough to let light filter through. Here, in the cool, dappled shade, she scratches out a patch of bare earth and constructs her finely lined, cup-shaped basket. By contrast, the grasshopper sparrow—a widely distributed but inconspicuous “little brown job” with a shrill, insectlike buzz—opts for more open habitats, with scattered clumps of tangled, mixed-height plants. Its nest is buried in a thicket of vegetation and domed over with grass, an object lesson in the arts of concealment.
Each species looks at the grasslands from its own unique point of view, seeking patches of vegetation that are the right height, density, and structure to meet its needs. Thus, although the grasshopper sparrow and the maddeningly similar Savannah sparrow both are found in open habitats, the former chooses sites with more patches of bare dirt (for catching insects), the latter those with less. And so it goes. The chestnut-collared longspur—so called for the rich splotch of color on the neck of the breeding male—seeks out closely cropped rangelands with the odd rose bush or tuft of grass for nesting cover. Meanwhile, its drabber but doughtier cousin, the McCown’s longspur, selects only the most barren prairie, where it nests in the shade of a prickly pear cactus or a cow pat. And the horned lark—another, quintessential short-grass species, much loved for its sweet, slurred song—prefers patches that have been thoroughly trampled, where it can hunker down in the dirt with little more than a stone or a tuft of grass for shelter.
Trying to remember the particulars of which species needs what and why taxes the memory. But for millennia, the turbulent dynamics of the prairies have provided for all these conflicting needs without any conscious intention or evidence of strain. Habitat simply happened. For all its chaotic uncertainties, the prairie ecosystem generated enough stability to sustain even those species with the most complex and demanding needs. The upland sandpiper, for example—a jaunty member of the shorebird tribe that has adapted to life on the arid plains—requires short vegetation with visible perches while courting, knee-high growth with a moderately open structure for nesting, and closely grazed pastures for feeding and rearing its chicks. What’s more, these resources must be provided generously enough to support not only a single pair but a loose colony of breeding birds. The sight of an upland sandpiper perched on a fence post is mute proof that these demands have been met in one way or another for thousands of years.
Today, however, two of the main forces that drive the prairie ecosystem have been muted. Wildfire has been suppressed. (On the tall-grass rangelands of Kansas and Oklahoma, annual burns still occur, but they are generally timed to promote the growth of grass and to reduce the diversity of “weeds” and other flowering plants.) In addition, the migratory ebb and flow of herbivores across the plains has been halted. Fortunately, even when confined by fences, cattle help to maintain patches of vegetation—clipped versus tall, open versus dense—and this effect can be enhanced by implementing an appropriate regime of management. By manipulating variables such as stocking level (number of cattle), season and duration of grazing, and rotations of grazing and rest, ranchers can manage the prairie to provide an array of habitats. The best and wisest land managers do exactly that because they understand that rangelands with a natural diversity of vegetation will outproduce and outlast those that are reduced to homogeneous spans of grass. But even with the most beneficent intentions, it would be a tall order to manage grasslands for commercial cattle production and, at the same time, to maintain the prairie in its free flow of creative change.
Dickcissel
There is little doubt that the populations of many grassland birds have declined in the last two hundred years. There is no doubt at all that those declines are continuing today. Data from the Breeding Bird Survey (a standardized count of birds that has been conducted annually since 1966) document recent declines in 32 of 37 grassland species. For many of them—including the bobolink, horned lark, and grasshopper, Savannah, and vesper sparrows, together with other favorite prairie species such as longspurs and meadowlarks—the losses are statistically significant. This finding removes any hope that the downturns are merely random blips on the graph. Overall, grassland birds have suffered steeper, more consistent, and more geographically widespread declines than any other group of birds in Canada or the United States. The causes of the losses are no doubt complex and likely include the continued conversion of native prairies to croplands and housing estates. (The less native grassland, the fewer grassland birds.) But these doleful statistics also help underline our failure, as a society, to reward ranchers for managing their land for “patchiness,” or biodiversity, and to pay them for producing bobolinks as well as T-bone steaks.
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> THE ROCKY MOUNTAIN LOCUST
Before the introduction of cattle, two very different herbivores dominated the Great Plains. One—the bison—is well known, but the other remains something of an enigma. A grasshopper in everything but name, the Rocky Mountain locust once made its permanent home on the sparse, dry grasslands of the west-central plains (from Alberta and Saskatchewan south to Colorado), where it fed on a mixed diet of buffalo grass and other native plants. Most of the time, it looked and acted like a typical hopper, with relatively short wings and little inclination to travel. But under certain conditions—for example, when the population was stressed by drought—a different breed, or phase, of the species began to hatch, one with longer wings and an unstoppable urge to migrate. Soon, silvery swarms of insects were moving east on the prevailing winds, rising to heights of 6,
000 to 10,000 feet (2,000 to 3,000 meters) and sweeping across large distances in a matter of hours. (Grasshopper Glacier, in the Beartooth Mountains of Montana, contains the remains of several million locusts that became embedded in the ice sometime in the distant past, probably when one of these migrating swarms was brought down by turbulence.)
