Recent studies in Southeast Alaska suggest the ecology of Glaucomys sabrinus differs from populations in the Pacific Northwest. In Southeast Alaska, densities were the highest reported for the species, populations were not as closely linked to old-forest attributes, and individuals had a more diverse diet that was less dependent on mycophagy. Pacific Northwest communities are comprised of several arboreal rodents; Southeast Alaska has a depauperate mammal fauna. I hypothesized that Southeast Alaska populations had a broader realized niche because of competitive release. The red squirrel (Tamiasciurus hudsonicus) is the only other arboreal squirrel and it is absent from the southern outer islands of Southeast Alaska's Alexander Archipelago. I compared demography and body mass of G. sabrinus on Prince of Wales Island to a population in sympatry with T. hudsonicus on a separate island (Mitkof). Home ranges were larger and population density, breeding female density, and juvenile recruitment of G. sabrinus were all lower in sympatry with T. hudsonicus. In a companion study, G. sabrinus on Prince of Wales Island used cavities for denning relatively more frequently than in sympatry with T. hudsonicus on Mitkof Island. Female G. sabrinus depend on cavities for natal dens, and breeding female and population densities are positively correlated with large snag or tree density. The presence of T. hudsonicus may influence G. sabrinus populations by limiting availability of cavities. Furthermore, variation in vertebrate assemblages among islands may influence realized niches of resident species, which manifest unique demographic profiles compared to populations of different ecological communities.