4 Extra Debut. Derek Mooney explores whether the maligned grey squirrel deserves its reputation as an unstoppable interloper. From 2016.

Telling the stories of non-native invasive species and our complicated attitudes to them and with an uncertain political future, how do we police our ecological borders?

The grey squirrel is considered one of the worlds greatest natural invaders. It's been on UK shores for over a hundred years and it's two million strong population dwarfs that of our native red squirrel.

It is maligned by many, but does the grey squirrel deserve its reputation as an unstoppable invader? Derek Mooney intends to find out.

Producer: Ellie Sans

First broadcast on BBC Radio 4 in August 2016.