Once almost extinct, beavers are making a powerful comeback across Europe and North America — and transforming ecosystems along the way. In Germany's Sophienfließ stream, their dams filter polluted water, raise groundwater levels and protect against floods and wildfires. Scientists call them "ecosystem engineers" for their ability to restore biodiversity and stabilize water supplies amid climate change. But as populations grow, balancing their impact with human needs is the next big challenge.

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