MINKCONTROL · Invasive predator control: response of American mink to eradication in relation to farm distribution

Invasive predator control: response of American mink to eradication in relation to farm distribution

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 Poland, as in other European countries, the introduction of American mink has led to a decrease in water bird numbers. Often eradication is impossible; however, the control of American mink can mitigate the impact on native species. Such mitigation of the negative impacts of invasive species requires information on the pathway of spread, and the population structure of these species. In this project, genetic diversity of feral and ranch American mink was used in order to understand the processes of invasion, and the possible influence of multiple introductions on the feral mink population in Poland. The data were collected in North Eastern and North Western parts of Poland. The results of genetic analyses showed that the feral American mink population in Poland is not genetically homogenous, but is characterised by a well pronounced genetic structure and a total of five clusters of feral mink and two clusters of ranch mink were identified. The spatial distribution of feral clusters partly reflected regional distribution but also suggested that there were other factors shaping mink genetic structure. The analysis showed positive association of pairwise genetic and geographic distances. Feral and ranch mink belong to two genetically separate clusters. In feral population, an average of 17 % of mink were assigned as escapees from farms; however, in some sites up to an 50 % frequency of escapees was recorded. The proportion of feral mink escapees corresponded to the size of breeding stocks in districts where sites were located, showing that, farm escapees could significantly supplement feral populations at the local scale. Analysing of mitochondrial DNA of American mink revealed very high genetic diversity. Overall, 31 haplotypes in the mtDNA CR, belonging to two lineages, were detected: 11 were unique to feral mink, and 12 to ranch mink, whilst and 8 were found in both feral and ranch mink. The genetic differentiation of feral mink from the separate trapping sites was high and significant, whilst that among ranch mink from various farms was moderate. There was no significant relationship between genetic and geographic distance. The number of trapping sites where given haplotypes occurred, correlated with the number of farms with these haplotypes. Comparison of mtDNA and microsatellite differentiation suggests male-biased dispersal in this species. American mink in Poland originate from different source populations of their native range, and the process of colonisation was triggered by numerous escapees from various Polish farms and by immigrants from Belarus. The genetic structure of local feral mink populations was defined by founder effects, multiple introductions, and the dispersal ability of mink. The genomic admixture that occurred during mixing of different populations might have increased the fitness of individuals and accelerated the invasiveness of this species. This may hinder efforts to manage this invasive species in Europe. Experimental mink eradication were conducted in two series of trapping in each year. Trapping took place in two sites: experimental area (were mink were removed) and a control area in which mink were marked and released. The lowest density of mink was in the Drawno NP (2.5 inds./10 km watercourse), and the highest was in the Narew NP (9 inds./10 km watercourse). During the spring and summer mink re-colonisation of the areas were monitoring. The rate of re-colonisation was related to the time since eradication and density of the mink in the national parks. The number of signs found in the eradication areas after six months of mink eradication was similar to the number of signs found in the control areas, suggesting a fast re-colonisation of the eradication area in all parks. In high density populations re-colonisation was faster than in areas where density was lower. The results show large differences in morphological traits and population parameters between the various national parks. The average body weight of males and females from the West of Poland was 1.35 kg and 0.6 kg respectively, and from the East of Poland 2.0 kg and 0.8 kg. We also analysed changes in body mass and length of American mink since introduction into a new area in relation to feeding habits, and progress in colonising the area at local and geographic scales. The body size of the mink decreased significantly during the period of population establishment within the study area, with an average decrease from 1.36 to 1.18 kg in males and from 0.83 to 0.70 kg in females. Diet composition varied seasonally and between consecutive years. The main prey consumed were mammals and fish in the cold season, and birds and fish in the warm season. The proportion of mammals and fish preyed upon increased, whereas birds decreased, over the study period. Following their introduction, the proportion of large prey decreased. The average yearly proportion of large prey and average-sized prey in the mink diet was significantly correlated with the mean body masses of males and females. Biogeographic variation in the body mass and length of mink was best explained by the percentage of large prey in the mink diet in both sexes, and by latitude for females. All these results showed that American mink rapidly adapted their behaviour and body size to local conditions. The results of the project implicate that ongoing introduction and reintroductions overwhelmed genetic structure, creating one homogenised population, where gene flow is human-mediated. This prevents distinguishing separate populations, isolated by landscape barriers reducing gene flow and in consequence from distinguishing potential management units within which mink could be controlled. Furthermore, increased genetic variation of the introduced population allowed the rapid adaptation of American mink morphology and behaviour to the new conditions. All these leads to decline in the effectiveness of local mink control programmes. Therefore, reducing number of escapees should be a priority management action to potentially reduce genetic variability of feral mink populations, decreasing its adaptive and invasive potential. In general, eliminating the vector and source responsible for introductions and expansion is an important prerequisite for developing successful management strategy. The development of mink farming in Poland, which provides ranch mink inflow to existing feral populations, has not only negative local environmental implications, but should be perceived in a larger, regional or even international perspective. In this project we have improved understanding of the population dynamics of the American mink and supported community-based American mink eradication program and birds conservation project in national parks. A. Zalewski, Mammal Research Institute, Polish Academy of Sciences, 17-230 Bialowieza, Poland; email: zalewski@zbs.bialowieza.pl

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