Stoat density, diet and survival compared between alpine grassland and beech forest habitats

In New Zealand, alpine grasslands occur above the treeline of beech forest. Historically stoat control paradigms in New Zealand?s montane natural areas have assumed alpine grassland is a marginal habitat that limits dispersal between beech forest stoat populations. We compared the summer-to-autumn (January?April) density, weight, diet and winter survival of stoats between these two habitatsduring years of low beech seedfall. Stoats were live-trapped, marked and released in alpine grassland and low-altitude beech forest in the Borland Valley, Fiordland National Park, during 2003 and 2004, and were caught and euthanased for necropsy in 2005. Stoat density was estimated using spatially explicit capture?recapture (SECR). The proportion of stoats marked in one year but recaptured in the next was used as a measure of ?observed survival?. Prey remains were identified from scats collected during 2003 and 2004 and stomachs from stoats killed in 2005. Stoat density was similar in both habitats over the two years, about one stoat per square kilometre. Observed survival from 2003?2004 was also similar, but survival from 2004?2005 was higher in alpine grassland than in beech forest. In 2003, male stoats were on average heavier in alpine grassland than in beech forest, although average weights were similar in the other years. Diet differed significantly between the two habitats, with stoats in alpine grasslands eating mainly ground weta (a large invertebrate) (72%) and hares (23%), while stoats in beech forest ate mainly birds (31%) and mice (19%). Collectively these results suggest that alpine grasslands are not a poor quality habitat for stoats. Traditionally it has been thought that stoats cannot survive on invertebrate prey alone. This research demonstrates that stoats relying largely on invertebrate prey can occur at similar densities and with equivalent survival to stoats relying on vertebrate prey.     

Lack of movement of stoats (

Little is known about the movement of stoats in alpine grassland, where several species of native birds, reptiles and invertebrates are potentially at risk from predation. Radio-tracking, live trapping and tracking tunnel techniques were used to sample stoats in two adjacent habitats to determine whether the home range of stoats in beech forest valley floors extends into neighbouring alpine grasslands in the Ettrick Burn Valley, Fiordland. If this is the case then trapping stoats in the more easily accessible beech forest valley floors might serve to protect endangered species inhabiting the adjacent but more remote alpine grasslands. Between December 2000 and March 2001, 415 radio locations were collected on 15 stoats and none were observed to make any significant movements between the two habitats. Stoats were active in alpine grasslands, and trapping in the adjacent beech forest valley would not have caught those stoats during the time-frame of this study. Further research is needed to determine long term impacts of trapping in beech forest on stoats in alpine grasslands. During the timeframe of this research stoats were more abundant in beech forest than in alpine grasslands, and tracking tunnels showed this trend to be consistent at other sites.     

An analysis of a recreational hunter's red deer tallies in the Tararua Ranges, North Island, New Zealand

Home range and diet of stoats inhabiting beech forest were examined by trapping and radio-tracking. Eleven stoats (6 female, 5 male) were fitted with radio-transmitters. Minimum home ranges of five females averaged 124 ± 21 ha and of four males 206 ± 73 ha. Range lengths of females averaged 2.3 ± 0.3 km and of males 4.0 ± 0.9 km. These differences were not statistically significant. Adult female stoats appeared to have mutually exclusive home ranges. Two females and one male had home ranges that were bisected by the Eglinton River. All three crossed the river regularly and could only have done so by swimming. Bird remains were found in 54% of stoat guts and scats examined, lagomorphs in 33%, and invertebrates in 34%. Australian brushtail possum remains were found in 11% of samples overall, but only in guts and scats from male stoats. A road through the study area affected the behaviour of stoats. Females avoided the road but males preferred it and were found scavenging road-kills, which may explain why they are more frequently found as road-kills themselves. In most years, New Zealand beech forest may be marginal habitat for stoats. No breeding was detected in the year of our study but there had been high productivity in the previous season. Stoats probably survive in this habitat because they are flexible in their diet and because their breeding biology allows them to respond rapidly to a sudden increase in food availability.     

