A common mammal along the ock valley walk is the grey squirrel (Sciurus carolinensis):
Introduced by our Victorian ancestors, they have become widespread and are an entertaining sight in most parks and gardens (this picture was taken in the Abbey Gardens in Abingdon).
But they are also seen as an eccological disaster (like most alien species) and held responsible for declines in dormouse numbers - by out competing them for food; killing ancient trees - hormonally charged males striping the bark, killing the tree; and reducing the song bird population - by eating eggs and fledglings (although research last year by the BTO seems to refute this: http://news.bbc.co.uk/earth/hi/earth_news/newsid_8448000/8448807.stm)
However, it's most evident victim is it's native british cousin - the red squirrel (Sciurus vulgaris). By out competing for food, habitat and spreading the parapox virus (fatal to red squirrels but grey squirrels are immune) the population has been reduced to 120,000 (compared to 6 million greys) with a majority being in Scotland and only fragmented populations in England - Thetford Forest (Norfolk), Brownsea Island (Dorest) and the Isle of White - where this picture was taken at the start of the year: