Mink Presence Research Articles

Comparing the effectiveness of environmental DNA and camera traps for surveying American mink (Neogale vison) in northeastern Indiana.

While camera traps can effectively detect semi-aquatic mammal species, they are also often temporally and monetarily inefficient and have a difficult time detecting smaller bodied, elusive mammals. Recent studies have shown that extracting DNA from environmental samples can be a non-invasive, alternative method of detecting elusive species. Environmental DNA (eDNA) has not yet been used to survey American mink (Neogale vison), a cryptic and understudied North American mustelid. To help determine best survey practices for the species, we compared the effectiveness and efficiency of eDNA and camera traps in surveys for American mink. We used both methods to monitor the shoreline of seven bodies of water in northeastern Indiana from March to May 2021. We extracted DNA from filtered environmental water samples and used quantitative real-time PCR to determine the presence of mink at each site. We used Akaike's Information Criterion to rank probability of detection models with and without survey method as a covariate. We detected mink at four of the seven sites and seven of the 21 total survey weeks using camera traps (probability of detection (ρ) = 0.36). We detected mink at five sites and during five survey weeks using eDNA (ρ = 0.25). However, the highest probability of detection was obtained when both methods were combined, and data were pooled (ρ = 0.47). Survey method did not influence model fit, suggesting no difference in detectability between camera traps and eDNA. Environmental DNA was twice as expensive, but only required a little over half (58%) of the time when compared to camera trapping. We recommend ways in which an improved eDNA methodology may be more cost effective for future studies. For this study, a combination of both methods yielded the highest probability for detecting mink presence.

Understanding ground-nesting habitat selection by waterbirds to prioritize invasive predator control on islands

The introduction of carnivores to islands affects the conservation status of native prey. For two decades, American mink (Neogale vison) has expanded its distribution in the sub-Antarctic archipelago of Austral Patagonia, while preying on nests of solitary ground-nesting birds. Here, upland geese (Chloephaga picta) and flightless steamer ducks (Tachyeres pteneres) are naïve to such predation risk because of the lack of native terrestrial predators. We used nest occupancy models to reveal preferred breeding habitats of those two ground-nesting waterbirds to prioritize areas for future mink control programs. We searched for nests along 80 transects on Navarino Island, southernmost Chile, and 11 transects on 10 nearby islets (3 ± 2.9 ha). Our results showed that islets were essential for breeding in comparison to the main island (5.3 nests/km versus 0.8 nests/km, respectively). The occupancy models revealed that islets were particularly important for flightless steamer ducks, endemic to western Patagonia. As breeding habitat, upland geese preferred medium-dense shrubland, and flightless steamer ducks preferred rocky coastlines. Nest detection was negatively affected by dense vegetation and, in the case of the upland geese, towards the end of the survey. Ten camera traps (n = 385 trap nights) revealed mink presence on all islets, highlighting their capacity to swim in cold water, up to 340 m distance from the main island. We conclude that islets represent breeding refuges for ground-nesting waterbirds and therefore advocate allocating resources for mink control during the warm season on islets. We also provide predictive maps of nesting habitat preference of upland geese and flightless steamer ducks for conservation managers to focus their often scarce human and financial resources on areas critical for breeding. Finally, we argue that detection probability should be an integral part of bird breeding studies, which can be methodologically achieved through targeting transects, not nests, as sample units.

Sexual differences in responses of meadow voles to environmental cues in the presence of mink odor.

In rodents, defensive behaviors increase the chances of survival during a predator encounter. Observable rodent defensive behaviors have been shown to be influenced by the presence of predator odors and nearby environmental cues such as cover, odors from conspecifics and food availability. Our experiment tested whether a predator scent cue influenced refuge preference in meadow voles within a laboratory setting. We placed voles in an experimental apparatus with bedding soaked in mink scent versus olive oil as a control across from four tubes that either contained (a) a dark plastic covering, (b) opposite-sex conspecific odor, (c) a food pellet, or (d) an empty, unscented space. A three-way interaction of tube contents, subject sex, and the presence of mink or olive oil on the preference of meadow voles to spend time in each area of the experimental apparatus and their latency to enter each area of the apparatus revealed sex differences in the environmental preference of meadow voles facing the risk of predation. The environmental preference of female, but not male, meadow voles was altered by the presence of mink urine or olive oil. A similar trend was found in the latency of males and females to enter each area of the experimental apparatus. These differences suggest that each sex utilizes different methods to increase their fitness when experiencing a predation risk. The observed sex differences may be explained by the natural history of voles owing to the differences in territorial range and the dynamics of evasion of terrestrial predators.

