Audrey J. Magoun

Characteristics of Wolverine Reproductive Den Sites

Successful wolverine (Gulo gulo) reproduction may be linked to the availability and quality of reproductive dens sites, but little is known about wolverine reproductive dens, especially in North America. We present descriptions of wolverine dens in Idaho and Alaska, compare them to dens in other regions, and propose factors that may influence den-site selection. Our study includes 8 den sites used by 3 females over 4 years in northwestern Alaska and 7 sites used by 2 females over 4 years in central Idaho. We categorized reproductive dens of wolverines as either natal (used during parturition) or maternal (used subsequent to the natal den and before weaning). Dens in Alaska were usually long, complex snow tunnels with no associated trees or boulders. In contrast, dens in Idaho were always associated with fallen trees or boulders. All dens were covered with at least 1 m of snow. With few exceptions, wolverine dens described to date have been located in alpine, subalpine, taiga, or tundra habitat. Reports of dens in low elevation, densely forested habitats are rare. Factors which may have influenced selection of den sites included thermoregulatory advantages, protection from predators, suitability of the site during the spring thaw, and location of rearing habitat. We recommend managers consider limiting wolverine harvests and reducing human disturbance in wolverine denning g habitat.

The bioclimatic envelope of the wolverine (Gulo gulo): do climatic constraints limit its geographic distribution?

We propose a fundamental geographic distribution for the wolverine (Gulo gulo (L., 1758)) based on the hypothesis that the occurrence of wolverines is constrained by their obligate association with persistent spring snow cover for successful reproductive denning and by an upper limit of thermoneutrality. To investigate this hypothesis, we developed a composite of MODIS classified satellite images representing persistent snow cover from 24 April to 15 May, which encompasses the end of the wolverine’s reproductive denning period. To investigate the wolverine’s spatial relationship with average maximum August temperatures, we used interpolated temperature maps. We then compared and correlated these climatic factors with spatially referenced data on wolverine den sites and telemetry locations from North America and Fennoscandia, and our contemporary understanding of the wolverine’s circumboreal range. All 562 reproductive dens from Fennoscandia and North America occurred at sites with persistent spring snow c...

SYNTHESIS OF SURVIVAL RATES AND CAUSES OF MORTALITY IN NORTH AMERICAN WOLVERINES

Abstract Understanding population vital rates is fundamental to the evaluation of conservation options for wolverines (Gulo gulo). We estimated survival rates and causes of wolverine mortality in trapped and untrapped populations within montane, boreal, and tundra environments using data from 12 North American radiotelemetry studies conducted between 1972 and 2001. Rates were based on data for 62 mortalities of 239 radiomarked wolverines. Mortalities included 22 wolverines that were trapped or hunted, 3 road or rail killed, 11 that were predated, 18 that starved, and 8 deaths of unknown cause. Annual survivorship rates were estimated for sex and age class using Kaplan-Meier staggered-entry techniques. Survival was substantially lower in trapped (<0.75 for all age–sex classes) than in untrapped (>0.84 for all age–sex classes) populations. Human-caused mortality was mostly additive to natural mortality for wolverines in a management context. Logistic growth rate estimates indicated that trapped populations would decline (λ ≅ 0.88) in the absence of immigration from untrapped populations (λ ≅ 1.06). We recommend a system of spatial harvest controls in northern, continuous populations of wolverines and reduction of harvest along with more spatially explicit conservation measures in southern metapopulations.

Density estimation in a wolverine population using spatial capture–recapture models

ABSTRACT Classical closed-population capture—recapture models do not accommodate the spatial information inherent in encounter history data obtained from camera-trapping studies. As a result, individual heterogeneity in encounter probability is induced, and it is not possible to estimate density objectively because trap arrays do not have a well-defined sample area. We applied newly-developed, capture—recapture models that accommodate the spatial attribute inherent in capture—recapture data to a population of wolverines (Gulo gulo) in Southeast Alaska in 2008. We used camera-trapping data collected from 37 cameras in a 2,140-km2 area of forested and open habitats largely enclosed by ocean and glacial icefields. We detected 21 unique individuals 115 times. Wolverines exhibited a strong positive trap response, with an increased tendency to revisit previously visited traps. Under the trap-response model, we estimated wolverine density at 9.7 individuals/1,000 km2 (95% Bayesian CI: 5.9–15.0). Our model provides a formal statistical framework for estimating density from wolverine camera-trapping studies that accounts for a behavioral response due to baited traps. Further, our model-based estimator does not have strict requirements about the spatial configuration of traps or length of trapping sessions, providing considerable operational flexibility in the development of field studies.

