Farther inland invasion of Finlayson’s squirrel Callosciurus finlaysonii (Horsfield 1823) poses a new conservation challenge for the endemic near threatened Calabrian black squirrel Sciurus meridionalis Lucifero 1907 (Rodentia: Sciuridae)

Understanding both native and non-native species ecology, including their distribution and interaction, is crucial for making informed decisions on conservation and management strategies, particularly for endemic threatened species. In this study, we report an update on the non-native Finlayson’s squirrel Callosciurus finlaysonii invasion of southernmost Campania (Italy), where the species is rapidly expanding its range towards inland areas. Our observational data confirmed the pattern of invasion for the Finlayson’s squirrel, extending the species’ presence 10 km further inland than what was reported before. Here, we also collected the first evidence of black morph squirrels’ occurrence potentially attributable to the Calabrian black squirrel, Sciurus meridionalis, a near-threatened and little-studied tree squirrel endemic of southern Italy. As a habitat of good quality, the area of potential new presence for this endemic squirrel represents an ecological corridor possibly enhancing individual dispersal from the neighbouring region into new suitable areas. However, due to the invasion of Finlayson’s squirrel, the two species likely became sympatric here, with endemic squirrels possibly suffering the competing effect of the alien species. We highlighted the importance of in-depth studies, including genetic surveys, to better understand the occurrence and ecology of the Calabrian black squirrel outside its historical range, and to plan effective control actions to prevent further expansion of the Finlayson’s squirrels.