Abstract
Unseasonably early blizzards in the northern Great Plains threaten large mammal populations unacclimated for variable and extreme winter conditions. This region averaged 22 blizzards per winter season during the 2010s, up from 6 during the 1960s, and is anticipated to average 32 blizzards by the 2050s. In early October 2013, the fatal Atlas Blizzard affected four livestock and captive species in 16 counties of western South Dakota. Expected one-week total death losses for the study area were estimated from national average background mortality rates: 161 cattle, 102 sheep, 9 horses, and 6 bison. However, observed death loss varied significantly (McNemar’s Test: p < 0.001) from the expected during the blizzard with: 35,682 cattle; 6428 sheep; 400 horses; and 40 bison. Observed proportional mortalities varied significantly from the expected proportional mortalities in cattle (83.9% vs. 58.0%); sheep (15.1% vs. 36.7%); horse (0.9% vs. 3.2%); and bison (0.1% vs. 2.1%; chi-squared goodness-of-fit: χ23 = 16.85, p ≤ 0.001). Husbandry practices, animal behavior, and physiology may also explain some of the inequitable death losses for each species. Bison appear to be resilient to blizzards and blizzards are expected to increase due to climate change, therefore, bison may offer viability for ranchers in the face of blizzards and more variable weather.
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