Diets of Native and Introduced Tree Squirrels in Washington

Competition for food resources can be a primary mechanism for displacement of native species by introduced species. Investigation of dietary partitioning between potential competitors and spatiotemporal variability in food resources can reveal contested food items and intensity of competition. Introduced eastern gray squirrels (Sciurus carolinensis) have been implicated as competitors with western gray squirrels (S. griseus) and Douglas’ squirrels (Tamiasciurus douglasii), but little is known about interactions among these species. We collected foraging observations and fecal pellets of sympatric gray and Douglas’ squirrels trapped and radio-tracked in western Washington, USA, from 2007 to 2012. We compared food resources consumed across species, seasons, and years to evaluate dietary overlap and the potential for competitive interactions. All squirrel species ate hypogeous fungi throughout all seasons; spores of several genera were present in 272 of 275 fecal samples and observed in all months. Rhizopogon, Geopora, and Melanogaster spp. occurred in most fecal pellets for all squirrels, but the diet of the Douglas’ squirrels had slightly lower richness of hypogeous fungi than gray squirrels. Although foraging observations suggest that eastern and western gray squirrels shared important dietary items such as acorns, strong differences in consumption of seeds of a common conifer may facilitate their coexistence. Our results suggest that dietary overlap among arboreal squirrels could lead to competitive interactions during periods of food scarcity, but subtle differences may be sufficient to permit long-term coexistence. Tree squirrels in Washington would likely benefit from forest management practices that promote or sustain robust crops of hypogeous fungi.