Ecological requirements and relative impact of threats affecting the Austral RailRallus antarcticus: monitoring methodology considerations for an imperative conservation status re-evaluation

SummarySince its rediscovery in 1998, two major threats have been mentioned for the enigmatic Austral RailRallus antarcticus: cattle management by burning grass and rushes and predation by American minkNeovison vison. Added to the lack of a protocol to monitor this secretive bird, ever-growing threats make it necessary to study its global situation in depth to take accurate and urgent management decisions. We firstly studied how threats to the Austral Rail currently impact their occupancy and relative density (RD) at a wetland scale and habitat features associated with its presence at survey site scale inside wetlands in Santa Cruz province, Argentina. We additionally developed a monitoring protocol to detect the rail effectively and promptly by studying responses to playback with two different vocalisations at different times of the day and season. Both threats evaluated were negatively related to occupancy and RD of the Austral Rail, especially mink presence appearing to have an additive effect. We propose American mink control as crucial for Austral Rail conservation, while it would also be necessary to conserve a portion of wetlands exempt from burning and cattle presence. At survey sites, its presence was positively related with c.1–1.5 m tall rushes, whilst rails avoided low- density rush areas that resulted after management of rushes with fire to create pasture. To detect rails, both vocalisations can be confidently used at any time of the day and season. The poor knowledge about species ecology, mismanagement of vegetation in wetlands, expansion of American mink in Patagonia, construction on two dams in the Santa Cruz river basin, added to the already modified humidity conditions due to global climate change, force us to suggest that the Austral Rail should be considered as globally ‘Endangered’ (EN), to ensure the consideration of the species in management decisions.

A familiar face with a novel behavior raises challenges for conservation: American mink in arid Patagonia and a critically endangered bird

Invasive American mink is a severe threat to endangered waterbird populations. In Europe and South America, control efforts have been increasing in the last decades. Mink has a proven acute impact on the Hooded Grebe, a critically endangered specialist of highland plateau lakes in Patagonia where a single mink attack can deplete up to 4% of the global population. We aimed to examine mink ecology in Austral Patagonia to inform ongoing control programs in the region and contribute to efforts elsewhere to counteract mink impact. We evaluated mink distribution at a regional scale and found that its distribution extends across the steppe-arid environment, often assumed to be an obstacle for mink colonization. Consequently, its current distribution fully overlaps with the Hooded Grebe breeding grounds. At a finer scale, we used 3 techniques to detect American mink at different times of the year and found a temporal sex-biased occupation-dispersion pattern: permanent presence of both sexes in areas around the plateaus used by grebes and upstream male biased dispersion in late summer (which leads young mink to the grebes' lakes). Therefore, plateau perimeters function as an invasion front that resets yearly. Lastly, from scat content analysis we detected a shift in mink diet from a generalist type in lowland rivers to a diet specialized on birds in highland lakes, where attacks are exclusively directed towards Hooded Grebes and where waterbird presence might be a determinant of mink presence (regardless of other prey). All of these findings have profound relevance for control schedules and priorities.

