Carolyn M. King

Techniques for trapping and tracking stoats (Mustela erminea); a review, and a new system

Systematic direct observations of the small, fast‐moving, and wide‐ranging stoat are rarely practicable. The simplest indirect methods of observation are kill‐trapping, live‐trapping, and footprint recording. The data obtainable and the advantages and disadvantages of these methods are reviewed. Two new kinds of traps and a footprint recording system are described; they are especially suitable for use in rugged field conditions far from base facilities. When operated together in suitable habitat, these techniques can provide useful information on the population structure, feeding habits, and natural movements of stoats. Together or singly they also have potential as management tools, especially in identification of nest predators and in faunal surveys of islands.

Population biology of the ship rat and Norway rat in Pureora Forest Park, 1983-87

Abstract Populations of ship rats (Rattus rattus) and Norway rats (R. norvegicus) were sampled over the five years 1983–87 at Pureora Forest Park, by Fenn and rat kill‐traps every three months. Fenn and rat traps recorded similar capture rates in comparable habitats, although Fenns caught more heavy and fewer young rats. Ship rats (n = 1793 collected) were more abundant, heavier and larger in native forest, regardless of logging history, than in exotic forest of any age. Young ship rats (age classes 1–3) were most abundant in unlogged interior native forest, and in autumn and winter after summer and autumn breeding. Capture rates declined after peaking in 1985, probably due to reduced recruitment of young rats following lower pregnancy rates in adult females. The irregular annual seasonal cycle of reproduction and abundance observed at Pureora is the same as that described for non‐commensal ship rat populations elsewhere in New Zealand and the world. Thirty five of 43 Norway rats collected came from a single trap by the Waipapa Stream, apparently set near a permanent colony. Pregnant female Norway rats were trapped in every season, suggesting year‐round breeding. This implies that both species can recover rapidly after control operations conducted at any time of year, but especially in spring and summer. Future research should include manipulative exploration of factors limiting ship rat abundance and Norway rat distribution.

Cohort variation in the life-history parameters of stoatsMustela erminea in relation to fluctuating food resources: a challenge to boreal ecologists

This paper reviews field evidence suggesting that periodic temporary population irruptions of feral house miceMus musculus in New Zealand have a substantial effect on the reproductive success of stoatsMustela erminea Linnaeus, 1758. Stoats born during the summer of a peak in numbers of mice are more numerous and have higher fecundity (ovulation rate) but lower productivity (independent offspring per female) and shorter longevity than those born when mice are not abundant. This reversed silver-spoon effect is apparently correlated with intense competition for food within a much larger than usual cohort of young stoats. However, both stoats and mice are introduced in New Zealand, so it is possible that these effects are not natural. The question could be resolved by data demonstrating similar cohort effects in stoats in the northern hemisphere, living in areas with fluctuating vole populations and limited alternative prey.

Estimating the potential for reinvasion by mammalian pests through pest-exclusion fencing

Pest mammals are completely excluded from Maungatautari Ecological Island, New Zealand, by a 47-km Xcluder pest-proof fence; however, they are commonly sighted directly outside, along the fenceline. Permanent pest exclusion relies on maintaining fence integrity, and enhancing knowledge of pest activity and behaviour at fenced reserves. We describe summer and winter periods of activity and behaviour of mammalian pests directly adjacent to the pest-proof fence. We (1) tested for the effects of adjacent habitat type, breach type and season on the rate of mammalian pest sightings directly at the fence, (2) determined how quickly pest mammals may locate a fence breach, and how likely they are to exploit it, and (3) developed a predictive model to help assess the probability of a pest gaining entry to the sanctuary if repair to a fence breach is delayed. Observations inside the rolled fence hood provided firm evidence that rats travel and forage extensively in this artificial although highly acceptable aboveground habitat, much more than on the ground. We confirm and emphasise that mammalian pests are constantly testing the pest-proof fence. Pests are very common directly outside the fence, and within 24h there is a very high likelihood that a fence breach will be located and exploited. The greatest threat of reinvasion comes (1) nocturnally, (2) from rodents and (3) in the summer; however, these results also confirm that there is constant risk from multiple pest species, regardless of time of day or season.

