Effects of grazing management on sharp-tailed grouse ecology in mixed-grass prairies
Milligan, Megan Cochran
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Many grassland species co-evolved with large herbivores and require habitats along the entire structural gradient created by grazing. Widespread declines of grassland birds, however, have prompted concerns about rangeland management. Conceptually, rest-rotation grazing functions as a conservation strategy to mimic historic disturbance regimes and create pasture-level heterogeneity in the absence of fire, but its utility for improving grouse habitat has not been tested. We evaluated rest-rotation grazing as a conservation management technique compared to traditional grazing systems, including summer rotation and season-long grazing, and assessed the effects on sharp-tailed grouse (Tympanuchus phasianellus), an indicator species for grassland ecosystems. We monitored radio-marked female sharp-tailed grouse in eastern Montana and western North Dakota during 2016-2018 to monitor nesting ecology, adult survival, and habitat selection. Both nest site selection and nest survival were directly related to vertical nesting cover, which was only weakly related to grazing management variables, including grazing system and stocking rate, at moderate stocking rates (< or = 2 AUM ha^-1). Cattle presence during the nesting period had a positive effect on daily nest survival, potentially because either the cow or rancher presence affected predator behavior. Grazing management did not have a meaningful influence on any aspect of the overall survival of adult female sharp-tailed grouse, although seasonal patterns of mortality risk differed among systems. More importantly, cropland increased mortality risk of adult female sharp-tailed grouse. At broad spatial scales, females selected for grassland habitats, but exhibited limited to no selection for either landscape or management variables when selecting habitat at smaller spatial scales. We found limited evidence that grazing management was a driver of habitat selection at either spatial scale examined. Furthermore, female sharp-tailed grouse exhibited strong individual variation in both home range size and third-order habitat selection. Taken together, our results suggest that rest-rotation grazing did not influence any aspect of sharp-tailed grouse ecology we studied relative to other grazing systems and did not increase pasture-level heterogeneity in relevant vegetation variables. Therefore, grazing management strategies with moderate stocking rates that preserve large intact grasslands are a better conservation strategy for sharp-tailed grouse than prescribing specific grazing systems.