| Summary: | Climate change and anthropogenic pressure can strongly impact food webs through modifications to species’ ranges and population foraging strategies, notably altering their exposure to contaminants. In northern ecosystems, boreal-forest species have expanded onto the tundra, where they may disrupt food-web interactions through competition or predation. Their lack of adaptation to the harsh tundra conditions, however, may severely constrain individual behaviors, increasing survival and reproduction costs compared to tundra endemics. A red fox (Vulpes vulpes) population that recently settled onto the coastal tundra of western Hudson Bay, historically occupied by Arctic foxes (Vulpes lagopus), provides a useful model to study the mechanisms and impact of generalist species expansion in Arctic communities. I used satellite telemetry and stable isotope analysis to investigate red fox spatial and dietary response to food scarcity, dietary mercury exposure, and interaction with Arctic foxes. Red foxes did not exclude Arctic foxes by interference, and their movement strategies differed from Arctic foxes, reflecting poor adaptation to food scarcity on the tundra but high behavioral flexibility. Arctic and red foxes’ diet tracked main prey densities. However, unlike Arctic foxes, which mostly consumed tundra rodents and switched to marine resources when rodent abundance decreased, red foxes consumed tundra, forest, and migratory prey in similar proportions. Those results suggested that the two species segregate resources to some extent and winter survival of red foxes relied on accessing forest prey. Both fox species consumed mostly terrestrial prey, explaining their generally low mercury intake, which increased with marine resource consumption. I also assessed the performance of keratinous tissues in predicting body-mercury burden, which despite being moderate at best, does not preclude using keratinous tissues to understand mercury intake while growing. Studying how expanding species respond to edge-habitat conditions ...
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