Understanding Waterfowl: Drought and Deluge

By Michael G. Anderson, Ph.D.

I'm a child of prairie pothole country, raised in southeastern North Dakota. My earliest memories are of gently rolling land, covered with grass in the summer and by wind-whipped flats and cornices of snow in the winter, with sloughs of various shapes and sizes filling the low spots among the knolls.

Farmers and ranchers, mostly second- or third-generation immigrants from northern Europe, owned the land where I grew up. For those who persisted through the dreadful 1930s, drought deeply affected their views of life on the land. No two years were the same-farmers described every year as too dry, too wet, or too cold. Rarely did the prairie porridge seem to be "just right." But I remember well the rebound years of 1965minus;1967, when I first began to hunt and there seemed to be ponds and ducks everywhere after the severe drought of the early 1960s.

Later, as a graduate student studying canvasbacks in Manitoba, I was much impressed by the profound impacts of droughts in the 1970s and 1980s, especially in 1977, when wetlands that produced canvasback nests the year before had dried to dust, few birds nested, and nesting success went from 51 percent in 1976 to 0! But the rain and snow returned the following year, and in 1978 canvasback nesting success was 76 percent, the highest recorded over a span of 30 years.

How could that happen?

Well, it starts with the boom and bust cycles of prairie precipitation. Driven partly by El Niño, La Niña, and other major oceanic influences, atmospheric circulation patterns can differ dramatically from one year to the next, with accompanying swings in annual precipitation of 50 percent or more. In fact, the prairies have the most variable precipitation of any region in North America.

These annual changes in precipitation, especially in winter and early spring, greatly affect the abundance, distribution, and nature of wetlands across the Prairie Pothole Region (PPR). The result is that pond numbers can vary dramatically from year to year and place to place. Since systematic surveys began, pond numbers in prairie Canada and the north-central United States have shown three generally wet periods, in the mid-1970s, the mid-1990s, and 2010minus;2012, with dry periods in the early 1960s, late 1980s and early 1990s, and the early 2000s. Total duck numbers in the areas traditionally surveyed by the U.S. Fish and Wildlife Service (USFWS) and Canadian Wildlife Service have followed much the same pattern, leading most scientists and waterfowlers to agree that as goes the PPR, so go populations of many of our most important game ducks.

It's not surprising that the number of ducks settling on the prairies in spring largely tracks the number of ponds, which in turn is determined mostly by the combination of carryover water from the previous year and winter precipitation. But what is it about changing water conditions that is so important to the ducks? A variety of factors come into play.

First, the plants and animals that thrive in prairie wetlands are beautifully adapted to wet and dry cycles. Most wetland plants that grow above the water's surface require the damp exposed soil of moderate drought conditions to germinate and restore healthy plant stands. Seeds can lie on the bottom of wetlands for years awaiting just the right conditions to sprout. Many (though not all) wetland invertebrates can survive drought, too, by encasing themselves or their eggs in water-tight structures and waiting for water to return. The combination of new wetland plants, annual seed-producing plants, and increasing invertebrate populations in newly flooded ponds means that wetland landscapes coming out of drought conditions are often dramatically more productive for breeding waterfowl than are the relatively static, open-water wetlands that typically exist after an extended wet period.

For diving ducks that nest over water on platforms made of old cattail, bulrush, or sedges, drying ponds leave the birds nowhere to build their nests and nowhere to hide. When it gets really dry, many ducks simply won't nest. And young ducklings, of course, need cover to escape bad weather and things that eat them. Again, flooded cover is key; without it, the risks to prospective breeders often outweigh potential rewards. Experienced canvasbacks, redheads, and their kin are likely to move on to better places or simply don't attempt to breed in really dry years.

In addition, it has been known for a long time, since Professor Paul Errington's work at Iowa State University in the 1940s and 1950s, that numbers of mink generally follow the abundance of muskrats. When it's really dry, these wetland-adapted animals suffer too (smaller litters, reduced survival). And when water returns to the prairies, the wetlands and the ducks can respond much more quickly than do the resident mammals.

Nearly 50 years ago, Johnny Lynch, a pioneer USFWS survey biologist, famously observed that wet and dry patterns on the prairies drove the boom and bust years for duck production, while the more stable water areas in the northern parklands and southern boreal forest provided more predictable, but on average less productive duck habitat. Many times the boreal forest has proved to be an important redoubt in dry years, but the likelihood of drought-displaced prairie ducks breeding there seems to be rather low.

In the late 1980s waterfowl biologists, thinking about all these factors, realized that the key to sustaining healthy duck populations was "keeping the table set" over large landscapes for those strong production years that come along now and then. If wetland basins are intact-and not ditched, tiled, or filled-they will return to their natural productivity whenever the snow and rain allow. Similarly, if native grasslands, forage crops, or Conservation Reserve Program cover are in place when the water returns-as illustrated graphically in the Dakotas in the mid-1990s-duck numbers can explode. The challenge is to affect the "table" on a scale big enough to matter. That means applying every tool we have to protect and restore pothole basins and doing everything we can to keep the grass right-side up across the prairie landscape.

Ensuring sufficient habitat to sustain healthy waterfowl populations will require broad public support for wetlands conservation. Fortunately, there is growing appreciation for the many benefits wetlands provide for society, especially during times of exceptionally wet weather. Intact wetlands can slow the flow of water over the land, giving creeks and rivers time to flow moderately and lessen the chance of catastrophic floods. Wetlands also take up excess nutrients like phosphorus and nitrates common in runoff from agricultural watersheds, minimizing harm to rivers, lakes, and municipal water supplies downstream.

For 10,000 years, since the Wisconsin Glacier retreated from the prairies of North America, duck numbers have likely waxed and waned due to dynamic forces that influence this region's wetlands and grasslands. If people use the land in a way that allows this natural system to continue to function, we should enjoy the magnificent company of wildfowl for generations to come. But that won't happen if we are careless, short-sighted, or complacent.

When DU celebrates its 100th Anniversary in 2037, what will future readers of this magazine think of our actions? Will they smile softly and thank us for getting it right? Or will they shake their heads and wonder what went so wrong? We can't know the answer, but we do know what we need to do today. If we keep wetlands and grasslands on the prairies, we will keep the ducks.