Hervé Bocherens, Keith A. Hobson, Dorothée G. Drucker, Agnieszka Sergiel, Jon E. Swenson, Andreas Zedrosser, Adrian Marciszak, Elisabeth Iregren, Leena Drenzel, René Kyselý, Grzegorz Lipecki, Daniel Makowiecki, Jan Wagner, Tomasz Zwijacz-Kozica, Susanne A. Fritz, Eloy Revilla, Nuria Selva

Bears are the largest terrestrial omnivores. Although omnivory is increasingly recognised as a dynamic stabilizing force under environmental change, we still know surprisingly little about how bear species respond to changes in terrestrial ecosystems. Across seven extant terrestrial species, we show that trophic position is not fixed but dynamically tracks environmental conditions. By integrating macroecological patterns with stable isotope data from modern populations and fossil records, we reveal a consistent pattern. Bears occupy higher trophic positions in low-productivity environments with short growing seasons and lower trophic positions in productive systems with longer growing seasons. Long-term isotopic records from European populations of brown bears (Ursus arctos) document a pronounced dietary shift towards herbivory from the Late Pleistocene into the Holocene, corresponding with increasing plant productivity and extended growing seasons. These results demonstrate that bears flexibly adjust their functional role within food webs in response to climate-driven changes in primary production and vegetation seasonality. Our findings position bears as climate-responsive omnivores whose trophic plasticity rewires food webs across spatial and temporal scales. Under ongoing global change, such trophic rewiring may modify bears’ functional role in ecosystems while simultaneously stabilizing food web dynamics.