Valsdad,

You might find this study in the American Society of Mammalogists on-line journal interesting:

www.mammalsociety.org/articles/origin-recently-established-red-fox

You have to scroll down a few articles to find it. DNA research found most red fox populations in North America are native, though a few are not.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck