Data from: 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 dietary niche
partitioning can facilitate coexistence between native and introduced
species. We also developed a novel approach to determine the influence of
fluctuating food availability on carbon enrichment in consumers. Mount
Graham red squirrels and introduced Abert’s squirrels partitioned the
dietary niche similarly to naturally syntopic populations. Removals had no
apparent effect. Diet of MGRS was more closely linked to availability of
resources than to presence of Abert’s squirrels. Flexible dietary niche of
introduced Abert’s squirrels may have allowed them to exploit a resource
opportunity in syntopy with MGRS. Variable food production of MGRS habitat
may intensify competition in poor years, and territorial defense against
non‐native Abert’s squirrels likely imposes fitness costs on individual
MGRS. Similarity in our model species’ diets may make MGRS more vulnerable
to competition if climate change eliminates the advantages of
larder‐hoarding. Where introduced populations of ecologically similar
species are better adapted to changing conditions, they may ultimately
replace southern peripheral populations of native species.