The life-history tactics of mustelids, and their significance for predator control and conservation in New Zealand

Abstract Intensive predator control on game estates in 19th-century England is believed to be largely responsible for the decline of two large native mustelids, the pine marten and polecat. Two small, related, species, the stoat and weasel, were also killed in large numbers but are still common in England, and have been introduced into New Zealand. The theory of life-history tactics offers an explanation for the different effects of persecution on large and small mustelids. It also suggests that stoats in New Zealand national parks cannot be exterminated by trapping, and that the first priority for the conservation of rare native birds such as the takahe is active management to stimulate breeding.

Tree-climbing capabilities of Norway and ship rats

Abstract Norway and ship rats (Rattus norvegicus and Rattus rattus) have invaded many habitats of conservation value worldwide, but Norway rats are widely assumed to be less of a threat to tree-nesting biota than are ship rats because they are less adept at climbing. We tested this assumption by measuring the capabilities of wild-caught captive rats of both species in reaching food rewards above ground, placed at fixed sites of increasingly difficult access. We confirmed that Norway rats were much slower and less agile than ship rats, but could in fact, given enough time, reach the same height above ground, run across the same thin ropes fixed at both ends and climb a real tree. However, they were more easily defeated by obstacles, more dependent on the availability of footholds, more vulnerable to falls, and those of >200 g body weight were significantly less likely to reach food rewards at the unsupported ends of small branches. We conclude that Norway rats seldom forage above ground, not because they cannot climb but because arboreal foraging is more risky and less likely to be rewarding for them.

Capture probability and heterogeneity of trap response in stoats (Mustela erminea)

The technique most widely used to control the stoat, an introduced predator in New Zealand, is to set Fenn (kill) traps, usually in lines or (less often) in grids. There has been no analysis of trap response in stoats, nor of the extent of potential variation in probability of capture. We report the results of an analysis of mark–recapture data recorded from stoats observed during a period of high stoat and mouse density in January 1980 in the Eglinton and Hollyford Valleys (northern Fiordland), using livetraps set in lines at one per 400 m over 14 km in each valley. Over 8 days of trapping (1–11 January), 89 stoats were tagged. The daily probability of first capture for all ages, both sexes, was 0.14 (with 95% confidence intervals 0.07–0.25) and of recapture 0.10 (0.07–0.14). We also analysed a new set of mark–recapture data collected during a period of very low mouse density in the Grebe Valley (southern Fiordland) in December 2000, using 19 live-traps set in a line at one per kilometre over 20 km. In this study 21 adult stoats and no young of year were tagged. The daily probability of first capture for adult males was 0.12 (0.04–0.31), and of recapturing them, 0.15 (0.10–0.23). A month later, in late January 2001, 68 Fenn traps set at four per kilometre caught 48 previously unmarked stoats, plus 12 of the 21 marked stoats released alive. Heterogeneity in probability of recapture was investigated by taking a longer subset of the 1980 data (1–17 January) and grouping individuals by sex and age. In the best closed-captures models, ranked using AICc, first-capture probability was similar for all stoats (0.17 (0.12–0.24)), and evidence of variation in the probability of recapture between age and sex classes was present but weak. The confidence limits around the recapture probabilities for adult males and females overlapped completely. Recapture probabilities for young-of-the-year males remained about the same (0.14 (0.11–0.19)), while the recapture probability of young-of-the-year females halved after first capture (0.07 (0.04–0.11)). Pledgers finite-mixture models demonstrating individual heterogeneity in trappability produced lower AICc values than the closed-captures models partitioning variation in recapture probability by age and sex alone. The observed heterogeneity in trap response is therefore not due only to variable individual response to traps, but is also to opportunity, as might be expected in data collected from a line of traps where the edge effect on trap-encounter rate is high. However, the extent to which trap-encounter rate helps to explain the observed heterogeneity is unknown. Indeed, there may be other sources of individual heterogeneity that are not related to age/sex or to trap-encounter rate, and this is a potential problem for wildlife managers using conventional trap lines to remove stoats to protect native species.

