Even changes that, at first glance, look like straightforward improvements often net a mixed bag of gains and losses. For instance, the Missouri, like many other prairie rivers, is naturally heavy with mud, the result of a long, slow journey through the dry and erodible soils of the grasslands. (They don’t call it the Big Muddy for nothin’.) Also, like other prairie rivers, it is typically shallow—“a mile wide and an inch deep,” as the settlers used to describe it. Warm, murky, and sullen, it is the antithesis of the proverbial babbling brook and, to some early observers, appeared to have few natural virtues. “Too thick to drink, too thin to plow,” the pioneers grumbled. But if you hold this same water behind a dam, it will clear, as the load of dissolved sediments settles to the bottom. This is what happens in present-day reservoirs, creating perfect conditions—cold and sparkling clean—for sight-feeding game fish such as the native walleye (now far more common and widespread in the Missouri basin than in times past) and the introduced northern pike, smallmouth bass, black crappie, chinook salmon, and others. In all, about half the fish species in prairie rivers these days are exotics, and they tend to be doing very nicely.
But what of the poor unfortunates that liked the turbid water of the Missouri exactly the way it was? What about creatures like the pallid sturgeon? One of approximately two dozen species of native Missouri River fish that are known to be in decline, the pallid sturgeon is among the unlucky few to merit inclusion on the federal list of endangered species. A member of an ancient order of cartilaginous fishes that has survived almost unchanged for some 70 million years, this is one weird-looking creature. It’s big (up to 6 feet long and 100 pounds in weight, or 1.8 meters and 45 kilograms) and looks vaguely menacing, with its gray, armor-plated body; small, beady eyes; and flat, shovel-shaped proboscis. Four fleshy whiskers, or barbels, dangle under this protruding snout and finger the water in front of the recessed mouth. These waggling appendages are sensory organs that, through a combination of touch, taste, and electrical cues, permit the sturgeon to locate food in even the siltiest waters. (Pallid sturgeon eat small prey such as insect larvae and worms, which they vacuum up off the river bottom.) Interestingly, researchers in North Dakota have recently discovered similar adaptations in two native prairie minnows—the sicklefin chub and sturgeon chub—which, though weak-eyed, feel their way through the gloom by means of special sensory bumps located on their heads, fins, and bellies and inside their mouths. Thus, there appears to be a guild of prairie fishes that are specially equipped to cope with the special challenges of prairie rivers.
As the character of the habitat changes—as a silty, warm, seasonal river is transformed into a clear, cold, managed flow—these specializations lose their survival value. What is the point of being adapted to heavy siltation or to a seasonal flood-pulse or to a dynamic flow of habitats if many of these natural processes have been altered? This appears to be the fatal question for many native prairie species of fish, including the pallid sturgeon. A survivor of the Ice Age, it has not been able to adapt to the changes brought about by a half century of human ambition. Yet if evolution cannot undo the damage, perhaps people can. Although major dams are unlikely to be removed from the Missouri system any time soon, conservationists are working on ways to modify the obstructions so the fish can regain access to the remaining stretches of free-flowing current.
Even without human remediation, the pallid sturgeon, together with a number of other declining species, such as the western silvery minnow and the flathead, sicklefin, and sturgeon chubs, are managing to hang on in a few restricted reaches of the Missouri River. These include, as a prime example, a short run of water just east of the Montana/North Dakota border and west of Lake Sakakawea, near the confluence of the Missouri and the Yellowstone rivers. The last major river on the Great Plains without a major dam, the Yellowstone not only maintains its natural vitality—with May and June rises, fast- and slow-water habitats, natural temperatures and turbidity, and shifting banks— but also possesses enough energy to reinvigorate the smaller and more docile Missouri. The fact that the native community of fish respond to this wild influence, however locally, suggests that restoring sections of the river is a credible strategy. One of the places in which this kind of restoration is being attempted is the Big Muddy National Fish and Wildlife Refuge in the state of Missouri. From the early 1990s onward, stretches of the river have been released from the demands of navigation and allowed to flood their bottomlands, thereby creating new sandbars and side channels. In 1998, these efforts were rewarded by the discovery of baby pallid sturgeon, no larger than tadpoles, in one of the restored backwaters. The first sign of natural reproduction by the species in the lower Missouri in at least fifty years, these bewhiskered little creatures were a welcome reminder of the synergy between life and the living waters of the Great Plains. Tragically, there has been no natural reproduction in the Upper Missouri in living memory.
