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

The invasion of Britain by American mink has had a catastrophic impact on w ater vote populations. We surveyed and live-trapped water votes over 2 year s 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 Gra mpian 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. Wate r votes occurred in small, discrete colonies. Median nearest-neighbour dist ance 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 fl uctuations in occupancy not caused by mink also occurred at the regional sc ales of the Grampian Mountains and Assynt areas. Localized mink invasions h ave fragmented a previously continuous metapopulation into smaller clusters and this may indirectly affect the likely persistence of water vote coloni es not directly exposed to predation by mink.