An invasive predator affects habitat use by native prey: American mink and water vole co‐existence in riparian habitats

AbstractThis study examines the occurrence of invasive American mink Neovison vison and native water voles Arvicola amphibius along the shorelines of eutrophic lakes and at small midfield ponds in the Mazurian Lakeland, north‐east (NE) Poland. The main hypothesis tested in this study is that built‐up areas and small midfield ponds are avoided by mink; therefore, they can serve as refuges that protect water voles from mink predation. Floating rafts were used to monitor mink and water vole distribution. Mink presence was negatively associated with urbanization at lake shorelines and midfield ponds, and these two characteristics positively correlated with occurrence of water voles in the study area. The probability of occurrence of water voles was significantly lower at sites where mink occurred. The occupancy rates for mink at lakes and at midfield ponds were higher in autumn than in spring and conversely the occupancy rates for water voles were higher in spring and lower in autumn. Our study shows that within a postglacial landscape, urban areas and midfield ponds distant from lakes are avoided by the American mink and thus these areas may function as refuge habitats for water voles and may maintain populations of this rodent at the landscape scale despite the presence of mink.

Range expansion of an invasive species through a heterogeneous landscape – the case of American mink in Scotland

AbstractAimThe impact of invasive species is one of the main causes of biodiversity loss world‐wide, and as a result, there is much interest in understanding the pattern and rate of expansion of species outside their native range. We aimed to characterize the range expansion of the American mink (Neovison vison) invading from multiple introduction points through a varied landscape bounded by coastline to better understand and manage its spread.LocationScotland, UK.MethodWe collated and used records of mink presence to calculate the historical range and rate of range expansion at successive time intervals. We used a presence‐only model to predict habitat suitability and a newly developed individual‐based modelling platform, RangeShifter, to simulate range expansion.ResultsRecords showed that mink were distributed throughout Scotland, except in the far north. We found that the rate of spread varied both spatially and temporally and was related to landscape heterogeneity. Habitat suitable for mink in west Scotland is restricted to the coast.Main conclusionsWe concluded that temporal and spatial variation in range expansion is attributable to heterogeneity within the landscape and also demonstrated that the potential for long‐distance dispersal does not necessarily facilitate range expansion when availability of suitable habitat occurs in narrow strips and/or is fragmented. We have highlighted methodological gaps in calculating rates of expansion in invasive species but have demonstrated alternative methods that successfully utilize presence‐only data. Our study reaffirms that invasive species will colonize less favourable habitats and highlights the need to remain vigilant of their potential for expansion even when distribution appears to be static for a time.

Tracking the expansion of the American mink (Neovison vison) range in NW Portugal

Invasive in many European countries, the American mink (Neovison vison) was introduced in Portugal in the late 1980’s, presumably escaping from Spanish fur farms close to the border. In spite of the biological richness of the invaded area, no study ever addressed the evolution of the invasion process. We aimed to investigate the current distribution and status of the mink in NW Portugal and discuss some contributing factors to explain the rate of invasion. We detected mink presence using floating rafts as footprint tracking devices, and scats as a molecular tool aiding in species identification. Results demonstrate a clear range expansion southwards, with mink already occupying most of the region’s hydrographic basins. After a first phase of slow expansion (55 km in 20 years), mink seems to have expanded its range quite rapidly in only 2 years (45 km). The initial delay could be due to local thriving otter populations, whereas the recent establishment of red swamp crayfish (Procambarus clarkii) in the area could be a plausible explanation for the acceleration in the mink’s expansion. Being a key food resource, crayfish may be playing an important role as an expansion facilitator. Mink eradication is probably no longer feasible since well established populations near the border continue to function as sources for the Portuguese population. Therefore, a control program should start immediately in the NW region, preferably in conjunction with Spanish authorities.

Controlling invasive species by empowering environmental stakeholders: ecotourism boat operators as potential guardians of wildlife against the invasive American mink

AbstractPeople who have a stake in their environment are more likely to volunteer to assist conservation but they must be empowered to do so. This study explored the possibility of harnessing volunteers in the control of an invasive predator, the American mink Neovison vison, which decimates seabird colonies in coastal west Scotland. A questionnaire was sent to ecotourism boat operators, a group assumed to have an economic interest in wildlife biodiversity and a stake in their environment, to gauge their opinion on lethal control of American mink. The majority (64%) of respondents were concerned about the presence of mink in their area, agreed with control in principle and were willing to become involved in a volunteer capacity. Respondents who would not volunteer but agreed with control (21%) might reconsider if mink had a visible impact on their local wildlife. The minimum level of support people expected was information on where to get, and how to deploy, monitoring and trapping equipment. This study confirms that people with an intrinsic interest in wildlife consider themselves willing to protect their local biodiversity, with only limited resource input, such as an information pack, from external sources.

