The influence of American mink odour on the spatial distribution and behaviour of water voles

The perception and assessment of predation risk often cause changes in the activities of animals and induce behavioural responses that may in turn affect their movements and distribution. To simulate high predation risk in a midfield pond riparian habitat, we used fresh faeces from ranch American mink Neovison vison and recorded behavioural responses of water voles Arvicola amphibius. In areas where mink odour was deployed, the numbers of captured vole individuals and their trappability were significantly lower than in control areas. Several voles migrated from the zones with deployed mink faeces to the areas without faeces, thus proving that increased predation risk affects the distribution of individuals in a population. The response to mink odour was much more pronounced in females than in males; in areas with deployed mink faeces, not a single female was trapped. We conclude that although American mink is a non‐native, invasive predator, water voles respond to mink odour by reducing their activity and/or by avoiding places with higher predation risk.     

Multiple captures provide evidence of small mammal social behavior

Social behavior of small mammals living under natural conditions often is inferred from live-trapping data, particularly from incidents in which two or more individuals are captured together in a trap. We examined whether multiple-capture data from a long-term study of prairie voles (Microtus ochrogaster) and meadow voles (Microtus pennsylvanicus) were consistent with well-known species differences in social behavior (whereas prairie voles are highly social and display monogamy, meadow voles are less social and promiscuous). When possible, we also examined multiple captures of two nontarget species, northern short-tailed shrews (Blarina brevicauda) and western harvest mice (Reithrodontomys megalotis). Percent of total captures that were multiple captures and percent of total adult captures that were male–female captures were highest for prairie voles and lowest for meadow voles; values for harvest mice and shrews were in between those of the vole species, but more similar to values for meadow voles. Repeat captures of the same male–female pair occurred most commonly in prairie voles, and multiple captures of this species typically involved individuals from the same social group. Multiple captures of adults and juveniles were more common in prairie voles than meadow voles, except for captures of at least one adult male and at least one juvenile, which did not differ between the two vole species. Multiple capture data for prairie voles and meadow voles were largely consistent with established species differences in social behavior, suggesting that such data can provide an accurate indication of social and mating systems of small mammals.     

Detectability of American Mink Using Rafts to Solicit Field Signs in a Population Control Context

Abstract: American mink (Neovison vison) are an ecologically damaging invasive species where they have been introduced in Europe. Effectiveness of mink population control by trapping has been difficult to assess, without knowing how efficiently mink are caught by traps or detected by other methods. Use of track-recording rafts to detect mink and guide trapping effort has proved efficient and leads to a supposition that no detection indicates absence of mink. To draw this conclusion with any confidence requires a measure of detectability. We applied occupancy models to data from an earlier study to estimate detectability of individual American mink on track-recording rafts. Estimated detectability of individual mink, per raft, and 2-week check period varied between 0.4 in late summer and 0.6 in late autumn. By inference, risk of failing to detect a mink that was present would be <5% given 4–6 independent opportunities to detect it. These opportunities could be created either by using a raft spacing that ensured multiple detections of each mink or by monitoring rafts through a succession of check intervals. Within certain simple constraints, raft location did not contribute substantially to detection probability. These findings will allow field operators, strategists, and funders to assess with confidence the success of efforts to control mink density. We expect the estimation of individual detectability to be similarly valuable in population control or eradication of other species.     

A mink-free GB: perspectives on eradicating American mink Neovison vison from Great Britain and its islands

