Climate change is altering the seasonal timing of life-cycle events in organisms across the planet, but the magnitude of change often varies among taxa. Unequal phenological responses can cause the temporal relationships among species to change, potentially altering the strength of interaction. A large body of work has explored what happens when co-evolved species shift out of sync, but virtually no studies have documented the effects of climate-induced synchronization, which could remove temporal barriers between species and create novel interactions. Using multiple sources of distribution data (aerial survey, GPS collar, and camera traps), we explored how a keystone predator, the Kodiak brown bear (Ursus arctos middendorffi), responded to synchronizing phenological shifts between its primary trophic resources, sockeye salmon (Oncorhynchus nerka) and red elderberry (Sambucus racemosa).
In years with anomalously high spring air temperatures, elderberry fruiting phenology occurred several weeks earlier in the summer, causing it to overlap with the spawning phenology of salmon. During years with synchronized resource phenologies, bears left abundant, high-protein spawning salmon to forage on berries on adjacent hillsides. Scat surveys corroborated that bears switched from salmon to elderberries, which have lower energy density than salmon, but likely offer higher net energy intake due to lower costs of assimilation. This prey switching behavior attenuated one of the most iconic predator-prey interactions in North America and certainly altered the many ecological functions and services that result from bears foraging on salmon. This is one of the first examples of how climate-induced shifts in resource phenology can restructure food webs through a mechanism other than trophic mismatch. The current emphasis on singular consumer-resource interactions fails to capture how climate-altered phenologies are rescheduling resource availability and in turn altering how energy flows through ecosystems.