This just reminded me of a time I was living in England in the late 90s, and a group of friends and I had found an injured grey squirrel. We called animal control for help, and their response was that if we decide to officially report it, they would have to put it down, because it's considered an invasive species. We ended up just letting the squirrel go, sorry England, for making your map just a tiny bit more grey.
We are in 2010 AD. All Wales is occupied by the Grey Squirrels. All? No! Because an island populated by irreducible Red Squirrels still resists the invader.
In 1876 a Victorian banker "decided to release into the wild a pair of grey squirrels he had brought back with him from a business trip to America. Other landowners, viewing the non-native species as a fashionable garden novelty, soon followed suit."
They were released everywhere in the US for a similar reason. Towns wanted squirrels for the furry aesthetic. Before squirrels just hung out in the forest.
They are. The big thing with invasive species out competing native ones is usually, however, due to bringing in different diseases and not having predators.