This paper examines the case for, and plausibility of, eradicating American mink Neovison vison from mainland Great Britain and its associated offshore islands. This invasive species causes extensive damage to native fauna throughout Europe, and the UK Government is obliged to eradicate it, if feasible, under the Bern Convention. Current mink control buys time, but is patchy and dependent on substantial funding in perpetuity. If enacted, eradication would be cheaper in the long term and much more effective in preserving native wildlife. The methodology of an eradication campaign is explored, together with risks, challenges, and a tentative timeline and cost. We judge that mink eradication is now logistically feasible, due to technological developments and experience gained from landscape-scale control. Using live traps fitted with electronic sensors – ‘smart’ traps – as the primary means of catching mink would render the campaign efficient, humane and free of non-target mortality and negative environmental impacts. The ecological benefits of mink eradication would be profound, including greatly improving prospects for water vole Arvicola amphibius populations. Reinvasion is highly unlikely. The greatest logistical challenge is probably removing mink from Scottish west coast islands. Eradication might take around a decade and be dependent on co-ordination between many conservation, fishing, farming, and water-related organisations, together with the consent of landowners. By adding alarms to existing mink traps, land and water managers can pave the way to eradication now. A mink-free Great Britain would plausibly cost tens of millions of pounds, against which could be set the limitless future costs of mink control. Such a campaign would be by far the world's largest invasive predator eradication project by geographical area and would set a precedent for citizen-led conservation action globally. Regional trials would be extremely valuable in determining the costs and practicality of a GB-wide campaign.     

Current status of American mink Neovison vison in Great Britain: a review of the evidence for a population decline

1. The American mink Neovison vison in Great Britain is an invasive alien species, with significant impacts on native prey species. There have been suggestions that populations of mink in Britain have declined since the 1990s.
2. Three nationwide data sets include data on mink distribution and abundance. Scat surveys and the National Game Bag Census suggest population declines, but the latter does not take account of survey effort (which may also have declined) and the former is misleading because there is evidence that mink change their marking behaviour in the presence of otters Lutra lutra. National Biodiversity Network data suggest an increase in mink numbers, but this can be explained by a concomitant increase in mammal recording.
3. Although intra-guild competition between invasive mink and native otters is likely, there is no evidence that otters have caused a decline in mink numbers. There is little information on the impact of disease, or exposure to rodenticides, on wild mink – both warrant further attention. Eradication efforts can have an impact on mink populations, but currently neither implementation nor monitoring is sufficient to generate effects throughout Great Britain or to assess the impact of cumulative local and regional efforts.
4. We conclude that it is not possible, on the basis of currently available data for Great Britain, to ascertain the status of mink or assess the underlying trend in their population. We stress the importance of collaboration, coordination, and record keeping (and sharing) in future, proper interpretation of existing data, and the use of alternative data sources. We call for greater, and better, effort in both mink management and monitoring of management in Great Britain.

     

Harmonia axyridis in Great Britain: analysis of the spread and distribution of a non-native coccinellid

Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) is native to Asia, and was widely introduced as a biocontrol agent of aphids and coccids in Europe and North America. In Europe H. axyridis is considered to be an invasive alien species. Although not known to have been deliberately introduced to Great Britain, it was first recorded there in 2004, in south-east England. Harmonia axyridis arrived in Great Britain by various means, primarily from mainland Europe, but also from Canada. Extensive national and local media coverage, and a dedicated website (), facilitated public involvement in recording H. axyridis in Great Britain; in excess of 4,000 verified records of the species were received between 2004 and 2006. Through detailed mapping, the objective of our study was to quantify and analyse the spread of H. axyridis in its early stages of invasion in Great Britain. Our data shows that between 2004 and 2006, the species spread north through Great Britain at the rate of 58 km year^-1 and west at the rate of 144.5 km year^-1. In England H. axyridis spread north to Yorkshire and west to Cornwall by the end of 2006, and also reached Wales. Recorded occurrence (of one or more H. axyridis individuals at larval, pupal and/or adult stage) in 10 km squares in Great Britain was: 2004—51; 2005—149; 2006—447. Records of juvenile H. axyridis extend through most of the recorded British range of the species, and we present evidence of bi-voltinism in the population in 2006.     

Empirical development of strategy for the control of invasive American mink by trapping

