Effects of cattle grazing on upland nesting duck production in the Aspen Parkland
Warren, Jeffrey Michael
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The beef industry is growing rapidly throughout the Aspen Parkland of Canada, leading to higher stocking rates on pastures and use of previously idled areas. Conversely, increased demand for pastureland has led to conversion of cropland that may have benefited upland nesting ducks by increasing the amount of perennial cover. We undertook the current study to evaluate the relationships of nest-site selection and nesting success of upland-nesting ducks to cattle grazing in the Aspen Parkland. Nearly 4,000 ha of upland cover were searched during the study. Vegetation physiognomy was quantified at each nest and at random points within each field. Despite extensive drought, nest searches located 309 duck nests. Grazing demonstrated a strong negative effect on duck nest densities, as did declining pasture health. Our best model of nesting success indicated an interaction between nest-site vegetation and residual cover. Across most values of residual cover, nesting success was positively influenced by nest-site vegetation. However, at high values of residual cover, nest-site vegetation negatively affected nesting success. Other negative effects on nesting success included cattle presence and grassland area within a 1-km radius. Field-scale wetland area, however, demonstrated a positive relationship with nesting success. Although previous work has demonstrated higher nesting success in pastures than other habitats in the Aspen Parkland, our study is the first that we know of that investigated grazing intensity as a continuous factor along the gradient from idle to > 90 % utilization across a large number of fields (n = 97).