With persistent inundation, the pioneering species—the ones that require drawdown to reproduce—gradually fade from the scene and are replaced by organisms adapted to long-term inundation. This category includes, for example, the larvae of tiger salamanders, large, slow-growing, tadpolelike creatures with a collar of fringed gills, which typically spend up to two years in the water before they metamorphose into gill-less, terrestrial adults. Indeed, some individuals avoid making this transition altogether and spend their entire lives in the pond, eventually reaching sexual maturity and even breeding in the form of oversized larvae, called paedomorphs. (Tiger salamanders frequently live to an age of eight to ten years on land, though little is known about the life span of the aquatic forms.)
Tiger salamander larvae
Yet the high times can’t last forever. Inevitably, whether in a matter of months or years, the conditions of life in the pothole will alter and the marsh will cycle on, passing from full flood to degeneration and from drawdown to rebirth.
Tiger salamanders spend most of their adult lives in damp crevices and underground dens, only emerging briefly when it rains.
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> “JESUS BUGS”
If you’ve ever seen swarms of spidery-looking insects zooming across a ditch or pond, then you have made the acquaintance of water striders. Members of the order Hemiptera, or “true bugs,” they are one of a small number of organisms specialized to live not in the water but on the water’s surface. Like skaters scooting over a frozen pond, they dart around on quiet streams and ponds, so light and quick that they scarcely make a dent in the surface tension. Buoyed up by a cushion of air trapped on the underside of their bodies in a layer of water-repellent hairs, they propel themselves forward with a jerky, oar-like motion of their spindly middle legs, while their long hind legs angle out to the rear for stability and steerage.
Although some species of water striders can fly and all can leap a short distance into the air, their lives are essentially focused on the two-dimensional surface. For instance, the diet of a water strider typically consists of terrestrial and aquatic insects and other small animals that either strike the surface or are unfortunate enough to be trapped in the surface tension. Like a spider in a web, the strider monitors the pond for ripples that indicate a potential meal is within reach. With a few deft strokes, it then spurts to the scene, grasps its prey with its clawed front feet, and stabs its sucking mouthparts into the body.
Water striders also use the water’s surface as a kind of drum for social communication. By jarring the water with their legs, they beat out rhythms at different frequencies, thereby conveying different meanings. For instance, a male water strider may send out a quick rat-a-tat to keep other adult males away from its mate but shift to a slow, steady pulse when it is courting. This “good vibration” seems to motivate the female to lay her fertilized eggs, which she attaches in neat parallel rows to floating leaves or stems just beneath the surface of the water.
Water strider
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Inside the Duck Factory
The wet-dry cycle provides a key to understanding how the Prairie Pothole Region is able to produce tens of millions of ducklings in an average growing season. The surge of life that follows drawdown, and the cycling of resources through the marsh, produces a periodic, self-renewing, superabundant crop of algae, minicrustaceans, insect larvae, and other high-protein food that is critical to the success of breeding waterfowl. Even species such as mallards and northern pintails, which are seed eaters for most of the year, switch to a diet of aquatic invertebrates during the nesting season. It is no small thing, after all, for a female duck to produce a clutch of nine or ten eggs, one a day for as many days, and then to start all over again if the first clutch fails to hatch. (Nest failures are common, and female mallards have been known to make as many as four nesting attempts in a single season.) Because regeneration is usually triggered by snowmelt in the spring, the pulse in the production of pond life is ideally timed to support egg laying and to sustain the hen through the month-long vigil of incubation. And there is still plenty to eat in midseason, when the ducklings peck their way out of their shells, follow their mother to a slough, and bob along after her in search of food.
Northern shovelers
Common mergansers
What’s good for ducks is also good for the people who admire them. The delight of traveling through pothole country, especially in a wet spring, derives not only from the numbers of ducks but from their variety. If one pond sports a pair of mallards, the next one will surprise us with blue-winged teals or American wigeons or lesser scaups, each appropriately dapper or dappled or iridescent. Here’s a pair of northern shovelers, slowly swaying their big, trowel-like beaks through the shallows, with much the same motion as if they were wielding metal detectors. (In fact, the birds are feeding on microscopic pond life, which they obtain by passing water over their tongues and out the sides of their bills, which are equipped with comblike filters.) In the next pond down the road, a pair of common mergansers may be swimming side by side, he resplendent in green, she with a tousled headdress of cinnamon, their beaks unexpectedly pointed and startlingly red. Hooked at the ends and serrated along both sides, these weapons permit mergansers to catch amphibians and (where the water is sufficiently oxygenated) minnows and other small fishes. No two potholes are likely to yield an identical list of species.