The effects of mice on stoats in southern beech forests

Introduced stoats (Mustela erminea) are important invasive predators in southern beech (Nothofagus sp.) forests in New Zealand. In these forests, one of their primary prey species – introduced house mice (Mus musculus), fluctuate dramatically between years, driven by the irregular heavy seed‐fall (masting) of the beech trees. We examined the effects of mice on stoats in this system by comparing the weights, age structure and population densities of stoats caught on two large islands in Fiordland, New Zealand – one that has mice (Resolution Island) and one that does not (Secretary Island). On Resolution Island, the stoat population showed a history of recruitment spikes and troughs linked to beech masting, whereas the Secretary Island population had more constant recruitment, indicating that rodents are probably the primary cause for the ‘boom and bust’ population cycle of stoats in beech forests. Resolutions Island stoats were 10% heavier on average than Secretary Island stoats, supporting the hypothesis that the availability of larger prey (mice verses wētā) leads to larger stoats. Beech masting years on this island were also correlated with a higher weight for stoats born in the year of the masting event. The detailed demographic information on the stoat populations of these two islands supports previously suggested interactions among mice, stoats and beech masting. These interactions may have important consequences for the endemic species that interact with fluctuating populations of mice and stoats.     

Non-invasive methods for genotyping of stoats (

Stoats (Mustela erminea) are a significant pest in New Zealand. A critical aspect of their management is the ability to identify individuals in order to estimate abundance or to determine the origin of residual animals after control, particularly as the trap-shy nature of stoats reduces the utility of trapping to gain this information. We investigated non-invasive ?capture? methods as an alternative to live-trapping or removal methods for estimating stoat abundance. First we determined whether sufficient variability exists at six microsatellite DNA loci to reliably identify individuals in the potentially bottlenecked, introduced stoat populations of New Zealand. In December 2001 we conducted a 7-night pilot field experiment using a modified hair-tube design, where we obtained a total of 64 hair samples. Sufficient DNA was extracted from 3?6 hair follicles to genotype a total of 51 samples. DNA quality declined if samples were left in the field for several nights before being collected, and daily checks proved best for maximising the quality of DNA obtained, while minimising the risk of multiple ?captures? of stoats. Conclusions were that non-invasive molecular sampling is likely to be a viable technique for estimating population density of stoats in New Zealand beech forest but that additional variable loci are required.     

Managing an invasive predator pre-adapted to a pulsed resource: a model of stoat (Mustela erminea) irruptions in New Zealand beech forests

The stoat (Mustela erminea) is a specialist predator that evolved to exploit the unstable populations of northern voles and lemmings. It was introduced to New Zealand, where it is pre-adapted to respond with a population irruption to the resource pulses that follow a heavy seedfall of southern beech (Nothofagus spp.). Culling stoats during an irruption is necessary to reduce damaging predation on nesting endemic birds. Culling might not reduce the stoat population long term, however, if high natural mortality exceeds culling mortality in peak years. During other phases of the beech-mast cycle, culling might have a greater effect on a smaller stoat population, whether or not damage prevention is critical. We developed a 4-matrix model to predict the effects of culling on λ, the annual rate of change in the size of the stoat population, through the four annual phases of an average masting cycle, explicitly distinguishing between apparent and real culling. In the Post-seedfall phase of the cycle, large numbers of stoats are killed, but little of this extra mortality is additive; in other phases, culling removes larger proportions of smaller total numbers of stoats that would otherwise have lived. Culling throughout all phases is most effective at reducing stoat populations, but is also the most expensive option. Culling in Post-seedfall plus Seed or Crash years is somewhat less effective but better than culling in one phase only. Culling has different short-term effects on stoat age distribution depending on the phase of the cycle when culling begins.     