The wolverine's niche: linking reproductive chronology, caching, competition, and climate

Abstract Wolverines are demographically vulnerable and susceptible to impacts from climate change. Their distribution is correlated with persistent spring snow cover, but food-based explanations for this relationship have not been explored. We synthesize information on the timing of both wolverine reproductive events and food availability to improve our understanding of the behaviors, habitat features, and foods that influence reproductive success. Wolverine births are constrained to a brief period of the year and occur at an earlier date than other nonhibernating, northern carnivores. Our examination suggests that this timing is adaptive because it allows wolverines to take advantage of a cold, low-productivity niche by appending the scarce resources available during winter to the brief period of summer abundance. The wolverines bet-hedging reproductive strategy appears to require success in 2 stages. First, they must fuel lactation (February–April) with caches amassed over winter or acquisition of a sudden food bonanza (e.g., winter-killed ungulates); otherwise, early litter loss occurs. Next, they must fuel the majority of postweaning growth during the brief but relatively reliable summer period of resource abundance. The 1st stage is likely dependent on scavenged ungulate resources over most of the wolverines range, whereas the 2nd stage varies by region. In some regions the 2nd stage may continue to be focused on scavenging ungulate remains that have been provided by larger predators. In other regions the 2nd stage may be focused on predation by wolverines on small prey or neonatal ungulates. During all seasons and regions, caching in cold, structured microsites to inhibit competition with insects, bacteria, and other scavengers is likely a critical behavioral adaptation because total food resources are relatively limited within the wolverines niche. Habitat features that facilitate caching, e.g., boulders and low ambient temperatures, are likely important and could be related to the limits of distribution. This “refrigeration-zone” hypothesis represents a food-based explanation for the correlation between wolverine distribution and persistent spring snow cover. Understanding regional differences in foods that fuel reproduction and underlying causes to the limits of distribution could be important for maintaining wolverine populations in the future.

Modeling Wolverine Occurrence Using Aerial Surveys of Tracks in Snow

Abstract We designed a novel approach to determining extent of distribution and area of occupancy for wolverines (Gulo gulo) by using aerial surveys of tracks in snow and hierarchical spatial modeling. In 2005 we used a small, fixed-wing aircraft with pilot and one observer to search 575 of 588 survey units for wolverine tracks in approximately 60,000 km2 of boreal forest in northwestern Ontario, Canada. We used sinuous flight paths to scan open areas in the forest in the 100-km2 survey units. We detected tracks in 138 (24%) of the 575 sampled units. There was strong evidence of occurrence (probability of occurrence >0.80) in 30% of the 588 survey units, weak evidence of occurrence (0.50–0.80) in 12%, weak evidence of absence (0.20–0.50) in 15%, and strong evidence of absence (<0.20) in 43%. Wolverine range comprised 59% of the study area and area of occupancy was 33,400 km2. With information on probability of occurrence and core areas of occupation for wolverines in our study area, resource managers and others can examine factors that influence wolverine distribution patterns and use this information to formulate best management practices that will maintain wolverines on the landscape in the face of increasing resource development. Comparing future survey results with those of our 2005 survey will provide an objective way to assess the efficacy of management practices.

Integrating motion-detection cameras and hair snags for wolverine identification†

ABSTRACT We developed an integrated system for photographing a wolverines (Gulo gulo) ventral pattern while concurrently collecting hair for microsatellite DNA genotyping. Our objectives were to 1) test the system on a wild population of wolverines using an array of camera and hair-snag (C&H) stations in forested habitat where wolverines were known to occur, 2) validate our ability to determine identity (ID) and sex from photographs by comparing photographic data with that from DNA, and 3) encourage researchers and managers to test the system in different wolverine populations and habitats and improve the system design. Of the 18 individuals (10 M, 8 F) for which we obtained genotypes over the 2 years of our study, there was a 100% match between photographs and DNA for both ID and sex. The integrated system made it possible to reduce cost of DNA analysis by >74%. Integrating motion-detection cameras and hair snags provides a costeffective technique for wildlife managers to monitor wolverine populations in remote habitats and obtain information on important population parameters such as density, survival, productivity, and effective population size.

