Interspecific competition between alien Pallas's squirrels and Eurasian red squirrels reduces density of the native species.
When alien species introduced into a new environment have a strong niche overlap with ecologically similar native species, interspecific competition can cause a decrease in abundance and distribution of native species. Pallas's squirrel (Callosciurus erythraeus) was introduced in Northern Italy where it currently co-occurs with native Eurasian red squirrels (Sciurus vulgaris). The alien species is known for its invasiveness but so far negative effects of Pallas's squirrels on native tree squirrels have not been demonstrated. Here, we compare demographic parameters of red squirrel populations between sites without (red-only sites) and with (red-Pallas's sites) C. erythraeus and present results of trapping and removal of Pallas's squirrel and its effects on red squirrel population dynamics. The native species was patchily distributed and absent in many trapping sites occupied by the Pallas's squirrel. Red squirrels occurred at much lower densities and showed reduced adult survival in areas of co-occurrence than in red-only sites, but there were no differences in reproductive rate. Removing invasive squirrels throughout the study period resulted in re-colonisation by the native species only in some trapping sites, and several alternatives to explain the lack of a marked increase in population size are discussed. This study is the first to provide evidence that presence of Pallas's squirrel reduces viability of local red squirrel populations.