Water vole in the Scottish uplands: distribution patterns of disturbed and pristine populations ahead and behind the American mink invasion front

The invasion of Britain by American mink has had a catastrophic impact on water vote populations. We surveyed and live-trapped water votes over 2 years in eight c. 25 km(2) blocks in the upland of Scotland behind and ahead of the mink invasion front. Water votes had a similar distribution in the Grampian Mountains of north-east Scotland, on the edge of the invasion front, and in the Assynt area of north-west Sutherland well beyond the front. Water votes occurred in small, discrete colonies. Median nearest-neighbour distance between colonies was 0.6-0.7 kin in both areas. Colonies experienced a high degree of turnover with extinction and colonization being commonplace and only a fraction of suitable sites were occupied at a given time. High dispersal rates connecting numerous (> 30) colonies over large areas (> 25 km(2)) enable water votes to persist in such circumstances. Synchronized fluctuations in occupancy not caused by mink also occurred at the regional scales of the Grampian Mountains and Assynt areas. Localized mink invasions have fragmented a previously continuous metapopulation into smaller clusters and this may indirectly affect the likely persistence of water vote colonies not directly exposed to predation by mink.