Including nanoparticle mixtures in human health risk assessment

This article presents an integrated analysis using a Monte Carlo exposure model, dose–response effects model and habitat,and population dynamics models, all of which allow us to quantitatively estimate the effects of polychlorinated biphenyl (PCB)exposure on American mink (Mustela vison) abundance at the Portland Harbor Superfund Site (Site), and the associated uncertainties. The Site extends from river mile 1.9 of the Lower Willamette River, near its confluence with the Columbia River, to river mile 11.8, just downstream of downtown Portland, Oregon. The potential effects of PCBs on the American mink population were evaluated in the Baseline Ecological Risk Assessment (BERA) due to the historical presence of mink in the are a and because mink are known to be highly sensitive to the effects of PCBs. Hazard quotients (HQs) calculated in the BERA indicated that PCB concentrations measured in Portland Harbor fish were above levels known to cause reproductive effects in mink. Further analysis was needed to evaluate the potential magnitude of effects on the Site mink population. The integrated analysis presented herein demonstrates that if an effect of PCB exposure is a less than 30% reduction in kit production, then PCB remediation is not expected to have any effect on mink abundance. This is a Site‐specific conclusion that depends on the quality, abundance, and distribution of mink habitat in Portland Harbor. The PCB dose associated with a 30% reduction in kit production was calculated as 101 mg/kg bw/d (90% CI ¼ 69–146 mg/kg bw/d). The mink PCB dose estimates from the Portland Harbor BERA indicate that if mink are present, their baseline exposure levels probably exceed 101 mg/kg bw/d. Therefore, some level of reduction in PCB exposure could be beneficial to the species if the study area provides sufficient habitat to support a mink population. This analysis demonstrates that risk analysis for population‐level assessment endpoints benefits from analyses beyond those that calculate exposure and predict organism‐level effects. Evaluation of population‐level impacts provides risk managers with a richer perspective within which to evaluate the environmental protectiveness and cost effectiveness of feasibility study alternatives across a range of potential remediation goals.

Assessing effects of PCB exposure on American mink (Mustela vison) Abundance in Portland Harbor

ABSTRACTThis article presents an integrated analysis using a Monte Carlo exposure model, dose–response effects model and habitat, and population dynamics models, all of which allow us to quantitatively estimate the effects of polychlorinated biphenyl (PCB) exposure on American mink (Mustela vison) abundance at the Portland Harbor Superfund Site (Site), and the associated uncertainties. The Site extends from river mile 1.9 of the Lower Willamette River, near its confluence with the Columbia River, to river mile 11.8, just downstream of downtown Portland, Oregon. The potential effects of PCBs on the American mink population were evaluated in the Baseline Ecological Risk Assessment (BERA) due to the historical presence of mink in the area and because mink are known to be highly sensitive to the effects of PCBs. Hazard quotients (HQs) calculated in the BERA indicated that PCB concentrations measured in Portland Harbor fish were above levels known to cause reproductive effects in mink. Further analysis was needed to evaluate the potential magnitude of effects on the Site mink population. The integrated analysis presented herein demonstrates that if an effect of PCB exposure is a less than 30% reduction in kit production, then PCB remediation is not expected to have any effect on mink abundance. This is a Site‐specific conclusion that depends on the quality, abundance, and distribution of mink habitat in Portland Harbor. The PCB dose associated with a 30% reduction in kit production was calculated as 101 μg/kg bw/d (90% CI = 69–146 μg/kg bw/d). The mink PCB dose estimates from the Portland Harbor BERA indicate that if mink are present, their baseline exposure levels probably exceed 101 μg/kg bw/d. Therefore, some level of reduction in PCB exposure could be beneficial to the species if the study area provides sufficient habitat to support a mink population. This analysis demonstrates that risk analysis for population‐level assessment endpoints benefits from analyses beyond those that calculate exposure and predict organism‐level effects. Evaluation of population‐level impacts provides risk managers with a richer perspective within which to evaluate the environmental protectiveness and cost‐effectiveness of feasibility study alternatives across a range of potential remediation goals. Integr Environ Assess Manag 2013;X:000–000. © 2013 SETAC Integr Environ Assess Manag 2013;9999:XX–XX. © 2013 SETAC Integr Environ Assess Manag 2014;10:60–68. © 2013 SETAC