In late 2009, the U.S. Army Corps of Engineers (the organization that was charged with “developing” the Missouri River all those years ago) embarked on a five-year, $25,000,000 study to reconsider the overall costs and benefits of their efforts. Laying aside their heavy equipment for the moment, they hope to initiate a national conversation about the Missouri’s health. In light of what we have learned about the ecological consequences of dams, do the original reasons for controlling the river command respect? This story is marked To Be Continued.
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> LAKE STURGEON
Four members of the sturgeon family are found in the rivers and lakes of the Great Plains, and all of them are wonderfully odd. They include the pallid sturgeon (now one of the rarest fish in North America), the shovelnose sturgeon (a smaller, darker version of the pallid, so similar that the two frequently interbreed), and the paddlefish (perhaps the strangest of the lot, with its long, spatulate snout—a third of its entire length—which picks up electrical signals from the schools of plankton on which it feeds). All of these species are native to the Missouri/Mississippi drainage.
Only the final member of the family has a range that extends beyond the Missouri system and into the Saskatchewan basin. Found in both the North and South Saskatchewan rivers, the lake sturgeon looks more or less like the pallid—the same armored body, beady eyes, and whiskered chin—but can be considerably larger. The biggest lake sturgeons on record reached lengths of 8 feet (2.5 meters) and topped the scales at 275 pounds (125 kilograms). Gentle giants that they are, they subsist on a diet of fish eggs, insect larvae, and other small organisms that they scoop from the bottom. Slow growing and slow to mature, they have been known to live for up to 150 years. Because of heavy commercial fishing and environmental damage caused by dams, the populations have declined drastically in recent decades, with reductions in the order of 50 to 80 percent since 1980, and only restricted sport fishing is permitted.
In his book Fishing in the West, writer David Carpenter has this to say about angling for lake sturgeon:
The standard method of catching sturgeon is to bring a fishing rod in one hand and a good book in the other to the edge of your favourite river. You cast your bait out to a likely spot, let it sink to the bottom and wait. Most anglers lay their rod in the crotch of a forked stick plunged into the beach, and some place a little bell on the tip of their rod to wake them if they’ve chosen the wrong book. (I would recommend Moby Dick.) . . .
It takes at least [twenty-five to thirty years] before these fish can spawn, so they are easily depleted. It behooves us to keep an eye on the populations of these leathery giants.
Lake sturgeon
Pallid sturgeon
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> COOL AS A CLAM
Just when you thought you had heard everything, someone tells you about the “unionid mussels.” Turns out they’re aquatic mollusks, or shellfish, that live in creeks and rivers across the Great Plains (among other places) and that produce the pearl-lined, ridged-backed, more-or-less-oval shells sometimes to be found around muskrat lodges or strewn on
beaches. Often referred to as “freshwater clams,” they are an important source of food not only for muskrats but also for raccoons, mink, otters, fishes, and certain birds. Humans can eat them, too, but only after they recover from the initial state of shock that inevitably follows from making their acquaintance.
At first glance, the life of a mussel sounds like boredom in the extreme. This is an animal that lies on the river bottom, half buried in gravel or sand, and pumps water in one end of its body and out the other. In the process, it filters out particles of food, mostly plankton or other bits of organic matter. And it goes on doing this continuously through a life span that, depending on the species, can easily extend from 10 to 100 years.
