Eastern Gray Squirrel as an invasive species

Gonzales, E. 2005. The distribution and habitat selection of introduced Eastern Grey Squirrels, Sciurus carolinensis, in British Columbia. Canadian Field-Naturalist, 119(3): 343-350.

(This peer reviewed article is not available online so no link, sorry)

The Eastern Gray squirrel was first introduced to Vancouver B.C. in the early 1900’s and soon after it thrived. The Gray squirrel inhabits deciduous forests typically filled with nut producing trees like oaks, hickories and chestnuts. They also prosper in residential areas, parks and cemeteries.  There has been concern as to whether or not these Gray squirrels are displacing native squirrel species, but the explanation for non-native and native squirrel species’ abundance is understudied. Further analysis will identify whether the culprit to the native species’ decline is competitive exclusion via interspecific interaction, or habitat modification upon forest fragmentation.

The Eastern Gray squirrel and several native squirrels have been known to co-exist in Vancouver over many years because of habitat segregation. Native squirrels typically inhabit coniferous forests and are more sensitive to anthropogenic activity than the Gray squirrel. Upon forestation it is the coniferous forests that commonly become logged and these areas are reconstructed into residential areas with deciduous trees. It was discovered that the Gray squirrel was very successful in human modified environments. Nesting was found in houses and trees and food was easily accessed through gardens, bird-feeders, garbage, and handouts. In order to control the ever growing population of Gray squirrels conifer forests must be maintained and large seed deciduous trees must be planted less frequently in these areas. It has also been suggested that large populations of Gray squirrels are detrimental to native, young Garry oak species as the squirrels frequently strip the plant’s bark. 

So, it is not the Eastern Gray squirrel that directly impacts native species’ persistence through competition of resources, but rather as development continues the native species’ population size declines due to loss of habitat as the invasive species’ habitat is favored and increases rapidly in response. These species will be able to co-exist further if sufficient suitable habitat is available for both natives and non-natives.


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