American mink (Neovison vison) are an ecologically damaging invasive species in Europe and Iceland where attempts to control them typically rely on trapping. The focus and efficiency of trapping can be improved by using track-recording mink rafts to identify where mink are present before traps are deployed. This paper describes development of operating strategy for the use of mink rafts with traps, to optimise capture efficiency against costs. We worked sequentially on two unconnected chalk streams in central southern England. On 17 km of the River Itchen, we operated a very high density of rafts (5.9 per km) through spring and summer to generate multiple detections of each mink present. All rafts recording mink tracks were armed with traps, and captured mink were euthanased. After removal of mink until no further detections were made, we calculated that each mink was detected 5.3 times at 5.1 raft sites, and on this basis, rationalised raft density to a standard one per kilometer of river. We set a trap deployment time (10 days) that encompassed the longest observed lapse from detection to capture (7 days), and extended the check interval for rafts in monitoring mode from 1 week to 2 weeks to further reduce costs. These operating rules were then deployed for 12 months on the 44-km River Wylye beginning in autumn. Rafts indicated that the river was cleared of mink through the capture of seven individuals, each of which was detected 3.6 times at 2.7 raft sites, on average, and was trapped within 6 days of detection giving a response time of less than 20 days. Although these operating rules may need refinement for other environments, we believe this is a sound basis for effective mink control.     

Rainfall, ambient temperature, and Clethrionomys californicus capture frequency

Sixty Western Red-backed Voles Clethrionomys californicus inhabiting an old-growth mixed conifer forest in north-western California (USA) were studied over 2 years to assess relationships among vole capture frequency, rainfall, and ambient temperature. Red-backed Voles were encountered more often and recapture rates per vole doubled with the tripling of yearly rainfall. In addition, more voles were captured during wet months than dry months. Analysed over weekly intervals, captures were positively correlated with the amount of weekly rainfall, a greater proportion of the known vole population was encountered during wet weeks than dry weeks, and increased weekly rainfall resulted in an increased proportion of the vole population captured. Significant relationships were not found between daily rainfall and vole captures, but both minimum and maximum daily ambient temperatures were lower when voles were trapped. Vole captures increased with decreases in daily and weekly minimum and maximum ambient temperatures. Weekly rainfall in combination with minimum weekly temperatures accounted for the greatest explanation of variability in vole capture frequencies. These results document that rainfall and ambient temperature have an impact on Western Red-backed Vole capture frequency; thus these weather variables should be addressed as covariates in analyses focused on capture data for this species.     

Assessing the control/eradication of an invasive species,the American mink,based on field data; how much would it cost?

Biological invasions are an important cause of biodiversity loss, American mink being one of the worst invasive species in Europe. We performed a 13-week control program of the species in the Butron river system (Northern Spain), where a natural population of the European mink is found. Three population estimates were considered: an absolute minimum, an intermediate scenario and a pessimistic one (n = 35, 49 and 70 animals, respectively). After 2,242 cage trap-nights, trapping success varied from 44 to 89% of these estimates. In addition, we evaluated the costs of eradicating the estimated populations; costs ranged between 652.5 and 2,970 € per mink, and would rise up to 83,462 € for the intermediate estimate under an exponential function linking captures and costs, or ca. 172.500 € to capture the highest estimate under a log-function. The implications of these numbers for the design and implementation of future control projects are discussed.     

Effects of Prebaiting on Small Mammal Trapping Success in a Morrow's Honeysuckle-Dominated Area

Abstract: Researchers have obtained mixed results in studies that use prebaiting to enhance small mammal trapping success. In 2004–2005 we tested the effects of prebaiting on small mammal trapping success in an exotic and invasive shrub community, Morrow's honeysuckle (Lonicera morrowii), using 4 80 × 120-m live trapping grids at Fort Necessity National Battlefield in Fayette County, Pennsylvania, USA. We randomly assigned traps to 1 of 3 trapping methods: we prebaited one-third of the traps 2 nights (n = 3,508 trap-nights), one-third one night (n = 3,492 trap-nights), and one-third had no prebaiting (n = 3,509 trap-nights). We compared small mammal richness, diversity, and relative abundance (no. captures/100 trap-nights) of white-footed mice (Peromyscus leucopus; n = 462 captures), meadow voles (Microtus pennsylvanicus; n = 89 captures), meadow jumping mice (Zapus hudsonius; n = 221 captures), masked shrews (Sorex cinereus; n = 87 captures), and shorttail shrews (Blarina brevicauda; n = 78 captures) among prebaited traps and non-prebaited traps. On the first day of trapping, as well as all 4 days combined, richness, diversity, and relative abundance for all species were similar among traps that we had prebaited for 2 nights, one night, and zero nights (P = 0.856). Moreover, total number of captures was similar among prebaiting treatments (P = 0.197). These results suggest that prebaiting does not enhance trapping success for small mammals in a landscape dominated by a dense, exotic shrub. We recommend that managers do not employ prebaiting in areas with similar small mammal species composition in an attempt to increase trapping success, as we did not record a difference in trapping success in prebaited traps compared to non-prebaited traps.     

