Management of American mink and interactions with native mustelids
American mink management
Invasive species are an important driver of global biodiversity loss. Recently, there have been an increasing number of successes in the eradication of invasive species from islands but eradications are rarely possible for established invasive species on mainlands and (for some species, eg. mustelids and cats) on larger islands, such as the UK. Nevertheless, under international legislation, the UK has an obligation to eradicate or to control the American mink.
We used spatially explicit, individual-based models and a large-scale field experiment to design, and test the effectiveness of, a control strategy for American mink on rivers in southern England. The field experiment was carried out in the Upper Thames valley concurrently with experimental water vole releases. We used mink rafts with integral tracking plates (designed by the Game and Wildlife Conservation Trust) to monitor the relative abundance of mink at paired removal and non-removal sites over a 4 year period. This work was completed in 2007. We found that mink removal could be effective in reducing the relative abundance of mink at small (20 km sites) with four months or less of trapping per year, and that this strategy was sufficient for water vole restoration. It is crucial, however, (particularly for small sites that are vulnerable to immigration) that monitoring is carried out intensively and frequently, and that trapping is flexible, and reactive to monitoring results.
Recently, we have used a similar modelling approach to investigate the feasibility of establishing a cordon-sanitaire to prevent the spread of American mink into the northern highlands of Scotland (an area that appears to currently be mink-free and hosts some of the last remaining extensive populations of water voles). On the basis of our recommendations, the cordon-sanitaire approach is currently being implemented by Scottish Natural Heritage. To maintain this area mink-free, however, it will be important to be able to predict the rate and direction of the spread of mink populations south of the cordon-sanitaire. Currently, it is not understood why mink have not spread further and faster in northern Scotland. We also do not know whether inland areas provide suitable habitat for self-sustaining mink populations, or whether mink are predominantly limited to coastal habitats. A new 3-year project aims to determine to what extent the marine environment subsidises highland mink populations, and to investigate the source-sink dynamics between coastal and inland mink populations.
Intra-guild interactions between American mink and native mustelids, the polecat and the otter
The American mink originally colonised the country at a time when both otters and polecats were largely absent due to pesticide poisoning, overhunting and persecution, respectively. Both native species are now recovering nationally. The otter is approximately seven times larger than the mink, the polecat is very similar in size, and all three species consume potentially overlapping diets, occupy similar habitats, and are generally nocturnal. Our studies of mink and otter diet in the Outer Hebridean islands in Scotland, and in southern England, show that in the presence of otters mink diet becomes more terrestrial yet the diet of otters changes relatively little, suggesting that, as expected from their relative sizes, otters are the dominant competitor and that where they are sympatric mink are forced to undergo a dietary niche shift. Indeed, both our work and that of others indicate that the presence of mink signs (scats) have declined nationally in recent years, in apparent response to a concomitant increase in the signs of otter. An experimental study of the impact of released otters on resident mink also suggested that mink densities may decline as otters recover. Given the pest status of the mink in the UK the relationships within this guild of predators are not only of theoretical interest; it may be that a practical way to reduce mink populations is simply to encourage otters. This idea has received much media attention. However, our more recent comparative study, comparing mink abundance and behaviour before and after the re-arrival of otters and polecats on the River Thames suggested that mink were still abundant in the presence of these native competitors, but that they had changed their activity patterns. Whereas mink in the 1990s (in the absence of otters and polecats) were generally nocturnal, mink in the 2000s (in the presence of otters and polecats) were predominantly active during the day. We hypothesise that this temporal shift may be an avoidance mechanism allowing the coexistence of mink with native, nocturnal competitors, although we are unable to attribute the shift to one or the other species.
Dr Laura Bonesi
Prof Xavier Lambin, University of Aberdeen