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

Abstract

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 the 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 supp lied carrion using camera traps. From 2006 to 2021, between 16 and 20 cameras were active for 2 months in late winter (288 camera-winters). Using a multi-species dynamic occupancy model at a week-to-week scale, we evaluated the 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 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.

Keywords: Arctic fox (Vulpes lagopus); intraguild interactions; occupancy; red fox (Vulpes vulpes); resource availability; scavengers; tundra.

FIGURE 1

Location of the Varanger Peninsula in northern Norway and map of the study area. Camera‐trap sites are shown with blue polygons and locations of small rodent trapping sites with white squares. The approximate location of feeding stations is shown with yellow circles. Roads are shown in red and forests in green. The altitude is represented in shades of gray, with darkest tones indicating low altitudes. The gradations are every 100 m.

FIGURE 2

Example of images from the camera‐trap survey on Varanger Peninsula. The images show, from top left to bottom, a red fox, Arctic and red foxes (red fox in the foreground and Arctic fox in the background), and an Arctic fox. Frozen remains of reindeer were placed in front of the cameras and are visible in the top right and bottom pictures.

FIGURE 3

Presentation of the multi‐species dynamic occupancy model used in this study: (a) Overview of the model. Panel A presents a state sequence at a camera station a given year. Primary periods of 1 week are shown with various colors (blue: week 1, orange: week 2, green: week 3, …) and the observed state is shown under the state sequence. The occupancy state is constant during each week and the observed state can vary every day based on the observation model (panel B). The transition model (panel C) describes how occupancy states vary between weeks; (b) Presentation of the model parameters.

FIGURE 4

Posterior distributions of average detection, initial occupancy, colonization and extinction probabilities for Arctic (blue) and red (yellow) foxes. All probabilities are calculated using the estimated intercept only, setting the value of covariate to their average value across the dataset.

FIGURE 5

Probability of carrion use for Arctic (blue) and red (yellow) foxes as a function of the positions on the coast to land (CLG) and tundra to forest (TFG) gradients, the supplementary feeding index and the rodent abundance. Solid lines represent posterior medians, shaded ribbons represent 50% and dashed lines 90% credible intervals. Red dots show the values taken by the covariate of interest in the real dataset. Carrion use is defined as the stationary occupancy probability, calculated as the steady state of the estimated transition matrix.

FIGURE 6

Arctic and Red foxes' colonization and extinction probabilities conditional to the other species' absence (green) or presence (magenta) as a function of rodent abundance and supplementary feeding. Solid lines represent posterior medians, shaded ribbons represent 50%, and dashed lines 90% credible intervals.