Middleton's observations therefore demonstrate a severe reversal of fortunes. Ecologists have reported troubled times for migrating animals all over the world, and attributed the problems to habitat changes wrought by human development and climate change. Middleton et al. point to the same influences, but draw a subtle distinction which they believe makes this a novel case study.
The Yellowstone elk enjoy some of the best open range of modern times, and a migratory path unimpeded by conspicuous physical barriers of modern infrastructure. Middleton et al argued that drought and the return of predators, specifically bears, to Yellowstone are causing the observed low pregnancy rate and low calf survival for migratory elk.
"Many of the forum commentaries discuss the implications of our work for management and conservation of large carnivores and their prey in Yellowstone, especially wolves," said Middleton. "However, a persistent focus on the impact of re-introduced wolves among scientists, wildlife managers, and the public misses key roles of grizzly bears and severe drought in limiting elk populations."
Over the last two decades, summers have been hotter and dryer in the summer range of the migratory Clarks Fork elk. Satellite imagery shows that the length of spring vegetation "green-up," a critical time for female elk to gain the fat the need to support reproduction, shortened by 27 days over 21 years. During the same time period, wolves were reintroduced to the park, and the numbers of wolves and bears are growing. Both pressures are ultimately the result of human choices through our manipulations of predators and increa
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Ecological Society of America