Scent marking by voles in response to predation risk: a field-laboratory validation

Predators use scent to locate their prey, and prey animals oftenalter their behavior in response to predation risk. I testedthe hypothesis that voles would decrease their frequency ofscent marking in response to predation risk. I conducted trialsin which prairie voles, Microtus ochrogaster, and woodland voles,M. pinetorum, were allowed to scent mark ceramic tiles placedin their runways in the field. The tiles were subjected to oneof three treatments: scented with odor from mink, Mustela vison(a rodent predator); rabbit, Oryctolagus cuniculus (a nonpredatormammal control); and no odor (control). No significant differenceswere found in the frequency of scent marking in response tothe three treatments for either species. To validate that volesdid not decrease their scent marking in response to predationrisk, I brought male prairie voles from the field site intothe laboratory and allowed them to scent mark white paper substratetreated with mink odor, rabbit odor, or no odor. No significantdifferences were found in the frequency of scent marks in responseto the three treatments. These results differ from what waspredicted based on laboratory studies with other species ofrodents that show avoidance, reproductive suppression, decreasedactivity, and reduced scent marking in response to odors ofpredators. Voles appear to scent mark different substrates andunder a wide variety of social and environmental situations,and this is not influenced by the presence of odor from a predator.     

Too hot to handle? Balancing increased trapability with capture mortality in hot weather pitfall trapping

Trapping at air temperatures close to, or exceeding, critical thermal maxima is important for comprehensive sampling of vertebrate assemblages and collection of sufficient data for impact assessment. However, pitfall trapping on hot days also potentially exposes trapped animals to stress or death through overheating or desiccation. We investigate causes of mortality from 14 305 captures over a 22‐year pitfall trap study in arid South Australia and compared mortality rates with maximum temperatures, solar radiation and rainfall. Overall mortality rate was 3.2% with chewing by rodents and handling accidents the most influential cause of death recorded. The highest mortality rates were experienced by the tiny skink, Lerista labialis, which was difficult to detect in traps each day and hence problematic to assess the effect of weather variables on capture mortality. For all other abundant species, high maximum temperature was only a significant explanatory variable for increased death rates of the house mouse Mus domesticus, and increased solar radiation was positively related to capture mortality for the house mouse, the frog Neobatrachus sudelli and the small skink Ctenotus schomburgkii. However, capture rates for these taxa and eight other common species would have been significantly lower if trapping did not occur on days of 40 °C or more. We conclude that trapping in hot weather is both desirable and justifiable and suggest techniques for further reducing mortality rates in pitfall studies.     

Feeding habits of three sympatric mammals in NE Spain: the American mink, the spotted genet, and the Eurasian otter

Diet composition, niche measures, and prey consumption of three sympatric species of carnivores, one non-native and introduced, the American minkNeovison vison Schreber, 1777, and two native, the spotted genetGenetta genetta Linnaeus, 1758 and the Eurasian otterLutra lutra Linnaeus, 1758, were studied in a Spanish Mediterranean area. The study was based on the analysis of prey remains in the faeces of the predators. Faeces of mink (n = 444), genet (n = 310), and otter (n = 108) were collected all year round for four years along the Llobregat (21 km) and Gavarresa rivers (12 km). Simultaneously, and in the same area, prey species density and weight were estimated by means of trapping. The diet of mink and genet was dominated by the American crayfishProcambarus clarkii, while the otter fed almost exclusively on fish. Compared to the mink, the genet seemed to specialise more on small mammals while the mink specialised more on aquatic birds, especially in autumn-winter. Niche overlap was higher between mink and genet than between either of them and the otter.     