The impact of the American mink (Neovison vison) on native vertebrates in mountainous streams in Central Spain

The American mink (Neovison vison) is responsible for the widespread decline of its prey species in the regions where it is an invasive species. The current expansion of the mink in the Iberian Peninsula has aroused concern among conservationists about its negative impact on the rich native fauna. However, evidence for this is still scarce, although there are several studies establishing a direct causal relationship between declining native species and the presence of the American mink. Thus, it is important to further investigate the responses of native species to the American mink in several habitats and locations to enhance our knowledge about the patterns of the effect of the mink in Spain, as well as to inform conservation actions. A field study of the impact of the American mink on a mountainous vertebrate community in central Spain is presented. We studied six species: two fish, one amphibian, one bird, and two mammals. The general results showed a species-specific sensitivity to mink presence, with the Mediterranean water shrew (Neomys anomalus) and the southern water vole (Arvicola sapidus) being the most affected because their ranges were significantly decreased after the introduction of the mink. Regarding the other species, neither their abundance nor range was apparently affected by the American mink. The predatory behavior of the mink and interactions with other carnivores could account for these results. These data aid in shedding light about the current impact of the mink on invaded areas of the Iberian Peninsula and highlight the variability of its effects, as well as the urgent need to establish a general program of control of the mink to avoid negative effects upon native prey communities. Furthermore, given the different responses of native species, we propose that measures to protect native species should be based on species-specific goals and attributes.

Effective control of non‐native American mink by strategic trapping in a river catchment in mainland Britain

The introduction of American mink (Neovison vison; hereafter mink) into Europe has had severe impacts on many native wildlife species, including the water vole (Arvicola amphibius) in mainland Britain. Although trapping has been widely used to attempt to control mink, managers have little direct evidence of its effect on mink density or distribution, particularly where immigration of mink from nearby areas is inevitable. Such evidence is needed to justify the use of lethal methods in conservation policy. During 2006–2010 we removed mink from the River Monnow Catchment in western Britain, using track-recording rafts to monitor continuously for mink presence, guiding a strategic trapping effort. The area monitored and trapped was increased in stages, from a core sub-catchment with 109 km of water-course in 2006, to a 421-km2 catchment with 203 km of water-course in 2009. In each successive sub-catchment, mink detection and capture rates declined rapidly to near-zero levels after trapping began. Detections and captures showed seasonal peaks in every year corresponding to known dispersal periods, but also declined steadily from year to year, with increasing periods in which we did not detect mink. These results suggested that each sub-catchment was cleared of mink within a few months, with subsequent captures attributable to immigration. On average, we detected each mink 5.1 times before capture (daily probability of detection = 0.059 per mink and raft), and trapped them 3.4 days after deploying traps in response. On average, mink entering the area were likely to have been present for less than 13 days before capture. Water voles had been extinct in the Monnow Catchment since the 1980s. During 2006–2008 (starting 6 months after mink trapping commenced), we released 700 captive-bred water voles into the treatment area to re-establish a wild population. Persistence of this population through the 4 years of the project was considered indicative of effective mink control. This study demonstrates that, even in a mainland context, a systematic trapping strategy can have a substantial impact on the density and distribution of a damaging species, in this case allowing the restoration of a native prey species. © 2013 The Wildlife Society

Understanding the inter-specific dynamics of two co-existing predators in the Tierra del Fuego Archipelago: the native southern river otter and the exotic American mink