But things start to get interesting when the mussels breed. The first thing that happens is that the male mussel (yes, there are two sexes) releases clouds of sperm into the water. When a female sucks in this seminal fluid, the sperm fertilize her eggs, which develop into larvae inside her body and attach themselves to her gills. To complete their development, however, the larvae have to abandon mother and launch themselves into the world by hitching a ride on a fish. Although some species of mussels can latch onto any available transport, others are specialists that can only accept the services of particular hosts.
How to make contact? In some species, the female simply turns the larvae loose and lets nature take its course. Either they find suitable hosts or they don’t. But many mussels are able to reduce the risk of failure by luring the fish in close. The female plain pocketbook mussel, for example, has a lip of tissue, or mantle flap, that looks astonishingly like a small shiner, complete with eyes and stripes. (Other species present lures that resemble crayfish and insect larvae.) By contracting this appendage, the mussel can wiggle it to draw in nearby bass, its host, and trick the fish into striking. Instead of a meal, the bass ends up with a mouthful of mussel larvae, which hook onto its gills or fins and grow to maturity. Once fully developed (usually without harm to the fish), the young mussels drop to the river bottom, where if conditions chance to be right, they settle into the humdrum routine of adult life.
Because they are highly sensitive to changes in their environment, whether through soil erosion, siltation, dredging, channelization, impoundments, or the expansion of exotic species such as zebra mussels, these wonderfully bizarre creatures are among the most endangered groups on the Great Plains and elsewhere in North America.
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PRAIRIE WOODLANDS
And I saw that the sacred hoop of my people was one of many hoops that made one circle, wide as daylight and as starlight, and in the center grew one mighty flowering tree to shelter all the children.
BLACK ELK, BLACK ELK SPEAKS, 1932
IF YOU PAINT the prairie with broad strokes, all you see are grass and sky—a ground of green or khaki, a wash of blue. But scan the scene more closely, looking for shadow as well as for light, and you will see that the face of the grassland is etched with trees and shrubs. Tangles of woody vegetation are scrawled along the contours of the Great Plains, defining coulees, streambeds, and ridges and generally adding emphasis to a landscape that suddenly reveals itself in three dimensions. Far from being characterized by the absence of trees, these so-called treeless plains are increasingly a land of many small, scattered woods, each a world in itself, that add complexity to the grasslands.
It takes a prairie dweller to fully appreciate the way trees change the world. To sense how they cut the wind and scatter the light. To smell the rising odors of sap and moistness and rotting earth. To hear the silvery rustle of leaves and the bright chorus of birds—warblers, thrushes, vireos, wrens—that flit and flash through the branches. Every prairie grove is a woody oasis in a sea of grass, a refuge for the hundreds of species of insects, amphibians, reptiles, mammals, and birds that are adapted to the shelter of woodlands. Yet as much as we enjoy this abundance, we seldom stop to think about the woody beings that stand at the center of things. What are these impressive organisms that we call trees and shrubs? And what are they doing on the grasslands?
To Be a Tree
The underlying principle of woody plants is height. Through evolution, their strategy has been to hoist their leaves up into the light atop either a single, towering trunk (thereby producing the form classified as a tree) or a cluster of often-shorter, multiple stems (a shrub). The “technology” that makes this possible is an elaborately modified sugar, or organic polymer, called lignin. Produced by a layer of cells just under the bark, this remarkable material—durable, springy, and tough—is laid down toward the center of the stem or trunk, where it bonds with other plant tissues to form a supporting core of wood. Remarkably, this adaptation has been achieved independently by many different kinds of plants, including grasses (bamboo), legumes (caragana), and asters (sagebrush), as well as by all the varied tribes of conifers and hardwoods. In every case, the benefit has been the same: to gain an advantage over ground-hugging grasses and forbs by reaching up and intercepting the sunlight in midair.