High mitochondrial DNA diversity of an introduced alien carnivore: comparison of feral and ranch American mink Neovison vison in Poland

Aim Invasive alien species usually exhibit very high adaptation and rapid evolution in a new environment, but they often have low levels of genetic diversity (invasive species paradox). Genetic variation and population genetic structure of feral American mink, Neovison vison, in Poland was investigated to explain the invasion paradox and to assess current gene flow. Furthermore, the influence of mink farming on adaptation of the feral population was evaluated by comparing the genetic structure of feral and ranch mink. Location Samples from feral mink were collected in 11 study areas in northern and central Poland and from ranch mink at 10 farms distributed throughout the country. Methods A 373‐bp‐long mtDNA control region fragment was amplified from 276 feral and 166 ranch mink. Results Overall, 31 haplotypes, belonging to two groups from genetically diverse sources, were detected: 11 only in feral mink, 12 only in ranch mink and eight in both. The genetic differentiation of feral mink from the trapping sites was high, while 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. The mink from two sites were the most divergent, both from all other feral mink and from ranch mink. Comparison of mtDNA and microsatellite differentiation suggests male‐biased dispersal in this species. Main conclusions American mink in Poland exhibit high genetic diversity and originate from different source populations of their native range. The process of colonization was triggered by numerous escapees from various farms and by immigrants from Belarus. The genetic structure of local feral mink populations was shaped by the founder effect and multiple introductions. The genomic admixture that occurred during mixing of different populations might have increased the fitness of individuals and accelerated the invasiveness of this species.     

Sex ratio in catches of American mink – How to catch the females

To protect seabirds breeding on small nearshore islands (“sites”) along the coast of west Scotland from American mink, continuous year-round conventional cage trapping was carried out at constant effort at ten sites for three years (2003–2005) and at five of these sites for 10 years (1997–2006). In both periods, trapped male mink outnumbered females for most of the year, usually by more than two to one, with peaks of males in April–May and in September–October. However, a striking change took place in August–September, when the sex ratio of catches became close to unity. Most of the trapped females were caught in these two months. The chance of catching a female by this method was seven or eight times greater in August–September than during the rest of the year. Relatively few mink of either sex were caught in November–February. These findings, particularly the greatly increased catch of females in August–September, may be useful to others working to control alien American mink.     

The stoat Mustela erminea population decline in northern Belarus and its consequences for weasels Mustela nivalis

Abstract

In northern Belarus, we have documented a decline in the local stoat Mustela erminea population following the naturalisation of the American mink Mustela vison. The most likely cause is the reduction in the density and distribution of the main prey of stoats, the riparian voles (the water vole Arvicola terrestris and the root vole Microtus oeconomus), due to excessive predation by mink. Since the stoat population has declined, the number of weasels Mustela nivalis in marshlands has increased and their mean body mass has increased, correlated with the higher number and mean weights of rodents available for weasels in marshland compared with forest habitats.     

Danish free-ranging mink populations consist mainly of farm animals: Evidence from microsatellite and stable isotope analyses

Two methods were used to separate free-ranging mink Mustela vison into wild mink and escaped farm mink. Analysis of stable carbon isotopes was performed on teeth and claws of 226 free-ranging mink from two areas in Denmark. A classification based on empirical data resulted in three groups (n=213); 47% were newly escaped farm mink and another 31% had been born in farms and lived in nature for more than ca. 2 months. The remaining 21% may or may not have been born in nature, but they had been free ranging for more than a year and were thus considered wild. A genetic analysis by means of microsatellites was performed on a subsample of the trapped mink (86 individuals) and on 70 farm mink, in order to assess the proportion of escaped farm mink in the free-ranging population. Strong genetic evidence for a high percentage of escaped farm mink in the free-ranging population (86%) was found, in agreement with the carbon isotope results. Both methods can be used to distinguish between farm and wild mink, but whereas microsatellites can only say whether a given mink originated from a farm or not, carbon isotopes can give some more detail on the period of time that a farm mink has been living in natural habitats. The high proportion of escaped farm mink in the Danish nature could have serious implications for the preservation of other vulnerable species and should be carefully considered when designing conservation strategies.     