Knowledge about interactions between endangered native southern river otters (Lontra provocax) and introduced American mink (Neovison vison) is essential for effective management of both species. We evaluated competition for spatial and trophic niches between otter and mink in overlapping and non-overlapping areas, comparing distribution, habitat preference, diet and mink marking behavior. We surveyed otter and mink signs along 250 km of Beagle Channel coastline. Habitat suitability models were constructed based on species presence/absence and habitat characteristics, using generalized linear models. Feces were collected for diet analyses. Otters used forested coasts with 12°–32° shoreline slope and without human influence, and our evidence suggests they were not affected by mink presence. Mink preferred forested and shrubland coasts with 10°–28° shoreline slope. Neither human influence nor otter presence affected mink habitat occupation, but in the presence of otters, mink left fewer signs. Otters consumed more aquatic prey than mink, and mink modified their diet in the presence of otters, consuming more exotic small terrestrial mammals and less fish as well as shifting to smaller and shallower fish species that are less consumed by otters. Mink showed more plastic, generalist behavior than otters, being more tolerant of human presence, using more habitat types and having greater diet breadth. At the same time, otters apparently affect mink adversely and could help limit their invasion in sympatric areas. Conservation and recovery of otters, therefore, may produce a secondary benefit of simultaneously reducing the effect of mink, thereby providing an additional way to control this exotic predator’s population.

Abundances of waterbird species on lakes in Argentine Patagonia as a function of season, lake size and the presence of mink

Regular counts from 2005 to 2009 were made of the waterbirds inhabiting lakes and ponds in Lanin National Park in the southwestern part of Neuquen Province, Argentina, a landscape dominated by Andean–Patagonian wetlands and forests. Bird surveys conducted on 21 wetland areas detected 8,311 individuals belonging to 27 species from 9 families. The most abundant and frequent species were Ashy-headed Goose (Chloephaga poliocephala), Speckled Teal (Anas flavirostris), Red-gartered Coot (Fulica armillata) and White-winged Coot (Fulica leucoptera). The presence and abundance of bird species with respect to wetland surface area was examined. Small lakes (<100 ha) had higher numbers of individuals, but some species such as the Neotropic Cormorant (Phalacrocorax olivaceus), Great Grebe (Podiceps major), Flying Steamer Duck (Tachyeres patachonicus) and Spectacled Duck (Anas specularis) were more abundant at larger lakes. Speckled Teal, Red Shoveler (Anas platalea) and Chiloe Wigeon (A. sibilatrix) were more common in small- and medium-sized lakes. Most waterbird species were found at a smaller percentage of the lakes where mink were present than at mink-free lakes. Although the ponds and lakes studied are protected within the network of this Argentine National Park, such protection is not implemented effectively. Hence, their future conservation faces several potential threats such as American mink expansion, tourism, fishing and hunting. This information could contribute to the development of management guidelines for the effective conservation of Patagonian wetlands.

A multi-scale analysis of the habitat associations of European otter and American mink and the implications for farm scale conservation schemes

Establishing relationships of species occurrence with environmental variables is important to define and develop species conservation schemes. The character of the riparian environment which supports populations of otter (Lutra lutra) and mink (Neovison vison) was investigated across spatial scales. Both species were positively associated with increased habitat diversity, the provision of natural land cover and a reduced level of urbanisation. However, mink were most closely associated with immediate riparian conditions whereas otters were associated more strongly with broad scale environmental characteristics. We argue that the benefits of habitat improvements may be seen more quickly in the occurrence of mink, a non-native pest species, than in the occurrence of the native otter. This has implications for the perceived benefits of conservation schemes which are applied at a fine scale, with particular relevance to farm-based conservation prescriptions such as agri-environment schemes. We show that mink presence is associated with higher levels of riparian bird diversity, a pattern which was not observed with occurrence of otter. It may be inevitable that conservation schemes benefit populations of both desirable and undesirable species. The present study demonstrates the importance of considering scale in animal ecology when developing conservation strategies. The use of specialist species with large home ranges to reflect the success of conservation schemes which are applied on a farm scale should be avoided. Effective conservation schemes may require specific pest species monitoring and management.