SELECTION OF POST-FIRE SERES BY LYNX AND SNOWSHOE HARES IN THE ALASKAN TAIGA

AssTRkcT Knowledge of lynx (Lynx canadensis) use of burned areas is desirable to anticipate the effects of fire management in the taiga. During 1991 to 1994 we tested whether L. canadensis and snowshoe hares (Lepus americanus) selectively used post-fire seres during winter in the Alaskan taiga and whether selection could be explained by population trends and the concept of refugia for L. americanus. Forest seral stages included early-successional tall shrub-sapling (1985 burn), mid-successional dense tree (1966 burn), and mature coniferous (100 to 115 yr old). Track counts, pelt sealing records, and anecdotal observations indicated that numbers of L. canadensis and L. americanus declined during 1991 to 1994. Tracks of both species were most abundant in the 1966 burn, which was characterized by dense coniferous and deciduous regeneration. Lepus americanus and L. canadensis aIso had the highest preference for the 1966 burn, which is consistent with the prediction that refugia for L. americanus would also contain most of the L. canadensis during the decline phase of the hare cycle. Fire is the predominant disturbance in upland taiga, and mid-successional forest is clearly important to L. canadensis and L. americanus. However, stands of mature forest may be important to L. canadensis for denning and as a source of alternative prey during declines of L. americanus. Thus, conservative fire management to benefit L. canadensis in interior Alaska should ensure some stands of mature forest to fulfill their potential habitat needs.

Coarse-Scale Distribution Surveys and Occurrence Probability Modeling for Wolverine in Interior Alaska

Abstract We determined wolverine (Gulo gulo) distribution and occurrence probabilities using aerial surveys and hierarchical spatial modeling in a 180,000-km2 portion of Interior Alaska, USA. During 8 February–12 March 2006, we surveyed 149 of 180 1,000-km2 sample units for wolverine tracks. We observed wolverine tracks in 99 (66.4%) sample units. Wolverine detection probability was ≥69% throughout the survey period. Posterior occurrence probabilities of whether a wolverine track occurred in a sample unit was dependent on survey timing, number of transects flown, number of neighboring sample units with detected tracks, percentage of the sample unit with elevation ≤305 m, and human influences. Our model indicated strong evidence of occurrence (>0.80) in 72% of the 180 survey units, strong evidence of absence (<0.20) in 12%, and weak evidence of occurrence or absence (0.20–0.80) in 16%. Wolverine area of occupancy made up 83% of the study area. Simulations illustrated that 2–4 survey routes were necessary for the survey technique to provide strong evidence of wolverine presence or absence in Interior Alaska if a track was not identified along the first route. The necessary number of survey routes depends on the occurrence probability in a sample unit. We provided managers with a map of wolverine distribution in Interior Alaska and an efficient and lower-cost method to detect coarse-scale changes in wolverine distribution. Our technique was effective in both Interior Alaska and Ontario, Canada, suggesting it would be effective throughout most of the boreal forest range of wolverines where tracks can be readily observed from the air. The technique requires a certain skill level in recognizing tracks; it is essential that tracks are identified correctly and training may be necessary depending on surveyor experience.

Comments on Brodie and Post: Climate-driven declines in wolverine populations: causal connection or spurious correlation?

The recent paper by Brodie and Post (“Nonlinear responses of wolverine populations to declining winter snowpack”, Popul Ecol 52:279–287, 2010) reports conclusions that are unsupportable, in our opinion, due to both mis-interpretations of current knowledge regarding the wolverine’s (Gulo gulo) association with snow, and the uncritical use of harvest data to index wolverine populations. The authors argue that, because the wolverine is a snow-dependent species, average annual provincial snowfall, based on weather station data, can be expected to correlate strongly and positively with wolverine population numbers, which in turn can be accurately indexed by trapper harvests. Thus, correlations between declines in wolverine harvests and declining average snowpack are interpreted to reflect a climate-driven decrease in wolverine populations. This conclusion overstates the nature of the wolverine’s association with snow, and makes unsupportable assumptions about the reliability of harvest data as a proxy for population size.