Prairie

by Candace Savage

  When feeding, the American avocet thrusts its bill underwater and swings it from side to side along the bottom, stirring up a flurry of tasty aquatic insects.

  Arthur Savage photo

  Short-eared owl

  But playas, above all, provide a haven for birds. During the breeding season, these unassuming wetlands serve as oases of nesting habitat both for marsh-adapted birds, such as ducks and red-winged blackbirds, and for a wide variety of typically terrestrial species, including short-eared owls, western meadowlarks, and ring-necked pheasants. The region also provides migratory stopover sites and wintering habitat for enormous numbers of waterfowl and shorebirds that breed elsewhere on the continent. Roughly 2 million ducks and geese crowd onto the playas each fall, including a gabbling contingent of mallards, northern pintails, American wigeons, and green-winged teal that nest in western Canada and the Dakotas. What’s more, playas provide off-season accommodation to flight upon flight of gurgling sandhill cranes—more than 300,000 in all, or 90 percent of the midcontinent population—which land on the mudflats and in the dwindling ponds to feed on the thick slurry of crustaceans. And this is still to say nothing of the stilt-walking American avocets, with their upturned bills, or the Wilson’s phalaropes, which spin round and round like tops to stir up food.

  Little understood and vastly underappreciated, the playas of the southern High Plains are a vital ecological resource that helps to sustain waterfowl and shorebird populations over much of the continent. This function is shared by a number of other equally important wetlands in other parts of the Great Plains, many of them, like the playas, out in the middle of the prairies. The Central Kansas Wetlands, for example, include a number of freshwater and saline marshes— notably Cheyenne Bottoms and the Quivira National Wildlife Refuge, both near Great Bend—that lead like stepping-stones through what is otherwise a vast plantation of corn, sorghum and soy. Located on a major midcontinental migration route, these wetlands annually play host not only to waterfowl by the millions but also to half of all the shorebirds in North America. Although significant populations of shorebirds breed in Kansas and elsewhere on the Great Plains, a much larger number beat northward to the Arctic coast, only to breed, turn around, and head back south, often to wintering grounds on the other side of the equator. Hence, the critical importance of refueling stops along the route.

  Wilson’s phalarope

  Just north of the Central Kansas Wetlands lies the Rainwater Basin of south-central Nebraska, a rolling landscape dotted with several thousand play-alike catchments. And then it’s on to the Platte River, with its sandbars and wet meadows, and, a little farther along, the clear, sparkling—utterly incongruous—ponds that spangle the Sand Hills of northwestern Nebraska. Filled to the brim by the Ogallala Aquifer (the vast reservoir of fossil water that underlies the south-central plains) and set against the silence of the grassy dunes, these wetlands are bursting with life and sound: the high-pitched complaints of long-billed curlews, the peeping of sandpipers, the metallic rattle of marsh wrens at nests in the cattails.

  Long-billed curlew

  The final entry on the list of major wetland resources on the Great Plains is the Prairie Pothole Region, an area of some 270,000 square miles (700,000 square kilometers, about the size of Texas) that angles northwest from north-central Iowa through western Minnesota and the Dakotas and across the Canadian prairies from Manitoba to western Alberta. This entire swath of country is pocked with millions of mostly small, knee- to waist-deep depressions— called potholes, or sloughs—that are a legacy of the most recent glaciation. As the glaciers decayed, chunks of ice became buried at random under layers of gravel and silt, and when these blocks eventually melted, the ground collapsed, turning ice mounds into small meltwater basins. In keeping with this Ice Age heritage, prairie potholes still depend not so much on rainfall as on snowmelt, trickling over frozen ground, to replenish their supply of water.

