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Impact of the Conservation Reserve Program on Duck
Recruitment in the U.S. Prairie Pothole Region

Introduction


During 1985-1992, mallard (Anas platyrhynchos), blue-winged teal (A. discors), and northern pintail (A. acuta) breeding populations reached their lowest levels since annual surveys began in 1955 (Dubovsky et al. 1997). Long-term population declines for these species coincided with large-scale declines in nest success throughout the Prairie Pothole Region of the United States (see Fig. 1 for area map) and Canada (Beauchamp et al. 1996). Sargeant and Raveling (1992) concluded that predation was the primary cause of duck nest loss in the Prairie Pothole Region. Klett et al. (1988) concluded that during 1980-84, nest success was inadequate to maintain populations of mallards and northern pintails in most areas of North Dakota, South Dakota and Minnesota and of gadwalls (A. strepera) and blue-winged teal in North Dakota and Minnesota. Long-term changes in predator demographics and lower nest success of ducks have coincided with the conversion of perennial grassland to cropland in the Prairie Pothole Region (Sargeant et al. 1993, Beauchamp et al. 1996). In North Dakota 67%, South Dakota 53%, and Montana 50% of grasslands have been converted, mostly to cropland, since settlement (U.S. Department of Agriculture [USDA] 1994). As grassland cover was converted to cropland, ducks increased their use of the remaining areas of cover for nesting, where predators such as red fox (Vulpes vulpes), striped skunk (Mephitis mephitis), and badger (Taxidea taxus) forage (Cowardin et al. 1983). Greenwood et al. (1995) found that duck nest success on study areas in prairie Canada was negatively correlated with the proportion of land annually cultivated and suggested that nests located in small blocks of nesting habitat were more likely to be destroyed by predators than those located in large contiguous blocks that were not fragmented by cultivation. Ball et al. (1995) reported high duck production rates on study blocks in north-central Montana where large areas of grass remained intact.

Efforts by the U.S. Fish and Wildlife Service (USFWS) to protect and increase grassland cover in the U.S. Prairie Pothole Region include purchasing nearly 112,000 ha of uplands on WPA (Cowardin et al. 1995) that are managed primarily for nesting ducks. Many of these WPA and adjacent private lands have high densities of wetlands that attract breeding ducks. However, studies have documented low nest success on some of these WPA (Klett et al. 1988, Sargeant et al. 1995), possibly due to their isolation in landscapes dominated by cropland. In 1985, Congress authorized the CRP as part of the Food Security Act (Public Law 99-198). The CRP is a national program administered by the USDA with objectives to reduce soil loss on highly erodible land, reduce crop surpluses, and improve wildlife habitat. Landowners contracted with USDA to convert cropland to perennial cover—such as grass or trees, usually for a period of 10 years—in return for annual payments. Enrollment began in 1986 and continued with periodic enrollments through 1995 when 14.7 million ha had been enrolled nationwide.

During 1992, North Dakota had 0.9 million ha and South Dakota and Montana each had 0.5 million ha enrolled in the CRP within the Prairie Pothole Region (USDA, Natural Resources Conservation Service, Bismarck, North Dakota, unpublished data). This land represented 98% of the final enrollment in these areas. Enrolled fields (hereafter CRP cover) in our study area were predominantly planted to a mix of grasses and legumes composed of wheatgrasses (Agropyron spp.), smooth brome (Bromis inermis), alfalfa (Medicago sativa), and sweetclover (Melilotus spp.). CRP cover planted to trees (approx. 1000 ha, [Moulton 1994]) was not included in our study. Haying, grazing, or other commercial uses of CRP plantings were not allowed unless authorized by the Secretary of Agriculture in response to drought or other agricultural emergency. Studies have shown that undisturbed cover of this type is attractive to nesting ducks (Duebbert and Lokemoen 1976, Reynolds et al. 1994, Greenwood et al. 1995, Renner et al. 1995). Klett et al. (1988) reported duck nest success in planted cover (plant composition similar to CRP cover) to be about average compared with other available common habitats. Other studies provided evidence that nest success in planted cover (including CRP cover) is high compared with other cover types (Duebbert 1969, Duebbert and Lokemoen 1976, Kantrud 1993, Reynolds et al. 1994). These findings led to speculation that the CRP would benefit upland nesting ducks.

We assessed duck use and nest success in CRP cover and planted cover on WPA (hereafter WPA cover) in the Prairie Pothole Region of North Dakota, South Dakota, and northeast Montana during 1992-1995. We chose planted cover on WPA as a reference cover because plant composition and structure were similar to CRP, historical nest success data are available for this cover (Cowardin et al. 1988, Klett et al. 1988), and WPA are present throughout our study area. Nest success in WPA cover provided a benchmark for comparing our findings from duck nests found in CRP cover. These results and other nest success and habitat data were used to estimate duck production in our study area under conditions with and without CRP on the landscape. Our objectives were to (1) compare DSR in CRP cover and WPA cover for 1992-1995; (2) explain variation in DSR in CRP cover using characteristics of surrounding landscapes (i.e., percent perennial cover, breeding duck population, number and area of ponds, indices of red fox and coyote [Canis latrans] abundance, and geographic location of study areas); (3) estimate average DSR for CRP cover and other major cover types during 1992-1995 (CRP period) in our study area and compare with estimates for 1980-1984 (pre-CRP); and (4) estimate duck recruitment and other population parameters in our study area during 1992-1997 and compare with predictions from models that simulated a scenario in which cropland had never been converted to CRP.

To address objective 4, the primary purpose of the study, we used habitat-production models to account for the distribution of CRP cover relative to geographic and temporal distributions of other habitats and duck populations.


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