Effects of resource availability and interspecific interactions on Arctic and red foxes' winter use of ungulate carrion in the Fennoscandian low-Arctic tundra

In the Arctic tundra, predators face recurrent periods of food scarcity and often turn to ungulate carcasses as an alternative food source. As important and localized resource patches, carrion promotes co-occurrence of different individuals, and its use by predators is likely to be affected by interspecific competition. We studied how interspecific competition and resource availability impact winter use of carrion by Arctic and red foxes in low Arctic Fennoscandia. We predicted that presence of red foxes limits Arctic foxes’ use of carrion, and that competition depends on the availability of other resources. We monitored Arctic and red fox presence at supplied carrion using camera traps. From 2006 to 2021, between 16 and 20 cameras were active for two months in late winter (288 camera-winters). Using a multi-species dynamic occupancy model at a week-to-week scale, we evaluated use of carrion by foxes while accounting for the presence of competitors, rodent availability and supplemental feeding provided to Arctic foxes. Competition affected carrion use by increasing both species’ probability to leave occupied carcasses sites between consecutive weeks. This increase was similar for the two species, suggesting symmetrical avoidance. Increased rodent abundance was associated with a higher probability of colonizing carrion sites for both species. For Arctic foxes, however, this increase was only observed at carcasses unoccupied by red foxes, showing greater avoidance when alternative preys are available. Supplementary feeding increased Arctic foxes' carrion use, regardless of red fox presence. Contrary to expectations, we did not find strong signs of asymmetric competition for carrion in winter, which suggests that interactions for resources at a short time scale are not necessarily aligned with interactions at the scale of the population. In addition, we found that competition for carcasses depends on the availability of other resources, suggesting that interactions between predators depend on the ecological context.

Methods
COAT is a project aiming to give robust documentation and predictions on climate change impacts on biodiversity, natural resources and ecosystem processes in the Norwegian Arctic, as a basis for management interventions and adaptations. This dataset is obtained from a camera trap survey conducted on Varanger Peninsula, Northern Norway between 2006 and 2021.

Each year, between 16 and 20 camera traps were active taking photos every 10 min for two months in late winter. We used several camera models with different fields of view (Camtrak; Reconyx Rapidfire, Hyperfire and Hyperfire 2). The cameras were painted in white, modified to have a flat front keeping snow from accumulating and powered by external batteries placed in a waterproof contained under the snow. Pictures were visually inspected and presence of species was recorded. Pictures with bad visibility were excluded. To estimate use of carrion, a block of ca 15 kg of frozen reindeer slaughtering remains (originally produced as dog food and consisting of tendons, fat, small entrail and meat fragments) was placed at 2-3 m in front of each camera and replaced two to three times during the season. For each photo, we recorded whether the carcass was present.