  Prairie potholes are proof of the adage that every little drop counts. Insignificant if considered in isolation, they gain significance when seen as elements of a sweeping, if fragmented, aquatic complex. The combined area occupied by potholes in the northern states, before settlement began, is estimated to have been 20 million acres (8 million hectares), giving them the same collective presence on the landscape as Lake Superior. And since 80 percent of the pothole region lies in Canada, this comparison barely hints at the wetlands’ original extent, much less at their ecological significance. Vitally important as a source of protein for both resident and migrant birds, these weedy, reedy marshes are perhaps most famous as the “duck factory” of North America. Although they only amount to about 10 percent of the waterfowl-breeding habitat in North America, they typically produce between 50 and 90 percent of all the ducklings that fledge in any given year. Clearly, prairie potholes are spectacularly productive. The obvious question is, how do they do it?

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  > BIRDS ON THE RUN

  Shorebirds, those long-beaked, long-legged stilt-walkers that feed on mud flats and shallow marshes, can be a tricky group for a bird watcher to bring into focus. They are like shimmering apparitions, caught in a dazzle of light and water. Almost frantically active, they peck, they probe, they wheel up and circle over our heads, sometimes uttering strange, quavering cries in their own strange languages. And then, faster than you can say, “black-bellied plover,” most of them have gone, either headed north to breeding grounds on the tundra or, a few short weeks afterward, flying south to far-distant climes for the winter.

  Of the thirty-seven species of shorebirds that have been sighted on the Great Plains, the great majority—twenty-three species, mostly sandpipers— were migrants noted in passing. Several of them, like the tawny little Baird’s sandpiper—no larger than a bluebird—make an annual round-trip journey of up to 20,000 miles (32,000 kilometers) from the High Arctic islands to the outer limits of Tierra del Fuego. Although Baird’s and several other species cut across the prairies on both legs of the trip, others—including the even tinier but otherwise confusingly similar semipalmated sandpiper—follow an elliptical route that crosses the interior of the continent on the northward journey and loops east to the Atlantic Coast for the return. In yet another variation, another close cousin, the western sandpiper, goes through this same pattern only in reverse, traveling north via the West Coast in spring and heading south across the Great Plains in the autumn. The result is a constantly changing playlist of shorebirds on prairie marshes and sloughs, as the sandpipers of the continent pass by us in revue.

  The remaining fourteen species of shorebirds, by contrast, are not driven to such extremes. Mainly short-to-medium-distance migrants, they tend to winter on the coasts of North or Central America and to nest in the temperate zone, including the wetlands of the Great Plains. This group includes such familiar birds as the killdeer, with its keening cry and broken-wing display; the American avocet and long-billed curlew, with their adroitly curved beaks (the former for scything through the water, the latter for probing); and the spotted sandpiper, the only “peep” that, as its name suggests, can easily be identified at first glance, at least during the breeding season. Spotted sandpipers are also noteworthy for their practice of polyandry, a mating system in which the male, as Mr. Mom, incubates and rears the nestlings, while the female freely pursues other matings and produces several broods.

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  How Prairie Potholes Work

  At first glance, a slough does not appear to have the makings of a biological powerhouse. What is it, in the final analysis, but a pool of stagnant water? Sunk into poorly sorted glacial rubble that inhibits organized flow, a typical pothole has only weak and fitful connections with the surrounding groundwater. Depending on the circumstances, any given pothole may receive water through seepage, lose it through underground leaks, or experience both input and outflow through different parts of the basin simultaneously. A slough that receives an influx of groundwater is also typically hit with a burden of dissolved salts
picked up from the underlying geological formations. If this briny solution is permitted to escape from the basin either through overbank flooding or underground leaks, the salts flow out with the water and tend not to accumulate. But if the basin has no outlet, the water gradually evaporates, the concentration of salts builds up, and the wetland becomes saline (with chlorides) or alkaline (with sulfates). Some prairie potholes are ten times as salty as the sea, surely pushing the limits of habitability.

