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Final Activity Report Summary - DISPERSE (Control of an invasive predator for conservation: Spatial density dependent dispersal in American mink)

The introduction of alien species in many parts of the world has had devastating impacts on native biodiversity and has often fundamentally changed ecosystems. While desirable, in many cases, complete eradication is not possible and sustainable mitigation of the impact of alien species must be implemented in order to preserve native species. In Britain, as in other European countries, the introduction of American mink has led to a catastrophic decrease in water vole arvicola terrestris numbers. In Scotland, the water vole has disappeared from the majority of lowland rivers and streams, even though numerous small populations persist relatively unaffected in the Scottish uplands. It is therefore necessary to reduce the magnitude and spatial scale of mink encroachment into these upland areas to help protect remnant water vole populations and to simultaneously improve the cost-effectiveness of control programmes.

In this project, metapopulation and population genetic theories were used to identify areas where populations of prey species might be maintained in protected refuges. We investigated the influence of population demographic parameters and landscape structure on mink dispersal into upland areas. Data was collected on mink distribution, demography and genetic structure in northeast Scotland.

Our results showed that American mink more commonly occupied relatively lush lowland areas than unproductive upland areas. Analyses of spatial distribution showed that distance from main rivers had the strongest influence on mink presence, suggesting that main rivers represented source populations, producing a surplus of individuals, while upland areas were population sinks, with a deficit of birth over death. The average age of mink in both areas was low, approximately 1.2 years, illustrating very high turnover in both populations. There was no difference in body length and body weight between individuals from lowland and upland areas. However, males in lowland areas were in significantly better condition than those in upland areas. Differences in body condition were related to differences in food abundance and diet composition. Indeed, mink diet was more diverse and contained more small, less profitable prey, in the uplands than in the lowlands.

At a large geographical scale, we found evidence of two genetically distinct groups of mink in eastern and western Scotland respectively, indicating that the mountain ranges separating the populations precluded dispersal. At a local scale, spatial autocorrelation analyses showed that mink were non-randomly distributed in space, although spatial structure occurred only over short distances, equal to 5 km per maximum, and, in general, populations from lowland and upland areas showed high levels of genetic mixing. The distances between parent-offspring pairs and full sibling pairs increased with time since birth, from on average 8 km in the first year to 16 km in the second year, for a maximum of 50 km. We also found evidence of male-biased dispersal. The effective dispersal distances computed using least-distance cost modelling, taking into account elevation, better correlated to genetic distances in the west coast area, showing that the landscape features, i.e. mountains, reduced dispersal. In eastern Scotland coast area, both Euclidean and distances calculated along rivers better explained variation in the genetic differentiation between mink, which suggested that very dense systems of rivers and streams in eastern Scotland might increase the spatial influence of mink on water voles. This clearly indicated that landscape features might affect the successful eradication and management of invasive mink.

Through this project we improved understanding of the metapopulation dynamics of the American mink and supported the community-based American mink eradication programme and water vole conservation project. Our results were anticipated to contribute to the mitigation of the impact of the American mink in the United Kingdom and other European countries.

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United Kingdom
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