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Greater Yellowstone Ecosystem Report

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The Greater Yellowstone Ecosystem (GYE) may support the densest populations of elk in North America, and a diverse community of large carnivores that prey on elk. From 1984 to 1996, the Jackson elk herd doubled in size exceeding its winter population objective of 11,000 elk by nearly 7,000 animals (Smith and Anderson 1998). In and adjacent to Grand Teton National Park (GTNP), mortality of neonatal elk averaged 15% annually during 1990-1992 (Smith and Anderson 1996). Predation by black bears and coyotes, when calves were less than 4 weeks of age, caused 68% of neonatal mortality. Calves killed by mountain lions were more than 4 months old. Except for hunting, predation was the largest source of mortality of radio­collared elk from birth through adulthood. After 1992, grizzly bear activity on federal cattle grazing allotments in eastern GTNP and the adjacent Spread Creek drainage of the Bridger-Teton National Forest (the East Study Area or East SA) markedly increased as did cattle losses to bears. Personnel of the Wyoming Game and Fish Department attributed 26 cattle losses (25 calves, 1 cow) in the East SA in 1993 to grizzly bear predation (Wyoming Game and Fish Department 1995). The Wyoming Game and Fish Commission, which compensates cattle producers for losses due to grizzly predation, initiated a study in 1994 to quantify the proportion of cattle losses in the East SA due to grizzly predation. One grizzly bear documented killing cattle each year, was captured and euthanized in GTNP in 1996. Grizzly bears are opportunistic omnivores that consume both plant and animal foods (Blanchard et al. 1992). Grizzly bear predation on elk calves less than 4 weeks of age was the primary cause of mortality of calves and a major factor regulating the size of Yellowstone National Park's northern elk herd (Singer et al. 1997). Smith and Anderson (1996) found no evidence of grizzly predation on Jackson elk during 1990-1992. However, mid-summer calf:cow ratios of elk in the Spread Creek area of eastern GTNP declined after 1993. We initiated this study in 1997 to compare causes of elk calf mortality in the grizzly-occupied East SA and the relatively grizzly-free area of GTNP west of the Snake River (West SA). The objectives were to: determine: 1) if grizzly bears were preying on elk calves, 2) whether such mortality was compensatory or additive to other sources of mortality, 3) if changes in neonatal mortality altered the harvestable surplus of elk from the Jackson elk herd for hunters, and 4) any influence of environmental factors, and physiological status on survival of elk neonates.