The preference of particular species of ducks for particular potholes is a sign of the subtle-but-significant differences between basins. Through the intricacies of the wet-dry cycle, slight variations in depth and water duration are translated into tangible differences in the amount and types of invertebrates, or duck food, in each wetland. When this slough-to-slough variability is multiplied by millions of basins and extended across the northern plains, it results in a varied landscape of wetland habitats that, collectively, are suited to meet the needs of a varied array of duck species. Pair by pair, the birds select the potholes that are best able to satisfy their specific requirements. Alternatively, if they can’t find one wetland that will do the entire job, the birds move from slough to slough, perhaps using one basin for feeding in early spring, another for nesting, and still another for rearing their ducklings. This ability to provide a variety of quality habitat is another reason that the Prairie Pothole Region is such a hotbed of duck production.
Not surprisingly, ducks are more searching in their assessment of wetlands than human observers are, and their reasons for preferring one slough over another are not always apparent to us. Nonetheless, some general patterns are fairly easy to discern. Most ducks that nest on the prairies fall into one of two main groups, or tribes, each of which is adapted to a particular lifestyle. On the one hand are the dabblers or, as they are more formally known, the members of the genus Anas, a lineage that includes the mallards, teals, wigeons, gadwalls, pintails, and shovelers. Often seen upended, tail waggling, in the slough, these ducks are adapted either to feed near the surface of the water (like shovelers) or, by tipping up, to forage on the slough bottom. Accordingly, they prefer shallow potholes, usually with an interspersion of open water and emergent plants. Since their preferred habitat is highly dynamic—flooded one year, dry the next—they tend to be opportunistic in their choice of breeding grounds, with each pair settling in the first unclaimed slough that they happen to come upon during the spring migration. Finally, because their chosen potholes are not deep enough to offer protection from terrestrial predators, dabbling ducks often nest in dense vegetation well away from wetlands and lead their ducklings on a long trek back to water.
The other tribe of pothole-nesting ducks is the divers. Less abundant and perhaps less familiar than the dabblers, this group includes two eye-teasing pairs of lookalikes—the canvasback and redhead, and the ring-necked duck and lesser scaup—all of them members of the genus Aythya. An unusual assortment of other, less closely related species can also conveniently be mentioned in the same breath, notably, common go
ldeneyes, buffleheads, mergansers, and ruddy ducks. (Ruddy ducks, in particular, are worth checking out, if only for the bright blue bills of the breeding males.) What these birds all have in common is their ability to dive, swim underwater, and exploit the resources of the deeper and more permanent prairie marshes. Unlike dabblers, which prefer ephemeral and seasonal basins, divers tend to be found in seasonal and permanent potholes with areas of open water. Although several of the diving species protect their eggs by nesting in hollow trees, others find safety for their broods on islands or in stands of emergent plants in the middle of the basin. And since their chosen waters are relatively reliable, as prairie potholes go, diving ducks tend to return to the same nesting sites year after year, rather than playing catch-as-catch-can in the way that dabbling ducks do.
Reliable, as prairie potholes go. The fact is, of course, that even the deepest and most permanent wetlands in the pothole region cannot be depended upon. A succession of snow-free winters and hot summers can dry the whole region out, including even the most extensive and durable marshes. Clearly, both dabbling and diving species are adapted to thrive in these dynamic environments when conditions are “ducky,” but what happens when they’re not? How do waterfowl cope when their habitat dries up?
The short answer is, they don’t. Instead, like other types of pond life, ducks have little choice except to get by as best they can. Most species of pothole-nesting ducks tend to overfly the dried-out potholes and continue on to the north, often settling down in the boreal forest. In this less-than-ideal environment, they produce a meager crop of young, often too few to sustain the breeding population. Thus, fewer sloughs translates directly into fewer ducks. This relationship between the number of ponds in the pothole region in May and the number of breeding ducks tallied the following spring has been documented repeatedly in data that go back to the early 1960s in Canada and the mid-1970s in the United States. Although the graphs do not line up precisely, because of the many complicating variables that influence the ducks’ success (including the timing of spring breakup, unseasonable blizzards, and so on), the parallel run of the lines from peak to trough is too striking to be questioned.
Already half-submerged, this eared grebe could slip underwater in an instant. These handsome birds often breed in large, noisy colonies on marshy, freshwater lakes where they dive to catch aquatic insects.
Arthur Savage photo
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> DRY OUT AND DIE
As the closest living descendants of the ancient, stout-limbed fish that first clambered onto dry land, amphibians retain a strong, lifelong allegiance to water. To paraphrase an old saying, you can take the frog out of the pond, but you cannot take the pond out of the frog.
All the amphibians on the Great Plains, including familiar species of salamanders, frogs, and toads, begin their lives in the water. Hatched from masses of jellied eggs (think eyeballs in aspic) stuck to submerged plants, they develop first into swimming larvae, or tadpoles, that breathe through gills. Then, after an aquatic stage that may last weeks, for frogs and toads, or years, for salamanders, they undergo metamorphosis and are spontaneously transformed into walking, hopping, terrestrial forms.
Yet even as landlubbing adults, most amphibians never dare venture far from a ready supply of moisture. Weak of lung, they supplement their supply of air by breathing through their skins, something that can only happen if their bodies are moistened. If they dry out, they die. This phenomenon explains why the tiger salamander, for example, spends the daylight hours hunkered down in a humid burrow, from which it emerges only at night or during showers. Species such as the northern leopard frog, by comparison, which are highly intolerant of drought, tend to focus their activities on the marshy fringes of streams, ponds, and other permanent sources of water.