Potential Impact of an Exotic Mammal on Rocky Intertidal Communities of Northwestern Spain

Being the interface of sea and land, the coast can be invaded by introduced species coming from either of these two worlds. Recent reviews of coastal invasions emphasize the human-mediated transport of non-indigenous marine plants and invertebrates, forgetting the potential role of invaders of terrestrial origin. By studying the diet of the introduced American mink (Mustela vison) on a rocky shore of southwestern Europe, we draw attention to the potential impact on intertidal communities of exotic species coming from inland. We analysed 199 mink faeces collected in August 1997 and August 1999 in Baiona, a coastal and urban area of northern Spain recently invaded by minks. The diet of the species was based almost exclusively on crabs (45.4% of individual prey) and fish (53.3%). Most crabs were marbled crabs (Pachygrapsus marmoratus) and most fish were adult blennies (Coryphoblennius galerita and Lipophrys pholis). Given its energy requirements (about 1250 kJ/day), a single mink will consume during the month of August approximately 945 blennies and 496 crabs. Although we lack accurate data on mink abundance, a cautious estimation (4 mink/km before dispersal), supported by field observations, suggests that predation in August may reach 3780 blennies and 1984 crabs per km of shoreline. This predation pressure could affect the numbers of blennies and (less probably) crabs, indirectly benefiting the populations of their prey, that is, sessile invertebrates and snails. More field research is needed, but our results suggest that an exotic non-marine top predator such as the American mink could affect intertidal communities in Eurasia.     

Voles on small islands: effects of food limitation and alien predation

Ecosystems of three trophic levels may be bottom-up (by food-plant availability) and/or top-down (by predators) limited. Top-down control might be of greater consequence when the predation impact comes from an alien predator. We conducted a replicated two-factor experiment with field voles (Microtus agrestis) during 2004-2005 on small islands of the outer archipelago of the Baltic Sea, south-west Finland, manipulating both predation impact by introduced American mink (Mustela vison) and winter food supply. In autumn 2004, we live-trapped voles on five islands from which mink had been consistently removed, and on four islands where mink were present, and provided half of these islands with 1.8 kg oats per vole. Body mass of female voles increased as a response to supplementary food, whereas both food supplementation and mink removal increased the body mass of male voles in subsequent spring. During winter, there was a positive effect of supplementary food, but in the subsequent summer, possible positive long-term impacts of food supplementation on field voles were not detected. Mink removal appeared not to affect density estimates of field voles during the winter and summer immediately after food addition. Trapping data from 2004 to 2005 and 2007 suggested, however, that in two out of three summers densities of voles were significantly higher in the absence than in the presence of mink. We conclude that vole populations on small islands in the archipelago of the Baltic Sea are mainly bottom-up limited during winter (outside the growing season of food plants), when food availability is low, and limited by mink predation during summer which slows population growth during the reproductive season of voles.     

Monitoring site occupancy for American mink in its native range

American mink (Neovison vison) are secretive, semi-aquatic carnivores that often require noninvasive methods based on field signs such as tracks and scat for determining their spatial distribution. Most previous assessments of survey methods for American mink have been conducted in the United Kingdom where mink are an invasive species. We evaluated survey techniques for American mink in riparian habitat in its native range in the midwestern United States. We used occupancy modeling to compare detection rates between walking surveys and mink raft surveys, and we evaluated the potential for environmental covariates and observer bias to influence detectability from walking surveys. Per-survey detection probabilities were greater for walking surveys (0.72) than for mink rafts (0.39). Walking surveys also were cheaper and easier to conduct in small streams prone to flooding when compared to mink raft surveys. However, detection probabilities from walking surveys were affected by observer bias, recent rainfall, substrate, and date. We recommend walking surveys for determining the distribution of American mink in riparian habitat in the Midwest if occupancy modeling is applied to adjust for environmental and observer effects on detectability. We used such an approach to demonstrate occupancy dynamics of mink were related to variable water depths, which has implications for how this carnivore might be influenced by climate change. Mink rafts standardize the substrate for recording mink tracks and reduce the likelihood of observer effects. For studies using many volunteers, we recommend mink rafts for determining site occupancy by American mink. © 2011 The Wildlife Society.