Diets of native and introduced tree squirrels in Washington

Abstract

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. © 2019 The Wildlife Society.

Differential Resource Use between Native and Introduced Gray Squirrels

“…Nevertheless, it was apparent that western gray squirrels do not require riparian areas to support populations in our study area. As in California (Byrne 1979), western gray squirrels on the Base, but not eastern gray squirrels, frequently foraged on seeds of conifers; however, both species consumed truffles with similar frequency in our study area (Johnston et al 2019). Many riparian areas on the Base had dense shrub cover, which was a top predictor for discriminating habitats of eastern and western gray squirrels in analyses of vegetation at field plots (Johnston 2013).…”

“…Eastern gray squirrels may be poorly adapted to conifer‐dominated forests with little diversity of deciduous trees and shrubs. Douglas‐fir and ponderosa pine were the dominant conifers on much of our study area, but we rarely observed eastern gray squirrels feeding on their seeds (Johnston et al 2019). In their native range, eastern gray squirrels often use bottomland hardwood forests and avoid uplands dominated by conifers (Fischer and Holler 1991, Riege 1991, Steele and Koprowski 2001).…”

“…Conifer mast failed the same year based on similar surveys of both Douglas‐fir and ponderosa pine on our study area (Vander Haegen and Orth 2011). During these mast failures, squirrels sought alternative food sources (Johnston et al 2019), including stripping of bark by western gray squirrels, indicating stressful conditions in 2010. Nevertheless, habitats of gray squirrels differed between species, despite periodic shortages in important food resources.…”

“…From 2007–2012, biologists augmented the population by releasing western gray squirrels translocated from other populations in Washington and Oregon, USA, and monitored their movements alongside resident squirrels with radio‐telemetry to investigate squirrel ecology and evaluate augmentation success (Vander Haegen and Orth 2011, Vander Haegen et al 2018). Concurrently, Johnston (2013) radio‐tracked eastern gray squirrels and experimentally removed some individuals to evaluate competitive interactions with western gray squirrels based on spatial partitioning, resource use, fitness correlates, diet (Johnston et al 2019), and behavior. In the present study, no translocated females had spatiotemporal overlap with radio‐tracked eastern gray squirrels and only 1 translocated female had spatiotemporal overlap with a resident, female western gray squirrel used in our analysis.…”