Phenological overlap of terrestrial and marine food resources did not reduce salmon consumption by Kodiak brown bears

Abstract

Climate change is altering the predictability of timing of resource pulses and how consumers interact with them, resulting in phenomena placed under the trophic mismatch hypothesis (TMH). This occurs when a consumer fails to overlap and capitalize on availability of a key food resource pulse. Such changes to predator-prey interactions can alter nutrient transfer, destabilize food webs, and reduce species reproduction and survival. Brown bears (Ursus arctos) on the Kodiak Archipelago, Alaska, USA, have purportedly altered their foraging toward increased use of terrestrial resources under TMH. We examined how the extent of within- and across-year overlap between abundances of Pacific salmon (Oncorhynchus spp.) and berries altered salmon assimilation by brown bears on the Kodiak Archipelago during 2015–2017. We further examined whether salmon consumption differed with brown bear sex and age. We found no decrease in salmon assimilation in response to variation in terrestrial and marine food overlap across and within years. Overall salmon assimilation was high, representing over 60% of brown bear diets in 2015 and 2017, with lower (44%) consumption of salmon in 2016 corresponding with a 28% reduction in salmon abundance. Within years, monthly salmon assimilation was lower in June when salmon began to enter the system, then increased and remained similar during July–October, despite 0–71% within month overlap in peak salmon and berry availability. Older brown bears, particularly males, consumed more salmon than did females and younger bears. While the potential ecosystem impacts of TMH may pose a lesser threat to brown bears than previously reported, possible future declines in Pacific salmon abundance due to climate change may adversely affect North American coastal brown bear populations by reducing abundance of this important food source.