Lauren A. Harrington

The American mink: The triumph and tragedy of adaptation out of context

Abstract The American mink has spread widely beyond its native North America due to the activities of fur traders. The mink is an extremely adaptable, generalist predator. Over two continents, the American mink is associated with problems in the conservation of local species because of their impact on both prey and competitors. Here, we review the impact of American mink on native species, stemming from over a decade of research, and concentrate on two examples: the impact of American mink on the water vole in Britain and on the European mink in Eastern Europe. While the near extinction of the water vole in Britain and that of the European mink in Eastern Europe are largely due to predation and interspecific aggression, respectively, both may have been affected by a multitude of factors acting synergistically. Terns, and other seabirds, are impacted by mink predation; the effect on riparian bird species is less clear but may potentially be high. Emerging principles, supported by preliminary evidence, suggest that the return of the larger otter in the United Kingdom may be detrimental to mink populations. The impact of, or on, the current recovery of the polecat in the United Kingdom is unknown but evidence from Eastern Europe suggests that American mink negatively affect at least female polecats. We conclude by briefly presenting two case studies of small‐scale mink removal and discuss considerations for future control projects. To mitigate mink impact on a larger scale we suggest a holistic approach, involving mink removal, habitat restoration, and recovery of native competitors.

The impact of native competitors on an alien invasive: temporal niche shifts to avoid interspecific aggression

The American mink, Neovison vison, is an established, alien invasive species in the United Kingdom that originally colonized the country at a time when two native mustelids (otters, Lutra lutra, and polecats, Mustela putorius) were largely absent. Both of these species are now recovering their populations nationally. We compared the relative abundance and the behavior of mink in the 1990s and in the 2000s in an area of southern England where both otters and polecats were absent in the 1990s but reappeared in the intervening years. We found that mink were still abundant in the 2000s in the presence of otters and polecats, but that they appeared to have altered some aspects of their behavior. In accordance with previous studies, we found that mink consumed fewer fish in the presence of otters. We also found that mink were predominantly nocturnal in the 1990s (in the absence of competitors) but were predominantly diurnal in the 2000s (in the presence of competitors). We hypothesize that this temporal shift may be an avoidance mechanism allowing the coexistence of mink with the otter and the polecat, although we are unable to attribute the shift to one or the other species. We also found that mink in the presence of competitors weighed less but remained the same size, suggesting the possibility of a competitor-mediated decline in overall body condition. This is one of very few field studies demonstrating a complete temporal shift in apparent response to competitors. The implications of this study are that recovering otter populations may not lead to significant and long-term reductions in the number of invasive mink in the United Kingdom as has been suggested in the media, although we cannot exclude the possibility of a decline in mink in the longer-term.

Spatial and Temporal Relationships between Invasive American Mink and Native European Polecats in the Southern United Kingdom

Abstract The spatial organization of a species on a landscape is influenced, at least in part, by the presence of sympatric competitors. Interspecific relationships can thus have direct effects on the carrying capacity of the landscape and have important implications for conservation. We investigated the spatial relationships and activity patterns of 2 similarly sized mustelids: the invasive American mink (Neovison vison) and the native European polecat (Mustela putorius) in lowlands in the United Kingdom. By radiotracking mink (n = 11) and polecats (n = 7) in autumn when individuals of both species hold stable home ranges, we found that individuals tended to have overlapping home ranges, both within and between species; and the size of overlap areas was similar, but generally small, within and between species (mean approximately 20%, although overlaps were higher among mink of opposite sexes). Mink shared their home ranges with 0.3–1.17 other mink (of both sexes) and 0.83–1.3 polecats; polecats shared their home ranges with 0.6–1.0 polecats (of the opposite sex) and 1.6–2.0 mink. Neighbors avoided simultaneous use of overlap areas; polecats were nocturnal and mink were predominantly diurnal. Our results are consistent with interspecific territoriality although we cannot distinguish between interspecific territoriality and niche differentiation. We suggest that there is habitat partitioning between the 2 species, but that this is incomplete and that temporal partitioning enables avoidance of interspecific neighbors within overlap areas. Niche partitioning by distance from water and time of day when active probably facilitates coexistence in the short term, but it is unclear whether coexistence is stable year-round or in the long term.