Population dynamics and resource use of red deer after release from harvesting in New?Zealand

Despite periods of extensive government-funded control, fluctuating commercial exploitation and ongoing recreational hunting, little is known about how red deer (Cervus elaphus scoticus L?nnberg) in New?Zealand respond to the cessation of harvesting in terms of population growth rate and resource use. We describe the population dynamics and resource use of red deer in a montane catchment over 5?years (1962?67) following cessation of intensive government-funded control in 1961. Locations and sex?age classes of deer were observed monthly along a fixed route in the Harper-Avoca catchment, inland Canterbury. A total of 2036 red deer groups were observed. The number of groups observed annually increased during the study but no trends in median (2 or 3) or modal (1 or 2) group sizes were found. Population growth rates (r) of deer were extraordinarily high in the first two years (e.g. 2.33 ? 0.22 for adult females and 1.61 ? 0.23 for adult males), but decreased in subsequent years and were not biologically possible without substantial immigration and/or changes in detectability of deer. Sexual segregation and selection of vegetation types (alpine grassland, montane grassland, and forest) and 10 topographic landforms showed stronger intra-annual than inter-annual patterns. Segregation was greatest in spring and summer, least in the rut, and variable in winter. In all seasons, sexual segregation was greatest at 25- and 50-ha scales, moderate at 100-ha, and absent at the 500- and 1000-ha scales. Selection of vegetation types also varied seasonally, with deer of both sexes preferring montane grasslands in spring and summer and alpine grasslands in the rut. Backslopes were preferred landforms in spring and summer, spurs during spring and the rut, and hollows during the rut. Our results highlight the need to consider spatial scale, immigration, and detectability in the design of red deer culling and harvesting programmes. Studies of home-range size and use, migration patterns, dispersal rates and distances are required to better understand the impacts of red deer on New?Zealand ecosystems and the effects of management on red deer populations.     

Using genetic techniques to quantify reinvasion,survival and in situ breeding rates during control operations

Determining the origin of individuals caught during a control/eradication programme enables conservation managers to assess the reinvasion rates of their target species and evaluate the level of success of their control methods. We examine how genetic techniques can focus management by distinguishing between hypotheses of ‘reinvasion’ and ‘survivor’, and defining kin groups for invasive stoats (Mustela erminea) on Secretary Island, New Zealand. 205 stoats caught on the island were genotyped at 16 microsatellite loci, along with 40 stoats from the opposing mainland coast, and the age and sex were determined for each individual. Using these data, we compare and combine a variety of genetic techniques including genetic clustering, population assignment and kinship‐based techniques to assess the origin of each stoat. The population history and individual movement could be described in fine detail, with results indicating that both in‐situ survival and breeding, and reinvasion are occurring. Immigration to the island was found to be generally low, apart from in 1 year where around 8 stoats emigrated from the mainland. This increased immigration was probably linked to a stoat population spike on the mainland in that year, caused by a masting event of southern beech forest (Nothofagus sp.) and the subsequent rodent irruption. Our study provides an example of some of the ways genetic analyses can feed directly into informing management practices for invasive species.     

Ecology of the stoat in

We studied the ecology of a high-density population of stoats in Fiordland, New Zealand, in the summer and autumn of 1990-91 following a Nothofagus seeding in 1990. Results are compared with findings from the same area in 1991-92, a period of lower stoat density. In the high-density year, minimum home ranges (revealed by radio-tracking) of four females averaged 69 ha and those of three males 93 ha; range lengths averaged 1.3 km and 2.5 km respectively. Neither difference was statistically significant. For combined sexes, average range area in the high-density year was significantly less, and range length was significantly shorter, than in the following year. When we compared stoat diet in the high-density year with that in the following two years, there were no significant differences in the frequencies of occurrence of birds or invertebrates in stoat guts. Overall, bird remains were found in 56% of guts, and invertebrates in 28%. Possum remains occurred in 6% of male stoats but were never found in females. Mice were only detected in stoats in the high-density year, when they occurred in 54% of guts. Lagomorphs occurred significantly more often in the guts of stoats during lower-density years (26%) than the high- density year (7%). Seedfall in Nothofagus forest is synchronous and periodic. Following seedfall, mouse density rises dramatically, followed by a sharp rise in stoat numbers. It has been suggested that mice feed on the abundant seed and that stoats in turn increase because of the large numbers of mice available to them. We suggest that the situation is more complex and that increases in not only mouse, but also bird (and possibly invertebrate), densities may contribute to the high productivity of stoats in the year following a Nothofagus seedfall.     

