Compound food failure on a deteriorated mast system drives Japan's bear conflict crisis.

Climate change is recognized as a key amplifier of human-wildlife conflict, yet the underlying mechanism remains poorly understood. Japan's 2025 bear crisis, which produced 232 casualties, has been widely attributed to rapid population growth, but demographic processes are far too gradual to explain the surge. Using an 18-year dataset across five Tohoku prefectures, we find that it reflects bears responding to a rhythm breakdown of the forest's food supply. Conflict rises sharply once mast production falls below a threshold. Around 2006, the mast system passed a break point, after which lean years grew more severe and crossed that threshold more often. Conflict is further amplified when the alternative food source, indexed by primary productivity, fails alongside mast, and the 2023/2025 outbreaks provide a natural experiment separating compound from single-source failure. Beyond the Japanese bear-mast system, our analysis yields a generalized diagnostic framework that considers failing food rhythms, compound shortages, and the thresholds at which animals enter human space.