Long-term fluctuations in an arctic fox (Vulpes lagopus) population: a matter of diet?

The long-term dynamics of predator populations may be driven by fluctuations in resource availability and reflect ecosystem changes in response to climate change. The Icelandic arctic fox population has known major fluctuations in size since the 1950s, starting with a decreasing period until late 1970s, followed by a six-fold increase until 2008 when the population dropped to half its size within five years until it recently bounced back again. Using stable isotope analysis of bone collagen over a long-time series (1979-2018), I aimed at identifying the main resources used by Icelandic arctic foxes during periods of growth and decline to assess if the recent variations in their population size are linked to fluctuations in the availability of resources. Building on the results of Carbonell Ellgutter & al. (2020), I hypothesized that the decline in seabirds abundance was responsible for the decrease in the fox population. Additionally, I expected that the preceding growth period led to increased competition among foxes, causing a rise in inter-individual variations in their diet, ultimately leading to variations in their isotopic niches at the population scale. The isotopic signatures of arctic foxes differed drastically between their habitats, as well as trends in δ13C and δ15N ratios over time. Inland foxes showed an overall shift towards more terrestrial preys, whereas coastal foxes displayed a relatively stable use of marine resources over the years. Stable isotopes mixing models suggested that marine resources and rock ptarmigans were the most important food sources, and highlighted a rather stable diet in coastal habitats compared to inland habitats where more fluctuations in dietary composition were observed. Isotopic niche breadths showed broader niches for coastal foxes than inland foxes, and also highlighted more variations in inland habitats where foxes seemed to diversify their diet during periods of growth, when their preferred prey became scarce and when the number of conspecifics increased. On the other hand, coastal foxes had a constant niche breadth throughout the study period and seemed to adopt a more generalist behavior. Fluctuations in the Icelandic arctic foxes population occurred without major changes in their dietary composition despite the variations in the abundance of their main resources