The biology of the stoat (Mustela erminea) in the National Parks of New Zealand VI. Infestation with Skrjabingylus nasicola

Abstract

The natural distribution of the parasite Skrjabingylus nasicola was surveyed in 1492 stoats from New Zealand’s National Parks. Infestation was rare in the young, so distribution was expressed as frequency of occurrence of skulls containing nematodes in samples of stoats more than 6 months old. Conservative estimates of incidence ranged from 0 to 37% in 27 local subsamples with at least 6 adults and subadults (mean 10%, n = 1005), sometimes varying substantially across short distances. Incidence was highest in beech forest and scrub/grassland habitats with annual rainfall less than 1600 mm. Subadult males (6–10 months old) were infested as frequently as adult males (older than 10 months), and there was no difference in incidence between the sexes. Worms recovered from 97 infested stoats occurred equally often in either side of the skull; were more often large (i.e., probably female) at all intensities of infestation; and numbered 1–73 per infested stoat (mean 12.9 in females, 14.2 in males; difference not significant). There was no evidence that infested stoats were smaller or lighter than uninfested onces.     

Spring home range, spatial organisation and activity of stoats Mustela erminea in a South Island Nothofagus forest, New Zealand

This radio-tracking study reports the spring home range, spatial organisation and activity of 11 stoats Mustela erminea in a New Zealand Nothofagus forest, 1,5 yr after significant seedfall when rodent density was low, but stoat density was high. The average home range of 4 male stoats was 223 (SE = 45) ha, significantly larger than the average range area of 94 (SE = 13) ha recorded for 7 female stoats. Stoats were generally tolerant of sharing space and did not maintain intra- or intersexual territorial spacing systems. There was no evidence of temporal avoidance with several stoats of the same sex showing slight attraction to one another. However, stoats still may avoid one another when in close proximity. Long-term radio-tracking studies are required to determine the general patterns of spacing behaviour in stoats, Male stoats showed higher levels of activity during daylight than females.     

Understanding contributions of cohort effects to growth rates of fluctuating populations

1. Understanding contributions of cohort effects to variation in population growth of fluctuating populations is of great interest in evolutionary biology and may be critical in contributing towards wildlife and conservation management. Cohort-specific contributions to population growth can be evaluated using age-specific matrix models and associated elasticity analyses. 2. We developed age-specific matrix models for naturally fluctuating populations of stoats Mustela erminea in New Zealand beech forests. Dynamics and productivity of stoat populations in this environment are related to the 3-5 year masting cycle of beech trees and consequent effects on the abundance of rodents. 3. The finite rate of increase (lambda) of stoat populations in New Zealand beech forests varied substantially, from 1.98 during seedfall years to 0.58 during post-seedfall years. Predicted mean growth rates for stoat populations in continuous 3-, 4- or 5-year cycles are 0.85, 1.00 and 1.13. The variation in population growth was a consequence of high reproductive success of females during seedfall years combined with low survival and fertility of females of the post-seedfall cohort. 4. Variation in population growth was consistently more sensitive to changes in survival rates both when each matrix was evaluated in isolation and when matrices were linked into cycles. Relative contributions to variation in population growth from survival and fertility, especially in 0-1-year-old stoats, also depend on the year of the cycle and the number of transitional years before a new cycle is initiated. 5. Consequently, management strategies aimed at reducing stoat populations that may be best during one phase of the beech seedfall cycle may not be the most efficient during other phases of the cycle. We suggest that management strategies based on elasticities of vital rates need to consider how population growth rates vary so as to meet appropriate economic and conservation targets.     

Change in Diet of Stoats Following Poisoning of Rats in a New- Zealand Forest

The abundance and diet of stoats (Mustela erminea) were compared before and after an aerial 1080-poison operation for possums (Trichosurus vulpecula) in a New Zealand podocarp- hardwood forest. Poisoning dramatically reduced ship rat (Rattus rattus) abundance. Although rats were the main prey item of stoats before the poisoning, stoat abundance was unaffected by the operation and there was a change in stoats' diet from rats to birds. The conservation benefits and risks of undertaking such operations are not clear. It is not known whether the predation risk for any particular species of bird (or other animal) will be higher or lower with fewer rats but the same density of stoats. As large-scale poison operations are now common in New Zealand forests, a better understanding of predator-prey relationships in these areas is required as soon as possible.     

