Experimental removals reveal dietary niche partitioning facilitates coexistence between native and introduced species

Niche overlap between native species and ecologically similar invaders can lead to competitive exclusion of threatened native species, but if two such species also co‐occur naturally elsewhere, interactions between native and introduced populations may mirror coevolved niche partitioning that reduces competition and promotes coexistence.
A single, insular population of Fremont’s squirrel (Tamiasciurus fremonti) the Mount Graham red squirrel (MGRS; T. f. grahamensis) in the Pinaleño Mountains, Arizona, USA, is critically endangered and resource competition with introduced Abert’s squirrels (Sciurus aberti) may threaten its long‐term persistence. The species are naturally synoptic in other mountain sites, and both consume diets comprised primarily of conifer seeds and fungi.
We conducted experimental removals of introduced Abert’s squirrels and used stable isotope analysis of diets before and after removals, and of diets in naturally syntopic populations to test the hypothesis that diet...