  Sloughs also present a variety of other potentially life-threatening challenges. In the swampy warmth of midsummer, as submersed plants die back and rot, the water becomes starved for oxygen—if there’s still water left to starve. Like playas, prairie potholes are notoriously unstable, typically filling in spring and dwindling over a period of weeks or months, with little chance of renewal before the next spring thaw. How long water persists varies from pothole to pothole and from year to year, in response to such variables as the depth of the basin, flow to and from the groundwater, and the uncertainties of the weather. Depending on its duration, a slough may be classified as ephemeral, that is, a thin layer, or sheet, of water that lasts for a matter of days; temporary, a puddle that persists into early summer; seasonal, a pond that dries out annually but that often remains wet till fall; or permanent, a wetland that withstands all but the most severe droughts. (This last category includes such birding hotspots as Delta and Oak Hammond marshes in Manitoba, Last Mountain Lake and the Quill Lakes in Saskatchewan, and Beaverhill Lake in Alberta, all of which have been honored as Wetlands of International Importance under the un’s Ramsar Convention on Wetlands.) And these distinctions are all overridden by the insult of the northern winter, which closes in on wetlands regardless of their status and deprives them of light, warmth, and other necessities.

  One of the largest shorebirds in North America, the marbled godwit feeds by probing the mud with its long, slightly curved bill, searching for mollusks, crustaceans, and worms. It nests on grassy prairie near the edges of sloughs and alkaline lakes in the southern Prairie provinces and the northern plains states.

  Yet the wonder is that, in one way or another, life goes on. For example, lack of dissolved oxygen is a crisis for many species that breathe through their skin (like flatworms) or with gills (like dragonfly larvae, or nymphs) and that accordingly are restricted to only the most permanent and well-aerated sloughs. But certain other species are adapted to cope with anaerobic or near-anaerobic conditions. Some daphnia, or water fleas, for instance, respond to oxygen-starved water by pumping up their production of oxygen-grabbing hemoglobin, an adaptation that not only turns them a surprising shade of pink but also enables them to take full advantage of every available molecule of dissolved oxygen. Mosquito larvae, or wrigglers, by contrast, live in the water but obtain their oxygen by means of breathing tubes that poke into the air, on the same principle as snorkelers. Similarly, a variety of aquatic bugs, such as waterboatmen and backswimmers, rise to the surface and pick up loads of air, which they carry down with them like scuba divers. Often you can see them scooting around beneath the surface of the slough, all shiny and silvered with bubbles.

  Dragonfly and emerging nymph

  So far so good. But what happens when the very medium of pond life—the water—disappears? This is a routine crisis for prairie potholes (and other transient wetlands on the Great Plains) and one that, to a remarkable extent, evolution has turned to advantage. Through thousands of reiterations, the so-called wet-dry cycle has given rise to an ecosystem that not only is adapted to cope with severe hardship but depends on it. Here’s how it works. If a basin remains full of water for even two or three years in a row, the community of life within it begins to deteriorate, becoming both dramatically less diverse and less productive. The first sign of this degeneration is a gradual die-back of cattails, bulrushes, and other emergent growth—those plants that are rooted in the muddy bottom, push up through the pond, and emerge crisp and spiky above the surface. With continuous inundation, this once-vigorous foliage either disappears from the basin over a period of months or else recedes toward the margins.

  Although the causes of this die-off are often mysterious (aging rootstocks or lack of oxygen in the mud?), the process is sometimes accelerated by an assault force of muskrats. Where conditions are suitable—namely, in well-vegetated potholes that retain free water throughout the year—these plump little herbivores often increase to such extraordinary numbers that they “eat out” large areas of marsh vegetation. (They are basically aquatic field mice and just as fecund.) Their habitat destroyed, the rodents also crash, leaving the wetland doubly diminished.

  One loss leads to the next. Fewer emergent plants means less habitat for all the little creatures that live on and around their leaves and stems, including algae, tiny crustaceans, snails, and insect larvae. In addition, the elimination of emergent vegetation allows the wind to stir through the pond, thereby muddying the water and reducing the amount of light available to plants that grow beneath the surface, including the slimy, undulating stems of sago pondweed and coontail. As this submersed vegetation begins to die off in its turn, the marsh completes its degeneration phase and reaches its lowest ebb—the lake phase—a stage that is characterized by large areas of open water and relatively low production of plants, microorganisms, insects, and everything that eats them. And what doesn’t?