Behavioural responses of voles to simulated risk of predation by a native and an alien mustelid: an odour manipulation experiment

Context. Potential mammalian prey commonly use the odours of their co-evolved predators to manage their risks of predation. But when the risk comes from an unknown source of predation, odours might not be perceived as dangerous, and anti-predator responses may fail, except possibly if the alien predator is of the same archetype as a native predator. Aims. In the present study we examined anti-predator behavioural responses of voles from the outer archipelagos of the Baltic Sea, south-western Finland, where they have had no resident mammalian predators in recent history. Methods. We investigated responses of field voles (Microtus agrestis) to odours of native least weasels (Mustela nivalis) and a recently invading alien predator, the American mink (Mustela vison), in laboratory. We also studied the short-term responses of free-ranging field voles and bank voles (Myodes glareolus) to simulated predation risk by alien mink on small islands in the outer archipelago of the Baltic Sea. Key results. In the laboratory, voles avoided odour cues of native weasel but not of alien mink. It is possible that the response to mink is a context dependent learned response which could not be induced in the laboratory, whereas the response to weasel is innate. In the field, however, voles reduced activity during their normal peak-activity times at night as a response to simulated alien-mink predation risk. No other shifts in space use or activity in safer microhabitats or denser vegetation were apparent. Conclusions. Voles appeared to recognise alien minks as predators from their odours in the wild. However, reduction in activity is likely to be only a short-term immediate response to mink presence, which is augmented by longer-term strategies of habitat shift. Because alien mink still strongly suppresses vole dynamics despite these anti-predator responses, we suggest that behavioural naiveté may be the primary factor in the impact of an alien predator on native prey. Implications. Prey naiveté has long been considered as the root cause of the devastating impacts of alien predators, whereby native prey simply fail to recognise and respond to the novel predation risk. Our results reveal a more complex form of naiveté whereby native prey appeared to recognise alien predators as a threat but their response is ultimately inadequate. Thus, recognition alone is unlikely to afford protection for native prey from alien-predator impacts. Thus, management strategies that, for example, train prey in recognition of novel threats must induce effective responses if they are expected to succeed.

Does removal of an alien predator from small islands in the Baltic Sea induce a trophic cascade?

Changes in carnivore abundance can alter the distribution and abundance of plants on a community wide basis, an effect known as a trophic cascade. Because alien predators can have a disproportionate impact, compared to native predators, on herbivore populations, they may induce stronger trophic cascades in plant communities than native predators. We studied the indirect effects of the removal of an alien predator, the American mink Mustela vison on plant communities on small islands in the Baltic Sea, SW Finland. Mink had been removed from a group of islands for 12 yr, while another group of islands with mink presence served as a control area. Field voles Microtus agrestis and bank voles Myodes glareolus exert strong grazing pressure on the island vegetation and are an important part of mink diet. On nine islands of the mink removal area and five islands of the control area we studied the vegetation in ten randomly chosen plots; five in herbaceous and five in woody (i.e. dwarf shrub) vegetation. We studied the cascading impacts of mink predation on grassy and woody vegetation using the Shannon diversity and equitability indices and comparing abundances of different species between mink removal and control islands. Diversity and equitability of plant communities were higher on mink removal islands. In grassy patches, abundances of several species differed between mink removal and control islands. Our results demonstrate, for the first time, that alien predator removal may induce a trophic cascade on small islands.

American mink control on inland rivers in southern England: An experimental test of a model strategy

Invasive species are an important driver of global biodiversity loss. Under international legislation, the UK has an obligation to eradicate or to control the alien, invasive American mink. Using a large-scale field experiment, we tested the effectiveness of a specified mink removal strategy, identified through earlier modelling work, in reducing the relative abundance of mink. We found that mink removal could be effective in reducing mink populations with four months or less of trapping per year, over only 2–3 years, but that for small sites (c. 20 km) a flexible, reactive approach, coupled with continual monitoring for mink presence is necessary. Survival of reintroduced water voles at four sub-sites within our mink removal sites suggest that the reactive mink removal strategy adopted in this study was sufficient for water vole protection. We discuss the use of an adaptive management approach in local mink management, and consider the wider implications of our results for invasive species control on mainlands.

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.