The Urbanization of the Eastern Gray Squirrel in the United States (2013)

On a slight tangent...grey squirrels are considered an invasive species in the UK. They were not originally native to the UK, but after their introduction from North America they quickly outstripped the native red squirrels in population numbers. They are heavier than red squirrels and outcompete them for food. They also carry a virus which is harmful to red squirrels but harmless to themselves.

Grey squirrels are absent from most of mainland Europe (red squirrels are the native species). However, there is a small population of grey squirrels in Italy. The concern is that these grey squirrels may spread into other continental countries and affect the native red squirrel populations.

Earlier this year, Imperial College London revealed details of a study into the genetic diversity of Italian grey squirrel populations:

"Genotyping of grey squirrels sampled from Italy and the UK shows a direct link between genetic diversity and their ability to invade new environments."

http://www3.imperial.ac.uk/newsandeventspggrp/imperialcolleg...