The accuracy of scat identification in distribution surveys: American mink, Neovison vison , in the northern highlands of Scotland

Distribution data for elusive species are often based on detection of field signs rather than of the animal itself. However, identifying field signs can be problematic. We present here the results of a survey for American mink, Neovison vison, in the northern highlands of Scotland to demonstrate the importance of verifying field sign identification. Three experienced surveyors located scats, which they identified as mink scats, at seven of 147 sites surveyed and “possible” mink scats at a further 50 sites. Mitochondrial DNA was successfully extracted from 45 of 75 (60%) scats, collected from 31 of the 57 “positive” sites; sequencing of amplified DNA fragments showed that none of these scats was actually of mink origin. We consider the implications of erroneous survey data and the potential waste of resources and misdirection of conservation/management actions. We discuss potential methods that may be useful to verify field sign identification, including the use of DNA analysis, and stress that verification is crucial to ensure rigorous and reliable survey data.

Conflicting and Complementary Ethics of Animal Welfare Considerations in Reintroductions

Despite differences in focus, goals, and strategies between conservation biology and animal welfare, both are inextricably linked in many ways, and greater consideration of animal welfare, although important in its own right, also has considerable potential to contribute to conservation success. Nevertheless, animal welfare and animal ethics are not always considered explicitly within conservation practice. We systematically reviewed the recent scientific peer-reviewed and online gray literature on reintroductions of captive-bred and wild-caught animals (mammals, birds, amphibians, and reptiles) to quantify the occurrence of animal welfare issues. We considered monitoring that could be indicative of the animals welfare status and supportive management actions that could improve animal welfare (regardless of whether the aim was explicitly animal-welfare orientated). Potential welfare issues (of variable nature and extent) were recorded in 67% of 199 projects reviewed; the most common were mortality >50%, dispersal or loss of animals, disease, and human conflict. Most (>70%) projects monitored survival, 18% assessed body condition, and 2% monitored stress levels. Animal welfare, explicitly, was referred to in 6% of projects. Supportive actions, most commonly use of on-site prerelease pens and provision of supplemental food or water, were implemented in 79% of projects, although the extent and duration of support varied. Practitioners can address animal-welfare issues in reintroductions by considering the potential implications for individual animals at all stages of the release process using the decision tree presented. We urge practitioners to report potential animal-welfare issues, describe mitigation actions, and evaluate their efficacy to facilitate transparent evaluation of common moral dilemmas and to advance communal strategies for dealing with them. Currently, comparative mortality rates, health risks, postrelease stress, effectiveness of supportive measures, and behavior of individuals warrant further research to improve animal welfare in reintroductions and to increase success of such projects.

Distinguishing tracks of mink Mustela vison and polecat M. putorius

Targeted trapping and monitoring methods for mink rely on the correct identification of mink tracks on tracking plates. Previously, there has been no reliable method by which mink tracks can be distinguished from polecat tracks. We present a simple discriminant function based on three measurements that can be used to distinguish the tracks of mink and polecats on clay-based tracking plates with classification success greater than 90%. The method could potentially be used in other circumstances.

Dive performance in a small-bodied, semi-aquatic mammal in the wild

Abstract Aquatic foraging is a fundamental component of the behavior of a number of small mammals, yet comprehensive observations of diving are often difficult to obtain under natural circumstances. Semiaquatic mammals, having evolved to exploit prey in both aquatic and terrestrial environments, are generally not as well adapted for diving (or for life in the water) as are fully aquatic species. Because dive ability also tends to increase with body size, small, semiaquatic mammals are presumed to have fairly limited dive ability. Nevertheless, diving plays an important role in food acquisition for many such species. We used time–depth recorders (TDRs) to measure and describe the dive performance of 9 female and 5 male free-living American mink (Neovison vison; body mass approximately 1 kg) on lowland rivers in the southern United Kingdom. We recorded dives up to 2.96 m deep (maximum depth X¯  =  1.82 m) and up to 57.9 s in duration (maximum duration X¯  =  37.2 s). Dive duration was approximately 40% of that predicted by allometry for all air-breathing diving vertebrates (as might be expected for a small, semiaquatic animal) but was twice as long as previously measured for mink in captivity. Mink performed up to 189 dives per day (X¯  =  35.7 dives/day), mostly during daylight, and spent a maximum of 38.4 minutes diving per day (X¯  =  7.6 min/day). Some individuals maintained particularly high diving rates over the coldest months, suggesting that the benefits of aquatic foraging in winter outweigh the costs of heat loss. We observed a number of very shallow dives (depth approximately 0.3 m) of particularly long duration (up to 30 s). The function of these dives is currently unknown, but possibilities include searching for prey, travelling, or avoidance of threats. There is only 1 other study of which we are aware that presents detailed measurements of dive performance in a small, shallow-diving, semiaquatic mammal.