Because the insects could not sustain themselves for long away from their usual range, these eruptions generally ended within a year or two at the most. But for the farmers who moved onto the plains in the 1870s, the year of the locusts could seem very long. The insects fell on gardens and crops like a Biblical plague, eating fruits, vegetables, grains, even the bark off the trees. Then, in the 1880s, the outbreaks inexplicably became less frequent and less widespread. By the early 1900s, the Rocky Mountain locust—a species once so plentiful that it had darkened the skies—had completely vanished.
No one knows why this happened, though the best guess is that cropping, haying, and trampling by cattle disrupted the moist swales and stream-edge habitats that the females needed for laying their eggs. But although the Rocky Mountain locust, Melanoplus spretus, is extinct, its lineage lives on in the lesser migratory grasshopper, M. sanguinipes. The swarming form of this species can sometimes still be seen high in the summer sky, as a vague shimmering blaze of silvered light.
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Food for Sex
Although cattle are the most conspicuous herbivores on the range, they are certainly not alone. Ounce for ounce or gram for gram, natural grasslands support a greater live weight of plant eaters than any other terrestrial ecosystem. (And this statement does not take into account the zillions of nematodes and other soil animals that dine on the plants from below.) The largest group of grassland herbivores, in numbers if not in mass, is insects. While a complete catalog of prairie insects has not yet been compiled, the list of the herbivorous species alone would certainly contain several thousand entries, perhaps even tens of thousands. There are insects that feed on plants by piercing through the cell walls and sucking juices out (aphids and “true bugs”); others that rasp a hole in the plant to obtain nourishing fluids (thrips); still others that are equipped to feed on nectar by lapping (bees), sponging (certain flies), or siphoning it up with their tubular tongues (butterflies and moths). Several groups have powerful jaws that permit them to gnaw on seeds, like the worker caste of harvester ants, or to feed on leaves and stems, like the hungry legions of sawflies, beetles, grasshoppers, and caterpillars. With their stabbing beaks, hairy tongues, jutting mandibles, and other specialized mouthparts, these insects look as strange, to human eyes, as the weirdest creations of science fiction.
Yet despite their otherworldly appearance, plant-eating insects are of down-to-earth importance in grassland ecosystems. On the one hand, like all herbivores, they prune and shape the vegetation, one miniature mouthful at a time, thereby contributing to the patchiness of the prairie. On the other hand, they inadvertently provide a generous source of convenient, bite-sized meals for a wide array of small-to-medium-sized meat eaters, including predacious beetles, ants, spiders, songbirds (even seed eaters start their young on an all-arthropod diet), kestrels and other small raptors, and a few oddball rodents—like the thirteen-lined ground squirrel and the northern grasshopper mouse—that are highly insectivorous. But the critical ecological function of plant-eating insects is also their most riveting. They often serve their food plants as sexual go-betweens.
Northern grasshopper mouse
To produce seeds, flowering plants must somehow transfer pollen from the male organ, or anther, to the top of the female organ, or stigma. While some flowers (including the closed, late-season blossoms of wild violets and narrow-leaved puccoons) can accept their own pollen and are thus self-pollinating, most must receive sperm cells from other members of their species. Unable to go a-courtin’, some of these cross-pollinated plants rely on the wind to distribute pollen for them. Self-pollination and wind-pollination are both common among grasses. Most wildflowers, by contrast, rely on insects (including beetles, beeflies, butterflies, moths, wasps, and bees) to carry their pollen from plant to plant in a transaction that could be summarized as food for sex. Insects that are enticed into flowers for a feed of nectar pick up sticky pollen grains, some of which they deposit in the next blossom that draws them in for a taste.
Although many nectar-and-pollen-feeding insects are generalists that visit a variety of different plants, some have very specific preferences. For example, of the 200 species of pollinating bees that are found on Missouri prairies, most are widely distributed in other habitats. But a significant proportion of the total—about 25 percent—occur only on native grasslands. Their allegiances are so specific that of the prairie specialists, 24 species work only the uplands, 13 species are restricted to sand prairies, and another 10 species are found exclusively on bottomlands. Different bees for different plants. Sometimes these alliances are easy to observe. In Missouri, for example, the handsome black-and-yellow bumblebee Bombus nevadensis is seen almost exclusively on the heads of wild bergamot, where it sends its long tongue into the equally long tubular corollas of the blossoms. By contrast, the all-yellow Bombus griseocollis, though more of a generalist, tends to favor plants like purple coneflower and other species of Echinacea that hold their nectar within reach of its midlength proboscis. A third species, Bombus impatiens, is attracted to the delicate fringe of blooms along the spike of Culver’s root, which can be penetrated by its short tongue. These teams of plants and insects have evolved together to meet the needs of both, providing the insects with a reliable (and sometimes almost exclusive) source of food, and the plants with an eager crew of sex workers. Through this symmetry of behavior and body parts, they have kept the grasslands buzzing and blooming for millennia.
