Biologists were unpleasantly surprised to discover that nutria, an invasive aquatic rodent, had returned to California in 2017 after being eradicated decades before. Now they’re in a high-stakes battle to remove nutria from the state’s waterways: Left unchecked, nutria could destroy crops, damage wildlife habitat, and wreak major havoc on dikes and dams, compromising California’s water infrastructure. So far, the quick-to-reproduce nutria are spreading faster than biologists can handle.