But every adaptation brings its own difficulties. The closer a plant is to the heavens, the farther it is from Earth, and leaves, no matter how lofty, still must be fed by their roots. Like the photosynthetic organs of all plants large and small, tree leaves need a supply of water and mineral nutrients that can only come from the soil. Woody plants respond to this challenge by drawing groundwater up to the leaves through slender tubes in their phloem, or inner bark. Since the roots cannot push the fluid up, it has to be drawn from the top, a feat accomplished through evaporation. As the sun warms the surface of the leaves, water vapor is drawn through their pores, or stomata, and lost to the atmosphere. This loss creates a partial vacuum in the leaves that causes liquid to rise up the phloem tubes, each molecule glued to the next, as if it were in an improbably long drinking straw. During the growing season, a large deciduous tree can suck up as much as 100 gallons (400 liters) of water in a day, just to keep its vital juices flowing.
But if the roots cannot find enough water to satisfy this hectic demand, the slender threads of liquid in the phloem will break, or cavitate—often with an audible pop—and circulation to part or all of the upper tree will be cut off. Death, or dieback, follows. This is exactly what happened, on a large scale, during the drought of the Dirty Thirties when, to cite one well-documented case, between 50 and 60 percent of all trees in Kansas and Nebraska succumbed to water stress, and most of the survivors were left stunted and partially blackened. In northern Texas and Oklahoma, where the death toll was even worse, as few as two or three trees in ten survived the decade of drought.
For a tree on the sunbaked prairies, survival is a question of supply and demand. On the supply side, trees reach out for water by producing ambitious systems of roots, which are often almost as massive as the plant’s aboveground growth. Contrary to a common misconception, most tree roots are relatively shallow—no more than a few yards, or meters, deep—and they rarely tap into the permanent moisture of the water table. (Even roots can drown if they’re held under water and deprived of oxygen for too long.) Instead, the plants push out laterally through the rain-watered upper layers of the soil until their rootstocks extend well beyond the circumference of the branches. Green ash, for example—an eastern woodland species that occurs midway across the plains—puts out roots to a distance of some 50 feet (15 meters), but rarely pushes them more than 3 feet (1 meter), deep. The more arid the habitat and the more drought-resistant the tree, the more far-reaching the roots tend to be. Thus, the gnarly little bur oak—one of the most drought-hardy species of trees on the tall-grass prairie and one of the few that carried on merrily during the thirties’ drought—typically produces roots that are as deep as the tree is tall and more than twice as extensive. To be specific: a 43-year-old bur oak that had struggled up to a height of 20 feet (6 meters) was found to have pushed its roots out laterally over a distance of 41 feet (12.5 meters).
The bur oak, a rugged, slow-growing
tree with a deep taproot and thick, fire-resistant bark, is found in tall grasslands and prairie-and-oak savannas. Although individuals may live for a thousand years and attain an enormous girth, the trees seldom exceed a height of 70 to 80 feet (20 to 25 meters).
On the demand side, trees reduce the rate at which water vaporizes and streams away from the surface of their leaves. Deciduous trees, for example, take the radical approach of shedding their leaves in autumn, entering near dormancy, and then bursting forth with new vigor at the time of the spring rains. Conifers have reduced the surface of their leaves to the barest minimum, thereby cutting their water throughput, slowing down their metabolism, and allowing them to photosynthesize sluggishly throughout the year. In addition, the leaves and needles of many species of trees are covered with a waxy, waterproof coating, or cuticle, that helps keep moisture from leaking out, except through the portholes of the stomata. And even these pinprick openings can be regulated to a certain degree—opened when the air is moist and cool or shut when it’s dry and hot—to minimize the risk of drying out.
Some species of trees, and even some geographical populations within a given species, are better than others at turning off the tap. For example, trembling aspens that grow on semiarid grasslands have a greater ability to adjust their stomata and reduce their water loss than do aspens from other, less-challenging environments. But despite these refinements, a typical tree is still a large and thirsty organism that often finds itself tested by the supply-demand equations of the open plains. This helps explain why prairie woodlands are often tucked into folds of the land, close to the water table and protected from sun and wind, where the margins of life, by prairie standards, are most generous.
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