There are, however, two species that are especially adapted to cope with arid conditions. They are the Great Plains toad—all wrinkled and warty and blotched—and the plains spadefoot toad, which has smooth skin and looks something like a bulky frog. Found in both desert and grassland environments from the southern part of the Prairie provinces to northern Mexico, they respond to drought by digging a shallow well and burying themselves in the bottom. Both species have spurs, or spades, on their hind feet, which they use to spiral down, rump first, to as much as a yard, or meter, beneath the surface. There they lie, their metabolisms set to idle, for weeks or months or years, until they are reawakened by an influx of rainwater. Then, suddenly, the toads are back on the surface, and the still-refilling rainwater ponds are alive with the quacklike grunts of spadefoots or the metallic, machine-gun-like clatter of Great Plains toads. Eggs are laid and within forty-eight hours develop into tadpoles, which mature into full-grown toads a mere three to six weeks later. Their reproductive cycle completed, the adults feed on insects and other small prey until the drying heat of summer forces them to bury themselves away.
Great Plains toad
Plains spadefoot toad
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Where Have All the Ducklings Gone?
The bleakest years so far on record for North American ducks extended from the mid-1980s to the early 1990s, a period during which the climate wavered queasily between wet and dry for several years before settling into the most intense drought since the Dirty Thirties. The dabbling ducks, in particular, were hit hard by this turn of events, as durable species like the mallard plunged toward the lowest numbers ever recorded. (In 1985, this most populous of North American ducks bottomed out at about 5 million birds, a worrying 40 percent below its long-term average numbers.) Worse yet, even during the occasional seasons when the prairie potholes did fill up, the ducks inexplicably failed to rebound. It seemed that some unknown difficulty was compounding the effects of the drought and preventing the ducks from recovering during the wet phase of the cycle. Alarmed that this downturn might be permanent, researchers embarked on an urgent search for answers.
One fact was immediately obvious. The Prairie Pothole Region was no longer the paradise for ducks that it had been at the beginning of the settlement era. Since 1900, somewhere between 20 and 85 percent of the original prairie potholes have been ditched, drained, dredged, filled, leveled, plowed, or paved; the exact percentage varies from district to district. (In general, losses have been lowest where land is used for grazing and highest on croplands and in urban settings.) Although the rate of wetland conversion has declined in recent decades—and the pace of restoration has picked up—the net balance is still negative. Between the late 1970s and the early 1980s, for example, an additional 1,500 square miles (4,000 square kilometers) of wetlands were appropriated for farming operations in the U.S. alone, and the losses in Canada were similar. This was the very period when the continental population of ducks began to slide into the basement.
Garter snakes are commonly found near wetlands and river valleys, on the prowl for frogs, earthworms, and small mammals.
Loss of wetlands is never good news for ducks, yet biologists were not convinced that it was a primary cause of the 1980s crisis. After all, the immediate problem wasn’t really a lack of potholes; it was the inability of the ducks to ride the wave of recovery following drawdown and fully reoccupy the wetlands still available to them. Some link in the chain between the wet-dry cycle and duck reproduction had been broken, so an abundance of duck food during wet years no longer resulted in an abundant output of ducklings. Could it be, the researchers began to wonder, that something was suppressing duck reproduction? Studies conducted in the early-to-mid-1980s quickly confirmed this disturbing hypothesis. Instead of the 15 to 20 percent “hatchability” (one or more hatchling per nest) that is considered a bare minimum to keep duck populations at a steady state, the results from the Prairie Pothole Region were dismal: an average of 7 percent nest success for pintails, 12 percent for shovelers, 14 percent for gadwalls, and 8 and 11 percent for mallards in two different areas. These figures meant that even when the ponds were crowded with ducks and the nests were brimful of eggs, the vast maj
ority of nesting attempts were failures. The famed duck factory of North America appeared to have gone bust, and a once-productive population was now merely going through the motions of reproduction.
One curious detail—a needle in the haystack of data—emerged from this research. The studies showed a strong and surprising relationship between the prospects of a given nest and the human land use in the countryside that surrounded it. Specifically, the greater the percentage of cropland in the area, the lower the likelihood that the ducks would be successful. This was partly because spring-seeding activities took a toll on nests that were situated in stubble fields, sites that are especially favored by pintails. But this kind of collateral damage did not appear to be a significant problem for other species of dabbling ducks, which nest in tractor-no-go zones such as ditches and patches of brush. The significant problem appeared to be not farming as such but one of its unintended consequences. In one way or another, agriculture had inadvertently boosted the populations of middle-sized predators, such as foxes, raccoons, and skunks. By the mid-1980s, this cadre of nest-raiding mammals (aided by bull-snakes in some localities) was making a meal out of both duck eggs and sitting ducks, and there didn’t seem to be much that anyone could do to stop them.
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