International commercial trade in live carnivores and primates 2006–2012: response to Bush et al. 2014

Bush et al.’s (2014) commendable and thorough analysis of the global exotic pet trade showed that mammals were traded in much smaller numbers than were birds and reptiles and that mammals traded legally under the Convention on International Trade in Endangered Species of wild fauna and flora (CITES) did not appear to be extracted directly from wild populations. Because CITES trade records do not explicitly include a category for the pet trade, Bush et al. took a conservative approach and interpreted “personal” trade in live animals as best representing those destined to be kept as pets. Other purpose categories used by CITES relevant to trade in live animals include breeding, educational, biomedical research, reintroduction or introduction into the wild, circuses and traveling exhibitions, scientific, commercial trade, and zoos. Commercial trade in live animals presumably includes the sale of animals for multiple purposes (e.g., crab-eating macaques [Macaca fascicularis] traded in tens of thousands annually are used in biomedical research [e.g., Eudey 2008]), but undoubtedly also includes the sale of animals into the pet trade (e.g., kinkajous [Potos flavus] are a popular pet in North America [CBS News 2011] and are traded under commercial rather than personal trade [Table 1]). There are 2 points worth adding to Bush et al.’s findings that arise from an examination of commercial trade in live animals recorded by CITES. First, the number of live mammals traded commercially (many of which may be destined for the pet trade) is much greater than the number traded for personal purposes alone. Second, some mammalian genera traded commercially (including those for which the most likely destination is the pet trade) are sourced predominantly from wild populations. Both points have potential conservation implications for the source populations and welfare implications for the individual animals involved.

Notes on the health status of free-living, non-native American mink, Neovison vison, in southern England

Haematological and serum biochemistry values were determined for 13 adult, free-living American mink, Neovison vison, in southern England live trapped as part of a longer term research project. Serum samples were tested for the presence of antibodies against Toxoplasma gondii, Aleutian disease virus (ADV) and canine distemper virus (CDV). Animals were examined to assess ectoparasite burden; faecal samples were examined for the presence of gut parasites and bacteria (identified via culture). Post-mortem examinations were carried out on four individuals shot during on-going control operations. Haematological and serum biochemistry values for most individuals were similar to published values for captive mink. Neutrophil/lymphocyte ratios were high in two animals (possibly due to trap-associated stress). Three individuals had high levels of creatinine, urea and the liver enzymes, alanine transaminase, aspartate transaminase, alkaline phosphatise and gamma-glutamyl transpeptidase. Six of 12 mink tested positive for antibodies to T. gondii and 8 of 12 tested positive for antibodies to ADV; none tested positive for antibodies to CDV. No significant bacteria or parasites were detected in faecal samples. Post-mortem examinations in three cases showed no significant lesions but the fourth animal had Skrjabingylus nasicola nematodes in the nasal passages, lung lesions suggestive of adiaspiromycosis, cholangiohepatitis possibly indicative of Pseudamphistomum truncatum infection and tubulointerstitial nephritis associated with renal calculi.

Sexual size dimorphism in musteloids: An anomalous allometric pattern is explained by feeding ecology

Abstract Renschs rule states that sexual size dimorphism (SSD) increases with body size in taxa where males are larger, and decreases when females are larger. The dominant explanation for the trend is currently that competitive advantage for males is greater in larger individuals, whereas female size is constrained by the energetics of rearing offspring. This rule holds for a variety of vertebrate taxa, and opposing trends are rare. We examine the allometry of SSD within the Musteloidea and demonstrate a hypo‐allometry contrary to Renschs rule, with lower SSD associated with larger body size. We provide evidence that feeding ecology is involved. Where diet promotes group‐living, the optimal strategy for the males of larger species is often not to attempt to defend access to multiple females, obviating any competitive advantage of relatively greater size. We conclude that the effect of feeding ecology on mating systems may be a hitherto neglected factor explaining variation in SSD.