The influence of large carnivore recovery and summer conditions on the migratory elk of Wyoming's Absaroka Mountains.

摘要

I studied the Clarks Fork elk herd, a population of 4,000 elk whose migratory individuals winter in outlying areas of the Greater Yellowstone Ecosystem (GYE), and summer in core areas of Yellowstone National Park (YNP). I first evaluated the long-term ecological context of the Clarks Fork herd's changing demography. Migratory have elk experienced a 21-year, 70% reduction in recruitment and a 4-year, 19% depression in the pregnancy rate, caused largely by infrequent reproduction of females that were young or lactating. Over the same period, resident elk have experienced increasing recruitment and a high pregnancy rate. Large-scale changes in predation and habitat quality appear responsible for the declining productivity of migratory elk. Migrants are now exposed to four times as many grizzly bears and wolves as residents. Both predators consume migratory elk calves at high rates in the Yellowstone wilderness, but occur less frequently in the year-round habitats of resident elk, due to lethal management and human disturbance. Migratory elk have also recently experienced a shorter spring green-up, consistent with recent drying and warming trends in the region. These findings suggest that large carnivore recovery and drought, operating simultaneously along an elevation gradient, have disproportionately influenced the demography of migratory elk. Next, I evaluated the hypothesis that the risk of wolf predation is contributing to the low productivity of migratory elk, via changes in winter behavior that reduce individual females' nutrition and pregnancy probability. Although such 'non-consumptive' effects (NCEs) of predators have been studied extensively in small-scale, experimental systems, NCEs have never been comprehensively evaluated among wide-ranging large mammals. I found that during the 24 h after wolves approached within 1 km, elk increased their rates of movement, displacement, and vigilance, but did not reduce their feeding rates or change their habitat use. The late-winter body fat and pregnancy probability of elk were not related to wolf predation risk, and were instead a function of autumn body fat and age. These findings suggest that wolves influence elk demography through direct killing, not NCEs. One of my observations -- that migratory elk experienced 1-km wolf encounters only once every 9 days -- suggests that the current conceptualization of risk effects, developed in smaller-scale study systems, obscures important effects of spatiotemporal scale. Next, I further explored the context-dependency of predation risk effects, evaluating the limiting role of prey body condition. Population- and community-level risk effects hinge on strong antipredator behaviors expressed by individual prey animals, but these behaviors might be limited by the countervailing influence of hunger and starvation, which can result from many biological and ecological processes that operate independently of predation risk. In a synthesis of the literature, I found that in 96% (44/46) of behavioral studies -- including experiments conducted with mammals, birds, insects, fish, reptiles, and amphibians -- prey body condition mediated the strength of antipredator behavior. This suggests that a predictive theory of predation risk effects can be improved by integrating the important role of individual prey condition, alongside other factors identified by prior work -- such as prey social behavior, predator hunting strategy, and spatial scale. Finally, I considered the influence of human-caused ecological changes on the predation rates of the dominant elk calf predator in the GYE, the grizzly bear. Over the past two decades, the availability of key grizzly bear diet items has declined. In particular, an invasion of non-native lake trout has driven a dramatic decline of native cutthroat trout that migrate up the shallow tributaries of Yellowstone Lake to spawn each spring. I explored whether this decline has amplified the effect of the omnivorous grizzly bear on populations of migratory elk that summer inside YNP. My synthesis of research conducted over three decades on grizzly diets and elk populations, including recent study of four elk migrations, indicates that the invasion by lake trout has contributed to increased predation by grizzly bears on the calves of migratory elk. Additionally, a demographic model that incorporates two independent estimates of this increase in predation suggests that its magnitude has been sufficient to reduce the calf recruitment (4-16%) and population growth (2-11%) of migratory elk. The disruption of this important aquatic-terrestrial linkage may hinder the restoration of historic species interactions in YNP, highlighting the urgency of efforts to suppress lake trout in Yellowstone Lake and the importance of preventing such invasions elsewhere. (Abstract shortened by UMI.)