Range expansion of Pallas's squirrel (Callosciurus erythraeus) introduced in southern France: Habitat suitability and space use

The study of space use and population density in different habitats is an important step in understanding the expansion process of an introduced species and in gathering useful knowledge for management actions. Pallas's squirrel (Callosciurus erythraeus) was introduced on the Cap d'Antibes (southeastern France) at the end of the 1960s. We used direct observations from a grid map centred onto the known historical distribution to document the expanding range of Pallas's squirrel. We assessed habitat suitability in the invaded area through distance sampling and nest counts and examined space use by quantifying the size of the home range and intra- and inter-sexual overlap based on radio-tracking in a suitable habitat. Our results confirm that Pallas's squirrel has expanded its range exponentially over the past two decades, although with low diffusion coefficients (0.08-0.20 km/yr). Squirrels reached the highest relative densities in suburban woodlands, in which females showed no territorial behaviour. Our results indicate that Pallas's squirrel was able to establish in various habitats in Antibes, preferentially in woodlands and gardens. Densely urbanised areas and the presence of the highway A8 (E80) acted as barriers that slowed range expansion. Such information is crucial to improve the control programme started in 2012 to limit the spread of this potentially invasive squirrel.