In 1995, the United States released 31 grey wolves from Canada into Yellowstone after the predators had been exterminated for nearly 70 years, hoping to rebuild a collapsing ecosystem; today, wolves have helped restore elk numbers, willow forests and beaver populations

For almost 70 years, one of the most famous American national parks lacked its primary predator. By 1926, grey wolves had been eliminated from Yellowstone National Park in accordance with government programmes aimed at controlling predators.

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