Browsing College of Natural Science and Mathematics (CNSM) by Author "Walker, Johann"
Nest and duckling survival of scaup at Minto Flats, AlaskaWalker, Johann (2004-05)To address the hypothesis that declines in recruitment were related to recent declines in abundance of lesser and greater scaup, I estimated variation in nest and duckling survival of these species at Minto Flats, Alaska (64°50'N, 148°50'W) during 2002-2003. I included nest survival data from two previous studies conducted during 1989-1993 in my analysis. Daily Survival Rate (DSR) of nests was variable within and among years and among habitats. Estimated nest survival of scaup ranged from 0.02 (95% CI: 0.00 to 0.06) in 1992 to 0.61 (95% CI: 0.50 to 0.74) in 1993. Predation was the primary apparent cause of nest failure, and flooding of nests was an important secondary influence. DSR of ducklings varied between years and increased with age of the ducklings and body condition of the brood female. Duckling survival to 30 days was: 0.24 (95% CI: 0.16 to 0.36) in 2002 and 0.03 (95% CI: 0.00 to 0.19) in 2003. I conclude that high temporal variability in production of scaup at Minto Flats was likely related to annual variation in the risks of predation and flooding and indicated that intermittent years of high production could be particularly influential to this population.
Survival of duck nests, distribution of duck broods, and habitat conservation targeting in the Prairie Pothole RegionWalker, Johann; Lindberg, Mark S.; Rotella, Jay J.; Ringelman, James K.; Hunter, Christine M. (2011-05)The Prairie Pothole Region (PPR) is an important breeding region for populations of upland-nesting ducks (Anas spp.). I studied survival of duck nests, distribution of duck broods, and conservation easement targeting in the PPR of North and South Dakota, USA. Nest survival of common duck species varied from 0.02 (ŜE = 0.01) to 0.23 (ŜE = 0.02) among study sites and years and was positively related to current pond density and primary productivity and negatively related to recent pond density and primary productivity. This result was consistent with the hypothesis that nest predators and thus nest survival were responding to changes in productivity induced by relatively short-term (1yr-2yr) precipitation cycles. Distribution of broods was positively related to wetland area and proportion of perennial grass cover on the study site. Estimated probability of wetland occupancy for a brood of a representative species, gadwall (Anas strepera), increased from 0.08 (90% Credible Interval: 0.07, 0.10) to 0.28 (0.24, 0.33) as wetland area increased from 0.19 ha to 2.12 ha. As proportion of perennial grass cover on a study site increased from 0.03 to 0.99, estimated probability of wetland occupancy by a gadwall brood increased from 0.12 (0.09, 0.16) to 0.20 (0.16, 0.25). These relationships identified wetland basins and landscapes with a higher probability of occupancy. The extensive repeat-visit brood survey was therefore a useful way to learn about the distribution of duck broods across a large geographic extent. I also investigated need and opportunity to refine current habitat protection strategies in the PPR. Area of habitat protected declined annually during 2000-2009 while cost of protection increased 248% from $210/ha to $730/ha and cropland rental rates increased 40%. Estimated area protected in 2009 was 210 km² (95% Confidence Interval: 133 km² to 287 km²), and 2,792 km² was protected during 2000-2009. Refocusing easement acquisition efforts on the 3,189 km² that was located in landscapes of highest priority based on pair density and was at relatively high risk of conversion, but was below the 25th percentile of cost provided a 24% reduction in per/hectare cost of protection and a 20% increase in area protected.