By Johann Walker, Ph.D.
During the past two years, breeding duck populations
have reached the highest levels observed since the U.S. Fish and Wildlife Service and Canadian Wildlife Service began jointly conducting annual waterfowl surveys in 1955. The record duck populations of 45.6 million and 48.6 million birds observed in 2011 and 2012 largely represented the bounty of three consecutive years of favorable wetland conditions in the Prairie Pothole Region
. During this period the number of May ponds surveyed on the prairies nearly doubled, increasing from 4.4 million in 2008 to 8.1 million in 2011.
A pond is simply a depression in the landscape that holds water for part or all of the waterfowl breeding season. Millions of these depressions, many of which are less than half an acre in size and a foot deep, are scattered across the Prairie Pothole Region. Why are these rather inconspicuous features of the prairie landscape so important to breeding ducks? It all comes down to habitat. Small, shallow wetlands, including temporary and seasonally flooded basins, provide much of the food and cover that breeding ducks need to successfully nest and raise their ducklings.
As waterfowl return to the prairies in spring, female ducks must acquire protein and fat to produce eggs and to sustain them during incubation. Seasonal wetlands provide nesting females with these nutrients in the form of abundant invertebrates and plant seeds, which become available as soon as wetlands thaw and the sun warms their shallow waters. To secure these nutrients, female ducks establish a feeding territory and males defend their mate and territory from intruding pairs of the same species.
When the prairies are wet, numerous seasonal wetlands provide an abundance of high-quality feeding territories for breeding pairs, and duck populations soar. During these years, food is abundant and the nesting effort is strong. In dry years, when seasonal wetlands are in short supply, female ducks must ”choose” either to migrate farther north in search of new breeding sites or make the best of local breeding opportunities. For the birds that choose the latter option, competition can be intense as breeding pairs attempt to establish and defend territories within limited habitat. This competitive behavior is likely part of the reason why waterfowl nesting intensity and reproductive success is lower in dry years than in wet years.
Later in the breeding season, as duck nests begin to hatch, seasonal wetlands provide important habitat for hens and broods. Research indicates that more ducklings survive during wet years, when seasonal wetlands are more numerous on the landscape, than in dry years, when water is only present in larger, more permanent wetlands. On landscapes with an abundance of healthy seasonal wetlands, brood-rearing females and their ducklings may not have to move as often or as far overland in search of food and shelter, reducing their vulnerability to predators and the elements.
Seasonal wetlands are an integral part of the prairie ecosystem, and the impacts of wet and dry weather cycles on these highly dynamic habitats affect a variety of species in addition to waterfowl. During wet years, seasonal wetlands not only support an abundance of invertebrates eaten by female ducks and broods, but also insects and small rodents, which are the primary prey of mammalian predators like skunks and foxes. During drought years, when seasonal wetlands are dry, populations of these predators and their prey decline on the landscape. In addition, dry wetland basins recycle dead vegetation and other plant biomass into nutrients that set the stage for an explosion of productivity when water returns. As a result, breeding ducks often experience increased nest success and productivity during the first few years of a wet cycle following an extended period of drought. Recent research indicates that this pattern, which is likely related to changes in the relative abundance of nest predators, primary prey, and nesting ducks, holds true even on intensively cultivated landscapes with low levels of perennial upland cover.
All of these ecological factors explain why duck populations boom when the prairies are wet. First, an abundance of seasonal wetlands attracts large numbers of breeding pairs. Nesting females then lay larger clutches and renest more persistently than they would in a dry year. More nests also hatch, and finally, more ducklings survive. These conditions—abundant wetlands, great numbers of breeding pairs, large clutches, high hatch rates, and increased duckling survival—are the equivalent of a straight flush for breeding ducks.
That is precisely what has occurred in recent years across the Prairie Pothole Region. Similar wetland conditions and duck population increases also took place during the 1970s, 1990s, and 2000s when the prairies emerged from drought. But the impressive fall flights witnessed during these banner years would never have materialized without millions of small wetland basins on the prairie landscape. Our most important game ducks, particularly mallards, blue-winged teal, and northern pintails
, are especially dependent on these habitats during the breeding period. As a result, the conservation of small, seasonal wetlands is crucial to sustaining duck populations at levels that can support hunting.
The negative impacts of wetland drainage (both surface and subsurface) on breeding waterfowl simply cannot be overstated. Small wetlands are disproportionally impacted by drainage, often leaving just a few large basins intact. This is a particularly troubling pattern from the perspective of breeding ducks. Studies show unequivocally that 10 one-acre ponds support three times more breeding pairs of ducks than one 10-acre pond. There is also growing evidence that an abundance of small wetlands will also support more broods than a few large wetlands. Consequently, conservation programs and policies that protect small wetlands from drainage are vital to the future of North America's ducks.
Dr. Johann Walker is director of conservation planning at DU's Great Plains office in Bismarck, North Dakota.