Large‐scale pest control in New Zealand beech forests

In 2014, baits laced with the poison sodium fluoroacetate (1080) were sown over 694 000 ha of mostly native beech forests in New Zealand to control rats, stoats and possums – a landscape‐scale pest control programme called ‘Battle for our Birds’. This large pest control operation was necessitated by the mast seeding of beech trees which led to irruptions of rodent and stoats which were predicted to lead to decreases in vulnerable native wildlife. In this article, we describe why and how this extensive pest control programme was developed and implemented. We describe the seedfall monitoring that was used to determine the need for large‐scale rodent and stoat control and the response of these predators to this control. We also provide a summary of the bird monitoring that was undertaken to demonstrate the effectiveness or otherwise of the programme.     

Using artificial nests to explore predation by introduced predators inhabiting alpine areas in New Zealand

Abstract

Many bird species endemic to alpine New Zealand are now at critically low densities and restricted in range, making predator‐prey research difficult. We used artificial nests in the Borland Valley, Fiordland National Park, to investigate (1) which introduced species is the most frequent nest predator in the two habitats, (2) whether nest survival differs between habitats, and (3) the utility of artificial nests for guiding conservation management. We used different types of artificial nest in 2 different years and undertook a calibration study of the two types. In 2003, survival of artificial nests containing wax eggs and chicken eggs was high in both habitats. In 2004, survival of artificial nests containing plastilina eggs and chicken eggs was low in both habitats, but was higher in alpine grassland compared with beech forest. Stoats and possums were the most frequent predators (36 and 22% respectively of artificial plastilina nests in alpine grassland and high‐altitude beech forest combined); these percentages did not vary significantly between habitats. Given the low density and sparse distribution of vulnerable species in much of New Zealand, data from artificial nests can be a useful tool for studying predation in these remote and difficult habitats, or at least, preferable to ignorance. However, the type of artificial nest used can strongly affect the rate at which they are destroyed.     

Conservation of New Zealand's tussock grassland moth fauna

New Zealand has a relatively rich moth fauna associated with tussock grasslands, reflecting the historic importance of plant communities containing native grasses in the New Zealand landscape. Extensive grassland communities were most common in the eastern and central regions of New Zealand’s South Island where what now remains of these communities is contained within more than 350 Pastoral Leases managed for pastoral farming. Because these various grassland communities are distributed from coastal areas to the limit of vegetation on mountains there has been an uneven human impact. Grasslands at sea-level (including those on sand dunes and alluvial plains) have almost been eliminated while those in montane valleys have until now escaped intensive development. Alpine grasslands, like those of montane areas are now much less woody as a result of both Polynesian and more recent burning. From the late 1970s, based on public concern that continuing development was eliminating the natural landscapes and conservation values of the distinctive tussock grasslands of the South Island, a concerted programme of identification of the broad conservation values of the remaining grassland dominated regions was initiated. Implementation of the recommendations in the resulting published reports relating to priority areas for conservation and the subsequent full-scale review of Pastoral Leases has resulted in the formation of large grassland Conservation Parks and the reservation of a multitude of smaller areas, based on a suite of recreation, landscape and biological values. This important ongoing Tenure Review process has important implications for the conservation of tussock grassland biota including numerous native moths and their habitats.     

