With only remnant patches of grassland remaining in many areas, ducks are forced to lay their eggs in fragments of prairie that have escaped cultivation. Predators, in particular red foxes, skunks, raccoons, badgers, and coyotes, can more easily encounter nests in these small patches. Moreover, changes in the landscape that accompanied European settlement -- extirpation of wolves and other large carnivores, farmsteads and culverts that provide den sites, and agricultural foods that better enable mammals to survive through the winter -- have also transformed the predator community in the PPR. Without wolves and coyotes to suppress their numbers, foxes are much more abundant than they were before settlement, as are skunks and raccoons. The dual effects of grassland fragmentation and alteration of the predator community have resulted in very low duck nest success rates and high mortality of nesting hens. Of all the demographic parameters that are responsible for change in mid-continent mallard populations, nest success and hen mortality during breeding are clearly of greatest importance (Steve Hockman et al. IWWR, unpubl. manuscript). There is strong evidence that these same parameters are driving other mid-continent dabbling duck populations as well as many other grassland bird species.
Although wetlands occur throughout most of the region, their density varies according to surface form patterns, relief, composition of glacial materials and human activities. About 40% of the PPR consists of ground or hummocky moraines, which produce landscapes pitted with numerous depressions of varying size. Wetland densities can vary from 0 to 74 ponds/km2. The flatter landscapes (60% of the region) are comprised of mainly lacustrine and fluvial material and wetland density averages <5/km2. In total, the U.S. portion of the PPR is estimated to contain 1,688,000 ha of wetlands (Prairie Pothole Joint Venture Concept Plan). In Canada there are estimated to be between 2 - 7 million wetlands, depending on weather conditions, and in wet years the area occupied by these basins can exceed 30 million ha (DUC wetland inventory).
The PPR has a continental climate, with extremes in both temperature and annual precipitation. Because prairie potholes are dependent on snowmelt, surface runoff, and direct precipitation as sources of water, the abundance of wetlands varies both seasonally and annually. Some larger PPR wetlands may be hydrologically connected to groundwater, but smaller wetlands are often perched above the water table. These small potholes are often linked hydrologically and thereby supply, obtain, or pass water to other, adjacent potholes. During springtime, ephemeral, temporary, and seasonal wetlands typically contain water for only days, weeks, or 1-2 months, respectively. Wetlands with longer hydroperiods, classified as semi-permanent or permanent wetlands, usually retain water throughout the growing season, although their water levels also decline due to evaporation, transpiration, and seepage. Under extreme drought conditions, entire wetland communities may dry up. More commonly, however, wetland densities vary from 0.8 potholes/km2 during a severe drought to 4.4 potholes/km2 during near average water conditions (Cowardin et al. 1995). This wet-dry cycle rejuvenates prairie potholes by exposing organic matter to aerobic decay, thereby making nutrients more available when the next wet period returns. Dry periods also allow the germination of aquatic plant seeds and set back wetland succession. The dynamic nature of wetlands in the PPR, combined with the high density of diverse wetland types, makes wetland communities of the PPR among the most productive systems on earth and ideal waterfowl breeding habitats. Wetlands are the magnets that attract waterfowl to the PPR.
The PPR is a large landscape, and in any given year weather conditions are not uniform across the region. Waterfowl respond to habitat conditions within the region, settling in areas with the best wetland and upland conditions. Although DU can not control annual precipitation, we can work towards conservation of wetland and uplands so that in wet years habitat in the PPR is in the best condition for maximum waterfowl response.
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