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

In western Canada, Eastern Grey Squirrels (Sciurus carolinensis) were introduced to Stanley Park in the city of Vancouver, British Columbia, shortly before 1914. The three to four pairs of introduced Eastern Grey Squirrels came from eastern Canada (Robinson and Cowan 1954). In the early 1980s, Eastern Grey Squirrels colonized the rest of the city of Vancouver, including many of its urban parks. Since then, Eastern Grey Squirrels have spread to many parts of the Greater Vancouver Regional District (Gonzales 1999, 2000). On the mainland in southwest British Columbia, the native tree squirrel is the Douglas’s Squirrel (Tamiasciurus douglasii); it is the ecological counterpart of the Red Squirrel (T. hudsonicus) that is widespread in temperate forests of North America (Steele 1998, 1999). The Eastern Grey Squirrel is native in eastern North America (Hall 1981) and its geographical range overlaps broadly with that of the Red Squirrel. Although Red Squirrels are associated with conifer forests and Eastern Grey Squirrels with mast-bearing hardwood forests, the two species often coexist in mixed forests or urban parks (Riege 1991). Forty years after Eastern Grey Squirrels were introduced to southwestern British Columbia, Robinson and Cowan (1954) conducted a live-trapping and observational study on Eastern Grey Squirrels in Stanley Park. They noted that, in deciduous habitat in Stanley Park, Eastern Grey Squirrels lived at high densities together with lower densities of Douglas’s Squirrels. They suggested that the Eastern Grey Squirrels outcompete Douglas’s Squirrels for food, and thus restrict Douglas’s Squirrels to areas of predominately coniferous habitat (Robinson and Cowan 1954). Whether competitive interactions between the two species still exist today (> 80 years since the introduction of Eastern Grey Squirrels) remains unknown. In Great Britain, Eastern Grey Squirrels were introduced at the end of the 19th century, and have caused the decline of the smaller native European Red Squirrel (Sciurus vulgaris) from its former range (Gurnell and Pepper 1993; Wauters and Gurnell 1999; Teangana et al. 2000). The mechanisms of replacement of European Red by Eastern Grey squirrels are not yet fully understood (Skelcher 1997). Juvenile recruitment of European Red Squirrels is typically lower when Eastern Grey Squirrels are present (Wauters et al. 2000, 2001). Also, diseases such as parapoxvirus have been hypothesized to facilitate the replacement of European Red Squirrels by Eastern Grey Squirrels in the United Kingdom (Tompkins et al. 2003). The Eastern Grey Squirrel averages 400–700 g in body mass (Koprowski 1984), and is larger than Douglas’s Squirrel, which averages 250-300 g (Steele 1999). Douglas’s Squirrels feed primarily on coniferous seeds, new shoots of conifers, green vegetation, acorns, nuts, mushrooms, fruits, and berries. Also, Douglas’s Squirrels actively defend food middens within their territory A Test of Interspecific Effects of Introduced Eastern Grey Squirrels, Sciurus carolinensis, on Douglas’s Squirrels, Tamiasciurus douglasii, in Vancouver, British Columbia