Eastern Gray Squirrel Ecology and Interactions with Western Gray Squirrels

Abstract

Populations of State-threatened western gray squirrels (<italic>Sciurus griseus</italic>) have declined in areas invaded by introduced eastern gray squirrels (<italic>S. carolinensis</italic>) in the Western United States, but little is known about competitive interactions between these species. The western gray squirrel is an ecologically important member of oak woodlands, and intensive efforts to recover this species are underway in Washington. We tracked eastern and western gray squirrels with radio-telemetry for over four years to investigate resource use and interactions between species on Joint Base Lewis-McChord near Tacoma, WA. Following a pre-treatment monitoring period of 1-2 years, we experimentally removed eastern gray squirrels from two sites and monitored both species at two control sites for up to 2.5 years to measure competitive effects on western gray squirrels based on measures of spatial-partitioning, body mass, fecundity, and survival rates. We also described dietary overlap between species based on foraging observations and examination of fungal spores in fecal pellets collected from squirrels during the study. Dietary overlap for most food resources was high between eastern and western gray squirrels but they did not share space and had little overlap in their use of habitat types. Western gray squirrels were found primarily in coniferous uplands with little cover of understory vegetation, whereas eastern gray squirrels were in riparian areas with deciduous trees and dense cover of understory vegetation. Following removal treatments, few western gray squirrels used areas formerly occupied by eastern gray squirrels, and we found no increases in body mass, fecundity, or survival for western gray squirrels. Although interspecies avoidance or competitive interactions may occur in habitats suitable for both species, such habitats were rare on our study area. Coexistence of eastern and western gray squirrels appears possible where distinctly different upland and riparian habitats occur in an area.