Next time you see a squirrel in Monroe Park, you might want to consider their origin. A new paper in the Journal of American History covers The Urbanization of the Eastern Gray Squirrel in the United States:

The urbanization of the gray squirrel in the United States between the mid-nineteenth century and the early twentieth century was an ecological and cultural process that changed the squirrels’ ways of life, altered the urban landscape, and adjusted human understandings of nature, the city, and the boundaries of community.
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Given the present ubiquity of gray squirrels, it may be difficult to believe that they have not always been common in American cities. In fact, they seem to have been entirely absent during the first half of the nineteenth century. The lack of systematic surveys before the twentieth century hinders estimates of the size of historical squirrel populations, which can fluctuate dramatically from year to year depending on food supplies, weather conditions, and other factors.