Sean M Johnson-Bice , Frank B. Baldwin, Evan Richardson, James D. Roth

Predator use of resource subsidies can strengthen top-down effects on prey when predators respond numerically to subsidies. Although allochthonous subsidies are generally transported along natural gradients, consumers can cross ecosystem boundaries to acquire subsidies, thereby linking disparate ecosystems. In coastal Arctic ecosystems, terrestrial predators can easily cross into the marine environment (sea ice) during winter, which is a foraging strategy that Arctic foxes (Vulpes lagopus) use to access marine subsidies – largely seal carrion leftover from polar bear (Ursus maritimus) kills – especially when rodent abundance is low. Terrestrial predator use of marine subsidies may strengthen the top-down control of tundra food webs, but this hypothesis had remained untested. We took an ecosystem-level approach towards evaluating tundra food web dynamics at the terrestrial–marine interface by assessing: (i) how winter environmental conditions affect rodent abundance and marine subsidy availability, (ii) the responses of the Arctic fox population to this winter food variability, and (iii) the subsequent effects of Arctic foxes on the reproductive success of other tundra prey (Canada geese [Branta canadensis interior]). Arctic foxes responded numerically to rodent abundance and marine subsidy availability, which were positively related to greater snow and sea ice persistence, respectively. Canada goose reproductive success, in turn, was negatively related to Arctic fox abundance. Long-term trends in Canada goose reproduction and snow persistence on the tundra also indicate an ongoing phenological mismatch between nesting initiation and the onset of spring. Our results reveal short-term apparent competition between rodents and geese through a shared predator, Arctic foxes, which contrasts with prior studies evaluating rodent–goose–predator relationships. Moreover, we establish a link between tundra and sea ice food webs by demonstrating seal availability has a negative indirect effect on goose reproduction via carrion provisioning from polar bears to Arctic foxes, both of which are undergoing long-term population declines evidently driven by climate-related changes in prey abundance and availability. Given the importance of the winter environment in mediating these trophic interactions, we contextualize our study within ongoing climate change and highlight the vulnerability of this likely widespread terrestrial–marine linkage in a warming Arctic.