Grey Squirrel Invasions in Western Canada; History Repeats Itself (Again and Again)
KARL W LARSEN
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Canada shares with the United States of America (USA) the dubious distinction of containing both native and introduced populations of the eastern grey squirrel (Sciurus carolinensis). One reason for this situation is that the two countries span the North American continent and thus encompass that region where the species is endemic. At the same time, geographic barriers, habitat discontinuity and the presence of species with overlapping niches historically have prevented the grey squirrel from spreading naturally into other areas of the continent that are now proving suitable for colonization. In Canada, as in other locations on the globe, introduction of this species into different regions has its origin in human-assisted transportation, in some cases quite intentional.
This chapter provides an overview of the establishment of eastern grey squirrels in western Canada, principally west of the continental divide and in the province of British Columbia (BC). Even within these political borders, the animal has been introduced on at least three separate occasions (the first being intentional), with the most recent occurring within the past decade. In all of these cases, the establishment of the animals took place in anthropogenic habitat, although subsequent spread into natural or semi-natural habitat is now occurring in at least two locations. BC contains a diverse array of ecosystems, and thus provides an interesting (if unwanted) opportunity to study where, and how, the animal may tolerate conditions outside of the urban environment.
A surprising lack of research, much less managerial intervention, typifies the grey squirrel introductions in BC, unlike that currently seen in western Europe. Thus, in this chapter I will not be able to draw heavily from the peer-reviewed literature, rather having to rely to a large degree on unpublished work and personal communications. Still, the goal of this chapter is to provide an overview of the grey squirrel introductions and the implications of such (realized, suspected, and predicted) in a more detailed fashion, adding in where possible information collected by myself and others in recent years. My intention is to add to the growing awareness of this species' capacity to invade a wide range of ecosystems and geographical regions, especially in the absence of pro-active or immediate managerial response.
Western Canada – repeated introductions of the eastern grey
The appearance of the eastern grey squirrel (Sciurus carolinensis) in western Canada took place via an intentional release of a small number of animals into Stanley Park in the early 1900s (see Steel et al. 1985, Gonzales 2000 and Nagorsen 2005). Stanley Park is a reasonably large (≥ 4 km2) municipal park within the sprawling city of Vancouver. It protects a remnant of the original west coast forest ecosystem, a mixture of conifer and deciduous trees and intensively cultivated parkland. The fact that the park is situated on a peninsula effectively 'blockaded' by the urban environment of Vancouver likely hampered the spread of the animals for over half a century. But, as detailed by Nagorsen (2005), during the latter part of the 1900s the animal began appearing in other areas of coastal British Columbia (BC), a region referred to in BC as the 'Lower Mainland'. This region has a relatively mild climate by Canadian standards, with winter temperatures rarely approaching freezing (Figure 1).
The Lower Mainland landscape also is densely populated by humans which, combined with the climate, has favoured the easterly (inland) expansion of the introduced grey squirrels for at least 75 kilometres (km). However, beyond this point the abrupt rise of the Coast Mountain Range provides a dispersal barrier to the animals, with the coastal environment quickly being replaced by expansive high-elevation conifer forests, considerably fewer human settlements, and more severe seasonal weather patterns. No such barrier exists to the south of the Lower Mainland, and with the Canada-United States of America (USA) border situated nearby (see Figure 2), the range of the eastern grey squirrel in western Canada has become essentially contiguous with the population of the animals introduced along the coastal region of Washington State and Oregon.
The second major introduction of eastern grey squirrels into BC took place on Vancouver Island (Figure 1). Over 30,000 km2 in area, Vancouver Island is less than 20 km distant from both the USA and the Lower Mainland of BC. Archipelagos of islands lie in the straits separating Vancouver Island from the continent, suggesting that in theory, grey squirrels may have been able to eventually reach the larger Vancouver Island by island-hopping, if not via the steady transport of motor vehicles by the large ferry fleet. Interestingly, an early attempt (≈ 1945) to establish grey squirrels in the largest municipal park in the Greater Victoria system met with failure (G. Smith, personal communication). However, an escape in the early 1960s by three captive animals led to the establishment of the current population on the southern end of the island (Nagorsen 2005). Eastern grey squirrels began appearing in city parks during the 1990s and by 2005 they were well established (G. Smith, personal communication). Whether the three founding animals were augmented by other individuals in subsequent years is not known.
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