Using genetics and Bayesian modelling to evaluate the eradication of stoats (

The New Zealand Department of Conservation recently (May 2008) began a programme to eradicate stoats (Mustela erminea) from Resolution Island (Fiordland, New Zealand) using kill traps. In conjunction with this eradication effort we have the following 3 objectives: (1) to measure the population abundance of stoats prior to trapping using hair tubes and forensic DNA methods; (2) optimise techniques for detecting individual stoats, in order to quantify the probability of stoat persistence given no detections after several months of trapping; and (3) use genetic analyses to identify the possible origins (mainland incursions or in situ breeding) of new stoats captured in a control zone. We present Bayesian modelling techniques used to determine the probability of stoat persistence on the island after the initial population reduction, when individual stoats are no longer captured in traps. We also provide details on an effective level of monitoring and trapping effort required to maintain a comfortable level of confidence that stoats no longer persist on the island. Improving these techniques adds to variety of valuable tools for management of invasive mammal species in a range of natural environments worldwide.     

Productivity and mortality of mohua (Mohoua ochrocephala)

Abstract

Mohua (Mohoua oehrocephala) breeding and mortality were studied in the Eglinton Valley, Fiordland National Park, New Zealand. Mohua on the valley floor (380 m a.s.l.) bred from early October until March, though birds at higher altitude in the Eglinton and elsewhere started later. In low altitude areas, most pairs raised two broods a year, but elsewhere they seemed to raise only one. Eggs were laid daily, and incubation began with the laying of the last egg and lasted about 20 days. The nestling period was about 23 days. Clutches contained 1–5 eggs, most often three. During three breeding seasons there was no significant nest predation by introduced mammals, but in 1990/91, when stoat (Muslela erminea) numbers were high, 67% of nests and 50% of nesting females were destroyed by stoats. When stoat numbers were low, the productivity and mortality of double‐brooded mohua were well within the ranges recorded for other forest‐dwelling bird species, and the productivity of single‐brooded mohua was lower than that of most other forest‐dwelling passerines. During years when stoat numbers were high, the productivity and mortality of even the double‐brooded mohua were lower than for most other forest‐dwelling passerines. There is some evidence that there were density dependent mechanisms influencing the productivity and survival of chicks.     

Do mallard ducks feature in the diet of stoats in an agricultural landscape?

ABSTRACT

Research on stoat diet composition in New Zealand has primarily focussed on consumption of indigenous fauna in largely unmodified landscapes. This study used stomach content and stable isotope (δ¹³C and δ¹⁵N) analysis to assess stoat diet in a highly modified agricultural landscape in Southland, New Zealand, focussing on stoat predation of the mallard duck. Stoats were captured in Lochiel, Southland during August–November 2016 and 2017. Stomach content analysis of 26 captured stoats revealed limited stoat predation of mallards (n?=?1) and mallard eggs (n?=?1). Using liver tissue, stable isotope mixing models suggested that bird eggs on average met between 73 and 85% of stoat metabolic requirements throughout the mallard breeding period. Furthermore, mixing model outputs suggested that bird eggs made up a substantial proportion (77–84%) of stoat assimilated diet early in the mallard breeding period, when mallard eggs are readily available. In contrast, isotope mixing models suggested that mallard ducks/ducklings did not make a large overall contribution to stoat diets (< 3%). This study shows that stoats are an egg predator in the Southland agricultural landscape and mallard eggs may contribute to stoat assimilated diet early in the mallard breeding season before alternative prey items become available.     

The biology of the stoat (Mustela erminea) in the National Parks of New Zealand III. Morphometric variation in relation to growth,geographical distribution,and colonisation

Abstract

A total of 1599 stoats were collected from 14 study areas (including all 10 National Parks) from 1972 to 1976. Samples were larger in summer, and contained more females. Young stoats are born in September-October, and females reach adult weight by the following March, though males not until after August. There was significant geographic variation in the body size of adult stoats sampled: males from lowland podocarp/broadleaved forests averaged 3% smaller than males from upland beech forests in skull length, and 4% smaller in head-and-body length. This pattern was repeated, less clearly, in females and in young (approximately 2–5 months old). In contrast with stoats in Britain, assumed to be still the same size as the colonising stock introduced into New Zealand in 1884 and subsequently, males from lowland podocarp forests were unchanged or possibly smaller, and males from upland beech forests were larger; females were larger in all habitats. In males, the extent of geographic variation is almost as great in New Zealand as in the whole of continental Europe. Possible explanations of this pattern are discussed.