A simplified protocol for detecting two systemic bait markers (Rhodamine B and iophenoxic acid) in small mammals

Abstract We developed a method of quantifying levels of fluorescence in the whiskers of wild stoats (Mustela erminea) using fluorescence microscopy and Axiovision 3.0.6.1 software. The method allows for discrimination between natural fluorescence present in or on a whisker, and the fluorescence resulting from the ingestion of the systemic marker Rhodamine B (RB), although some visual judgement is still required. We also developed a new high performance liquid chromatography (HPLC) protocol for detecting the systemic marker iophenoxic acid (IPA) in the blood of laboratory rats (Rattus norvegicus) and wild stoats. With this method, the blood of an animal that has consumed IPA can be tested for the presence of the foreign IPA compound itself. This is a more reliable test than the previous method, which measured the raised level of natural blood protein‐bound iodine correlated with IPA absorption. The quantity of blood required from animal subjects is very small (10 μl), so the testing is less intrusive and the method can be extended to smaller species. The extraction technique uses methanol, rather than acids and heavy metal salts, thereby simplifying the procedure. Recovery of IPA is quantitative, giving a highly reliable reading. In experiments on captive rats the IPA method proved successful. Of 12 positively marked carcasses, two that had not been frozen for the 24 h before blood samples were taken showed relatively lower IPA levels. The same IPA detection method, as well as the whisker analysis for RB, was applied successfully to a population of wild stoats to which both Rhodamine B and IPA were made available at bait stations. The presence of both bait markers was detectable in rats for at least 21 days and in stoats for at least 27 days.

Can a predator see 'invisible' light? Infrared vision in ferrets (Mustelo furo)

Infrared (wavelengths >750 nm) light-emitting equipment is commonly used worldwide to monitor nocturnal predator and prey behaviour. However, it is possible that the infrared (IR)-light wavelengths emitted from the equipment are so close to the spectral threshold of some key species that the light may be detected. An operant procedure was used to test whether five male ferrets (Mustela furo) could see an IR light with peak wavelengths of 870 and 920 nm. First, the ferrets were taught to press a lever under a lit white light for food reinforcement (overall mean response accuracy was 89%). Changing the properties (wavelength and intensity) of the light did not disrupt the ferrets’ abilities to perform the learned task. When the light was changed to IR (870 nm), four of five ferrets responded to the light at levels significantly higher than chance (mean = 68%, n = 4188, P < 0.01). When glare from a red trial-starting light was removed, two of the five ferrets (S3 and S4) showed strong evidence (response accuracies of 84% and 78%, respectively, P < 0.01) that they could see IR at 870 nm; however, S3 definitely could not see IR at 920 nm (n = 124, mean = 47%, P = 0.53). We conclude that at least some ferrets can see the light emitted from standard monitoring equipment that uses IR wavelengths of ~870 nm. To ensure nocturnal predator and prey behaviours are not altered by IR surveillance, field programs should use only high-wavelength IR diodes (at least 920 nm).

Functional responses of an invasive top predator Mustela erminea to invasive meso-predators Rattus rattus and Mus musculus, in New Zealand forests

Context Management of suites of invasive mammal species can lead to perverse outcomes, such as meso-predator release, or can achieve desirable reductions in the abundance of top-order predators by controlling their prey. Predictive models for predator–prey systems require estimates of predator functional responses, i.e. predation rates as functions of prey density. Aims In New Zealand, estimates of the functional responses of stoats (Mustela erminea) to mice (Mus musculus) and ship (black) rats (Rattus rattus) are required to improve management models for these invasive species. Methods We derived fitted relationships between the presence or absence of mouse or ship-rat remains in stoat guts and corresponding indices of prey abundance in beech and podocarp forests, respectively. To convert field data on stoat-gut contents to minimum kill rates, we used data on feeding activity and estimates of gut-passage time, observed in captive stoats. Key results The most parsimonious fitted curves were Type II functional responses, with a steeper stoat–mouse curve for autumn–winter, indicating a more specialist feeding habit than that in spring–summer. Estimated kill rates of mice per stoat per day reached an asymptote of 1.13 during autumn–winter. Our maximum observed kill rate for spring–summer was 11% less than the extrapolated upper limit of 1.04 mice per stoat per day for New Zealand ecosystems. No asymptote was reached within the limits of the data for the stoat–rat relationship. Conclusions Recent models for trophic interactions between stoats and the primary rodent prey have overestimated kill rates by stoats in forested ecosystems, particularly at very low and very high densities of mice. We show how data on stoat-gut contents can be rescaled to estimate minimum kill rates of rodent prey. Implications The functional-response relationships we have derived can be used to improve modelled predictions of the effects of natural or management-driven perturbations of invasive stoats and their primary rodent-prey populations.