A painted lady butterfly visits blue vervain flowers at the Konza Prairie Research Natural Area near Manhattan, Kansas.
A Prairie Dog Home Companion
After insects, the second-most populous group of herbivores on the grasslands is rodents. Although all the members of this tribe are equipped for a life of gnawing with a pair of big, self-replacing front teeth, different species have become specialized to gnaw on different types of plants. Pocket gophers, for example, are stubby, squirrel-sized mammals with small, weak eyes, that spend most of their lives beneath the surface of the soil. There, they subsist mainly on the roots of perennial forbs, such as yarrow and goldenrod, which they locate by tunneling through the ground. As they “swim” through the dirt, they first kick the soil behind them, then turn around and, using their flat foreheads, shove it back along the burrow and onto the surface. These mounds of loose soil, as tousled as unmade beds, provide openings for annual plants, like bluebur and small lupine, and warm, moist daytime hideaways for tiger salamanders and shiny, low-slung lizards called prairie skinks.
The “pocket” in the pocket gopher’s name refers to the fur-lined pouches on either side of the face, in which the animal carries loads of cut-up stems and roots. The same characteristics are repeated in miniature by the pocket gopher’s relatives, the pocket mice. These dainty creatures (tail and all, they’d fit into the palm of your hand) also excavate shallow tunnel systems and seal themselves in with dirt, but unlike pocket gophers, they venture out under cover of night to search for food. Their primary resource is seeds, which the prairie showers on them like the proverbial rain of gold. A stand of little bluestem, for example, will typically produce roughly 200 pounds per acre (about 200 kilograms of seed per hectare). To harvest this bounty, the sleek olive-backed pocket mouse (typically found on thin, sandy grasslands across the northern plains) sits up on its hind legs and sifts seeds out of the dirt, then stuffs them into its pouches with its forepaws. Weeds like Russian thistle and pigweed and native plants like butterfly weed and June grass are among its favorites. Seeds that aren’t eaten immediately are stockpiled underground. In the fall, the animals move their caches below the frost line and hole up, drowsy but still conscious, for as long as six
months. During all this time, they do not have to drink because they can extract water from their diet of dry seeds. Ord’s kangaroo rat, a bigger, bouncier, desert-adapted species found in the Great Sand Hills of Saskatchewan and other ultradry grasslands, shares this metabolic talent.
Plains pocket gopher
But of all the rangeland rodents—pocket mice, harvest mice, jumping mice, pygmy mice, ground squirrels, voles, and more—none is more remarkable than the black-tailed prairie dog of the western plains. Before the prairies were settled, this was the prairie dog’s world. Its population ran into the billions, and if all the colonies, or towns, had somehow been amalgamated into one great metropolis, they would have occupied an area as big as Texas. In reality, prairie dog towns were, and in places still are, dotted across the short- and mixed-grass prairies, moonscapes of closely cropped grass marked by craters of bare earth. Each of these earthworks marks the entrance to a shaft that plunges 6 to 10 feet (2 to 3 meters) below the surface and then levels out to lead to a complex of nest chambers and latrines. Prairie dogs descend into their burrows to sleep, give birth, nurse their young, and escape from predators, but they spend much of their lives on their earthworks—sprawled out on their bellies in the sun or upright and alert, ready to bark a warning at the first sign of danger.
Ord’s kangaroo rat
On a warm summer day, a prairie dog colony is alive with the hustle and bustle of urban life, as youngsters chase each other, neighbors meet and “kiss,” heads pop in and out of holes, and individuals waddle away from their mounds to graze on grass and forbs. But this apparently free-wheeling sociability masks a complex social order based on kinship. Each prairie dog lives in a small, tightknit family group, or coterie, that usually consists of one adult male, three or four breeding females, and nonbreeding youngsters. Young males disperse as juveniles to seek acceptance in another group, but females generally settle down in their natal coterie, close to their mother, sisters, and aunts. Although coterie members defend their space against intruders from neighboring clans, they live companionably among themselves, sharing burrows and other resources—with one significant lapse. During the birthing season, nursing females vigorously defend their nest burrows even against their kin, and for very good reason. The greatest danger to infant prairie dogs is their adult female relatives, who nip into unattended dens and kill the helpless huddles of pink pups. Yet once the pups emerge from their burrows, harmony is restored and adult females take turns nursing youngsters that, only a few days before, they would have attacked with vigor.
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