Estimating Red Fox Density Using Non-Invasive Genetic Sampling and Spatial Capture-Recapture Modeling

Abstract Spatial capture-recapture modelling (SCR) is a powerful tool for estimating densities, population size and space use of elusive animals. Here, we applied SCR modeling to non-invasive genetic sampling (NGS) data to estimate red fox (Vulpes vulpes) densities in two areas of boreal forest in central (2016 - 2018) and southern Norway (2017 - 2018). Estimated densities were overall lower in the northern study area (mean = 0.04 foxes per km2 [95%CI: 0.02-0.09] in 2016, 0.09 [0.05-0.18] in 2017 and 0.07 [0.04-0.13] in 2018) compared to the southern study area (0.16 [0.09-0.26] in 2017 and 0.10 [0.07-0.16] in 2018). We found a positive effect of forest cover on density in the northern, but not the southern study area. The absence of an effect in the southern area may reflect a paucity of evidence caused by low variation in forest cover, but could also be due to climatic differences (e.g., winter severity) between the two areas. Estimated mean home range size in the northern study area was 45 km2 [34-60] for females and 88 km2 [69-113] for males. Mean home range sizes were smaller in the southern study area (26 km2 [16-42] for females and 56 km2 [35-91] for males). In both study areas, detection probability was session-dependent and affected by sampling effort. This study highlights how SCR modeling in combination with NGS can be used to efficiently monitor red fox populations, and simultaneously incorporate ecological factors and estimate their effects on population density and space-use.