Fig. 3.1 shows a red squirrel, Sciurus vulgaris. This species is native to the British Isles, meaning it has lived there for at least 10000 years. In the 1800s a related but slightly larger species, the grey squirrel, Sciurus carolinensis, was introduced from North America. [Figure 3.1] A history of the interaction between red squirrels and grey squirrels includes these facts: • Grey squirrels, which occupy a similar niche to red squirrels, expanded their range rapidly after introduction. • Grey squirrels carry a virus that does not harm them, but which kills red squirrels. • Red squirrels became extinct in many parts of the British Isles. • Recently some areas have seen a reversal of this trend. There has been a decrease in numbers of grey squirrels. Red squirrels have re-colonised their former areas of habitat. • This has happened in areas where protection of a native predator species, the pine marten, Martes martes, has resulted in an increase in the number of these predators. • Analysis of pine marten faeces shows that they catch and eat many more grey squirrels than red squirrels.