. The bison of Yellowstone National Park. American bison; Mammals. Mortality Factors 73 1966), the predators in this case had alternate prey (elk) to sustain their numbers as bison decreased (from poaching). If bison had been easy or preferred prey, their increase, once they were protected from poaching, might have been prevented. Other Causes Winterkill, probably from the combined effects of climatic stress, forage availability, and physiologi- cal condition of individual ani- mals, was the main cause of ob- served mortality (see Other Mor- tality). Death usually occurred af- ter prolonged we
. The bison of Yellowstone National Park. American bison; Mammals. Mortality Factors 73 1966), the predators in this case had alternate prey (elk) to sustain their numbers as bison decreased (from poaching). If bison had been easy or preferred prey, their increase, once they were protected from poaching, might have been prevented. Other Causes Winterkill, probably from the combined effects of climatic stress, forage availability, and physiologi- cal condition of individual ani- mals, was the main cause of ob- served mortality (see Other Mor- tality). Death usually occurred af- ter prolonged weakening, often in late winter (March and early April). A few animals died annual- ly, but the number increased greatly with severe winters. This suggested that climatic influences (long winters, periods of pro- longed cold, deep and sometimes crusted snow) acting directly and indirectly on the bison were the most important mortality factors. The most apparent direct effect of the winter environment, above the energy levels required to main- tain body temperature, was the effort required to travel between foraging areas. In deep snow the mixed herd groups usually trav- eled in line, plunging to create trenches several feet deep (Fig. 25), frequently for more than a mile. Variations in topography crossed added to their Fig. 25. Trench left in soft snow by traveling bison. Observations during the mild and average winters of the study period indicated that snow depth did not limit forage availability. Bison commonly cleared fairly deep snow (by swinging the head in a sideways motion) with appar- ent ease (Figs. 26, 27). McHugh (1958) observed bison feeding in snow up to 4 feet deep. At higher population levels, snow depth may limit access to forage. However, the size and location of the most used foraging areas suggested that this might happen only under the most extreme conditions (see Use of Forage Areas). Susceptibility to winterkill var- ied according to physical cond
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Keywords: ., bookauthorunitedstatesnatio, bookcentury1900, booksubjectmammals
