Wetlands: Boom and Bust

The prairie potholes offer a good example. These wetlands speckle the landscape between north-central Minnesota and central Montana, and extend north well into Canada. This vast, sparsely populated region, often called the duck factory, is the ancestral breeding grounds for a sizable percentage of all North American duck species. Potholes come in all shapes and sizes, with an equally varied array of lifespans. But the ducks make use of them all.

When teal, gadwall, mallard, wigeon, and other ducks complete their northward migration, the larger potholes are still cold and dormant. The smallest potholes, often called “pair ponds,” are not. Filled with snowmelt or spring rains, these may be no larger than a few feet across and three to six feet deep—a mere watery dimple on the rolling grasslands or a crop field, with no cattails or any other “wetland” plant life on the edges. But these shallow pools warm rapidly. In no time an array of insects has laid eggs in the water. Other micro-organisms jump to life. Just when they need it most, the ducks feast on this protein-rich broth, recover from the stress of migration, and conduct their breeding rituals. In two to three weeks, most all of these wetlands vanish.

The midsize model pothole will hold water through midsummer or thereabouts—just enough time to allow a hen duck to incubate and hatch her brood. The nest is tucked away in the lush surrounding vegetation, which tends to be a mixture of prairie grasses and in some cases a bit of what we think of as “wetland” plants, such as arrowleaf. Still, most of these wetlands are not deep or long-lived enough to grow cattails or bulrush. Freshwater shrimp, large and small insect larvae, and nymphs live and reproduce in these wetlands, which provide breeding and feeding grounds for other creatures—frogs, salamanders, and snakes, which in turn attract such mammals as raccoons, skunks, minks, and red foxes. As the wetland dries, a progressively larger ring of exposed muck appears. This ephemeral habitat is prized by shorebirds, which flock to the mudflats, probing for invertebrates.

Not long after the ducklings have hatched, many of these seasonal wetlands have dried completely. It's time then for mother hen to lead her brood to the last, most enduring prairie pothole. Often called a “permanent” wetland, the largest of these potholes may go dry only once in ten years. They may appear as a sizable lake, though most are only about ten feet deep. Here the ducklings find open water well away from the dangers of the shoreline; the new year's crop of aquatic vegetation, such as pondweed, is ready for hungry ducklings and their parents. And here they will stay through late summer and early fall, growing and learning, preparing for their first flight south.

The “boom” of prairie potholes, as well as several other types of wetlands, would never happen without the cyclical “bust” of going nearly or completely dry. How come? The reasons have to do with soil, geography, and the properties of water itself. For different wetlands in different regions, these elements all vary. But one basic concept applies: The same old water sitting in the same old container is going to lose its vitality over time.

If a wetland doesn't receive an influx of fresh water, several things may happen. Oxygen levels will steadily decrease. Bacteria, some varieties of which are harmful to waterfowl or other aquatic life, can multiply to unhealthy levels. Or the water may absorb excessive amounts of salt or other natural elements occurring in the soil. In all cases, aquatic life large and small, from insect larvae to plants, is no longer able to flourish. The wetland sinks below its potential.

Nature's solution is to let the wetland dry up. Plants drop seeds into the muck and wither away. Shrimp and other tiny things lay eggs and die. Salamanders and frogs and turtles may bury themselves in the mud or hit the road in search of another oasis. The wetland may remain dry for a year or even longer, depending on the season's rain or snowfall. All that's needed to trigger a new “boom” is the water that is bound to return; water that is bound to be rich in oxygen, to start the cycle over.