  Daphnia

  Waterboatman

  Backswimmer

  The slough persists in this depressed condition until it is revived by drought. To regenerate, it first has to dry out or, as the biologists put it, experience drawdown. Although the plants that are at home in prairie potholes typically require standing water to grow, their seeds are only able to germinate on terra firma. So while the brittle mud of a dried-out slough may look dead, it is actually a green-and-growing marsh just waiting to happen. A small square of exposed slough bottom (a square yard, or meter, on a side, dug to the depth of your thumb) can be expected to contain somewhere between 20,000 and 45,000 seeds for as many as two dozen species of plants. The exact nature of this seedbank depends on the history of the pond and the species that set seed in it during the preceding wet-dry cycle. Some of them—like the barbed fruits of nodding beggar-ticks—have been lying there since the last time the slough dried out, anywhere from one to thirty years earlier. As a mud-flat specialist and an annual, the nodding beggar-tick germinates at drawdown, produces a profusion of yellow blooms, and sets and deposits a stock of fresh seed. When water returns to the pond, the mature plants are drowned out, but the seeds of the future lie waiting in the ground.

  Floodwater mosquito

  For cattails and other emergent plants, by contrast, this same inrush of returning water is the stimulus for vigorous growth. Seedlings that germinated on the dry mud flat now sink their roots into the gooey muck, pushing out runners in every direction and sending up juicy young shoots. Then, once the marsh is fully reflooded, pondweed and other submergents begin to germinate in their turn, as the marsh flows into its regeneration phase. Meanwhile, various forms of animal life are also emerging from the mud, in a kind of ritual reenactment of creation. The first to crawl out of the ooze are water fleas and other tiny crustaceans that hatch from eggs, laid the last time the pond was wet, that have remained buried and dormant in the sediments. Some species are so deeply attuned to the wet-dry cycle that they cannot reproduce unless their eggs are first desiccated and then reflooded. This is also true of the “floodwater mosquitoes” (genus Aedes), which lay their eggs on plant stalks in dried-out meadows, leave them high and dry all winter long, and rely on spring flooding to revive them. The transition from dormant egg to flying, blood-sucking adult can be achieved in a mere ninety-six hours. Instant mosquitoes: Just add water.

  Once the water begins to flood into the pond, life returns in a rush, as these pioneering species soak up the nutrients deposited during the preceding degeneration phase, or die-off. How long this surge of life persists depends, more than anything else, on h
ow long water remains in the basin. If the slough only stays wet for a matter of weeks, the recovery is cut short and is limited to species that can hatch, mature, and deposit new eggs on a tight schedule. But if the wetland is seasonal or permanent—if it lasts for several months—the recovery is enriched by a wave of colonists. These are mobile species of insects, reptiles, amphibians, mammals, and birds that escaped from the drawdown by moving to deeper, more permanent sloughs and that begin to find their way back as the habitat improves. For example, many kinds of water bugs and beetles cope with the dying-and-reviving nature of wetlands by making regular reconnaissance flights, scanning the landscape for reflective surfaces of an appropriate tint and size. By responding to the glint of water, they can home in on regenerating habitat or, if their instincts happen to fail them, occasionally on the roofs of blue Datsuns! Sometimes they bring stowaways with them, in the form of aquatic mites and crustaceans that have tucked themselves under a wing and that join in the reconstitution of the marsh community. Ducks and other waterfowl also provide transportation from pond to pond, carrying in algae, snails, and other undeclared cargo. (Of the two dozen species of snails found in prairie potholes, several can survive drawdown by burrowing into the bottom or sealing themselves inside a mucilaginous coating. Most, however, can’t cope with dry conditions and have to be reimported with each successive cycle.)