Positive relationship between non-native and native squirrels in an urban landscape

Paradoxically, non-native species sometimes displace native species that appear to be well adapted to local landscapes. That many landscapes have been altered by humans, creating habitat suitable for non-native species, helps explain this apparent paradox. We asked whether the abundance of native Douglas (Tomiasciurus douglasii (Bachman, 1839)) and northern flying (Glaucomys sabrinus (Shaw, 1801)) squirrels was best explained by the abundance of non-native eastern grey squirrels (Sciurus carolinensis Gmelin, 1788), the proportion of urban development, or both using available squirrel abundance data from wildlife shelters and land-use maps. There was no evidence that non-native squirrels replaced native squirrels given that their abundances were positively related, whereas native squirrels varied negatively with the amount of development. The best model explaining variation in the abundance of Douglas and northern flying squirrels incorporated both eastern grey squirrels and development, which is consistent with the-hypothesis that regional declines in native squirrels are more likely to be predicated by the alteration of native conifer habitats by humans independent of the effects of non-native squirrels.