The native Red squirrel has suffered a dramatic decline due to the spread of the invasive, non-native Grey squirrel. The Grey squirrel, which was introduced from America, out-competes the Red squirrel for food and living space and can carry Squirrelpox virus - which, if contracted by Red squirrels, is fatal.

In Tayside, Grey squirrel control is focused across the 'Highland Line', ensuring that the Grey squirrel population does not expand northwards, where Highland and Grampian Red squirrel populations would come under threat.