Marine resources alter tundra food web dynamics by subsidizing a terrestrial predator on the sea ice.

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 like Arctic foxes (Vulpes lagopus) cross into the marine environment (sea ice) during winter to access marine resources. Arctic foxes kill seal pups and scavenge seal carrion (often remains 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 remained untested. We evaluated tundra food web dynamics at the terrestrial-marine interface from an ecosystem-level perspective by assessing: (1) how winter environmental conditions affect rodent abundance and marine subsidy availability, (2) the response of the Arctic fox population to this seasonal food variability, and (3) the subsequent effects of Arctic foxes on Canada goose (Branta canadensis interior) reproduction. Arctic foxes responded numerically to rodent abundance, which was positively related to snow persistence. Arctic fox abundance was positively related to polar bear body condition metrics, which were used as a proxy for marine subsidy availability. Canada goose reproductive success, in turn, was negatively related to Arctic fox abundance. Long-term trends in goose reproduction and snow persistence also indicate an ongoing phenological mismatch between nesting initiation and spring onset. Our results reveal near-term apparent competition between rodents and geese through a shared predator, Arctic foxes, contrasting with prior studies evaluating rodent-goose-predator relationships. Moreover, we establish a link between tundra and sea ice food webs by demonstrating how seal availability can affect goose reproduction indirectly by increasing Arctic fox predation on goose nests via a population response of foxes to marine resources. These marine resources are often provisioned by polar bears, and with both Arctic foxes and polar bears undergoing long-term regional declines evidently driven by climate-related changes in prey abundance and availability, we contextualize our study within ongoing climate change and highlight the vulnerability of this likely widespread terrestrial-marine linkage in a warming Arctic.