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, recurrent periods of food scarcity force predators to rely on a wide variety of resources. In particular most predators use ungulate carcasses as an alternative food supply, especially in winters when live preys are scarce. 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. Here, 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 the outcome of competition for carrion depends on the availability of alternative food resources, such as rodents. We monitored Arctic and red fox presence at experimentally supplied carrion using camera traps, between 2006 and 2021 in late winter. 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 primarily affected carrion use by increasing both species' probability to leave occupied carcasses to a similar extent, suggesting a symmetrical avoidance. Rodent abundance was associated with an increase in the probability of colonizing carrion for both species. For Arctic foxes, however, this increase was only observed in carcasses unoccupied by red foxes, showing greater avoidance when alternative preys are available. Contrary to expectations, we did not find strong signs of asymmetric competition for carrion in winter. Our results suggest 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.