Neonatal Mortality of Elk in Wyoming: Environmental, Population, and Predator Effects
Public concerns over large losses of wild ungulates to predators arise when restoring large carnivore species to former locations or population densities. During the 1990s, mountain lion (Felis concolor) and grizzly bear (Ursus arctos) numbers increased in Jackson Hole, Wyoming, and gray wolves (Canis lupus) were reintroduced to the Greater Yellowstone Ecosystem. We investigated effects of these predators, as well as black bears (Ursus americanus) and coyotes (Canis latrans), on mortality of an abundant and increasing prey species, elk (Cervus elaphus). We captured, radio-instrumented, and monitored survival of 145 elk neonates from 3 cohorts during 1990 - 1992, and 153 neonates from 3 cohorts during 1997 - 1999 when grizzly bears and lions were likely more common than during the earlier period of study. Neonatal (birth through 31 July) mortality of elk due to predation, disease, and accidents increased from 15.2 % to 27.5% (P = 0.01). Sixty-eight percent of all mortality during 1990 - 1992 resulted from predation by black bears and coyotes, compared to 76% during 1997-1999 by black bears, coyotes, grizzly bears, and mountain lions, a non-significant difference (P = 0.49). Weight gains of calves during the first week, but not birth weights, declined from 1990 - 1992 to 1997 - 1999. April temperatures were cooler, delaying spring green-up, and elk numbers were larger during 1997 - 1999 when weight gains and survival of calves declined. Calves that died were more likely to be male, below average birth weight, and had inferior serum nutritional indices. The change in neonatal calf survival reduced the annual growth rate of the Jackson elk herd from 1.26 to 1.23, yielding a decline in the annual increment of approximately 500 animals in a preparturition herd of 11,000 elk. Changes in mid-summer calf:100 cow ratios indicated a 39 - 45% greater decline in neonatal survival than measured among the radioed calves. We suggest increasing predation during the study was partially compensatory, given predator selection of inferior calves and increased mortality of cohorts with reduced first week growth rates. Reduced rate of first week weight gains of elk calves extended the duration of neonatal mortality by one month during 1997 - 1999, and may be as important in predisposing calves to predation and other mortality as low birth weights. Consequently, we conclude that increased predation was a proximate not an ultimate cause of declining neonatal survival during the 1990s. We recommend careful evaluation and hypothesis testing of predator effects on elk as restoration of large carnivores continues.