A Test of Interspecific Effects of Introduced Eastern Grey Squirrels, Sciurus carolinensis, on Douglas's Squirrels, Tamiasciurus douglasii, in Vancouver, British Columbia

Abstract

We compared the effects of absence and presence introduced Eastern Grey Squirrels (Sciurus carolinensis) on the demography of native Douglas’s Squirrels (Tamiasciurus douglasii) in two urban parks in Vancouver, British Columbia: Ecological Reserve #74 in Pacific Spirit Regional Park (Douglas’s Squirrel only) and Stanley Park (Douglas’s and Eastern Grey squirrels). Based on the exploitative competition hypothesis, we predicted that in the presence of introduced Eastern Grey squirrels, Douglas’s Squirrels would occur at lower densities, have larger home ranges, lower body mass, and poorer reproduction. Using mark-recapture methods, we found no differences in density, home range, or body mass of Douglas’s Squirrel between parks. However, the proportion of breeding Douglas’s Squirrels was higher in Ecological Reserve #74 in the absence of Eastern Grey Squirrel, than in Stanley Park. We found no evidence that Eastern Grey Squirrels are displacing Douglas’s Squirrels in Stanley Park, but less conspicuous negative effects such as reduced breeding propensity may still reflect the competitive interactions of the two squirrel species.