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题目材料：
In 1995, after an absence of nearly 10 years, wolves were reintroduced into Yellowstone National Park. During the wolf-free era, heavy browsing of aspen trees by elk populations spelled doom not only for the trees themselves but for a host of other creatures dependent on them, such as beavers, whose population in Yellowstone crashed after wolves were removed. Without beavers to create ponds, wetland ecosystems—aquatic plants, amphibians, birds—were devastated. When wolves retuned, grazers and browsers resumed normal patterns of behavior, preferring safer, open areas over the dense cover and streamsides where carnivores can lurk. Keeping elk wary and on the move, wolves gave aspen and other young trees the opportunity to grow and become reestablished.
以上解析由 考满分老师提供。
