Mitchell K. Taylor

Genetic structure of the world’s polar bear populations

We studied genetic structure in polar bear (Ursus maritimus) populations by typing a sample of 473 individuals spanning the species distribution at 16 highly variable microsatellite loci. No genetic discontinuities were found that would be consistent with evolutionarily significant periods of isolation between groups. Direct comparison of movement data and genetic data from the Canadian Arctic revealed a highly significant correlation. Genetic data generally supported existing population (management unit) designations, although there were two cases where genetic data failed to differentiate between pairs of populations previously resolved by movement data. A sharp contrast was found between the minimal genetic structure observed among populations surrounding the polar basin and the presence of several marked genetic discontinuities in the Canadian Arctic. The discontinuities in the Canadian Arctic caused the appearance of four genetic clusters of polar bear populations. These clusters vary in total estimated population size from 100 to over 10 000, and the smallest may merit a relatively conservative management strategy in consideration of its apparent isolation. We suggest that the observed pattern of genetic discontinuities has developed in response to differences in the seasonal distribution and pattern of sea ice habitat and the effects of these differences on the distribution and abundance of seals.

Modeling the Sustainable Harvest of Female Polar Bears

We explored ttir boundaries of sustainable harvest of polar bears (Ursus maritimus) by considering a range of values for population parameters in a discrete, age specific model structured to mimic polar bear life history. Survival rate of adult females is the predominant factor affecting population growth rate and sustainable harvest of polar bears although other factors may also be significant; e.g.. cub survival, litter size, and age of 1st reproduction. The parameter of least imimrtance is litter production rate. Deferred reproduction has a small effect 011 population growth rate. ’fliese findings are consistent with theoretical predictions for populations experiencing density indeimideiit mortality mainly restricted to juveniles. ‘rhe critical issue. when considering the long-term effect of any harvest, is the effect on numbers of breeding females. Under optimal conditions tlw siistainalde yield of adult female polar bears is typically <1.6% of the total popillation. J. WILDL. MANAGE. 51(4):811-820 W e use a model to explore the constraints that polar bear life history s t ructure places on sustainable rates of harvest. W e develop a mathematical description of polar bear life history, including harvest as a separate source of mortality. By simulating several harvest types, t h e model yields information on t h e effects of harvest types and o n the sensitivity of the harvest to changes in vital rates (parameter values). Additionally, a “best case” scenario can be developed using maximum values for survival a n d reproduction. Funding for this work was provided by the Norsk Polarinstitutt, Dep. Renewable Resour., Gov. Northwest Territ. , the Univ. British Columbia, a n d t h e Mich. State Agric. Exp. Stn. W e thank S. C. Amstrup, M. C. S. Kingsley, T. Larsen, L. J. Lee, j. W . Lentfer, P. E. Mills, N. A. Oritsland, M. A. Ramsay, I. G. Stirling, a n d K. Ugland for their comments a n d for discussions 812 POLAII BEAR H A I I V E S I ~ Taylor et al. J . Wildl. Manage 51(4):1987 that led to this work. M. Bromley, N. J . Lunn, and J . S. Pin edited the text. C;. L. Locke arid J . E. Troje prepared the figures. l’he life history pattern of polar bears is typical of species in which environrnerital fluctuations strongly affect recruitment rate antl survival of the young (tlairston et al. 1970; Schaffer 1974a,b; Stearns 1977; llorn 1978; Goodman 1979, 1981). Polar bears are a long-lived, late maturing species with a low rate of annual recruitment (Dehbster and Stirling 1981). Polar bears exhibit “birth pulse” (Cauglilry 1977) reproduction. Typically, a small fraction of polar bear females breed for the 1st time at age 3, and slightly more begin breeding at age 4. Generally all females breed at adult rates from age 5 onwards (initial age = 0). However. age specific litter production rates vary according to environmental conditions (Stirling et al. 1975, 1977, 1978). During the 1st 2 years following birth, cubs remain with the female and she is unavailable for breeding. Some females with cubs lose their litters and become available for breeding at the next season. Females with 2-year-old cubs are ready for breeding because virtually all cubs are weaned at 2.5 years (Stirling et al. 1975, DeMaster and Stirling 1981). In arly given year, however, 30-60% of the available adult females do not breed or are not impregnated (Lentfer et al. 1980; I . Stirling, pers. commun.) The breeding season for polar bears is from early spring to early summer. Cubs are born in late December or January (L0ne 1970, Lentfer 1976) antl are called cubs-of-the-year or COY’S to distinguish them from older cubs. Data from captive polar bears suggest that, typically, 2 young are born (Kostyan 1954). However, because of intrauterine and den mortality, the average litter size of adult females ranges from 1.58 to 1.87 (L0n

Population delineation of polar bears using satellite collar data

To produce reliable estimates of the size or vital rates of a given population, it is important that the boundaries of the population under study are clearly defined. This is particularly critical for large, migratory animals where levels of sustainable harvest are based on these estimates, and where small errors may have serious long-term consequences for the population. Once populations are delineated, rates of exchange between adjacent populations can be determined and accounted/corrected for when calculating abundance (e.g., based on mark-recapture data). Using satellite radio-collar locations for polar bears in the western Canadian Arctic, we illustrate one approach to delineating wildlife populations that integrates cluster analysis methods for determining group membership with home range plotting procedures to define spatial utilization. This approach is flexible with respect to the specific procedures used and provides an objective and quantitative basis for defining population boundaries. See ful...

INFLUENCE OF SEA ICE DYNAMICS ON HABITAT SELECTION BY POLAR BEARS

Polar bears live in high-latitude environments characterized by cyclic variation in form and extent of sea ice. From 1991 to 1995, we used radio telemetry and monthly satellite images to compare patterns of ice selection by 110 female polar bears, relative to two geographic regions and four seasons. We hypothesized that extreme seasonal changes in ice characteristics in the Baffin Bay region, including a period of open water, may limit polar bear density despite supporting greater prey density than the Archipelago region, where ice is present year-round. Using cyclic time series analysis to model seasonal variation, we found differences in level, amplitude, and phase between sea ice characteristics and habitat selection by polar bears of the Arctic Archipelago and Baffin Bay regions. Polar bears not only followed seasonal changes, but they anticipated seasonal fluctuations, e.g., polar bears were found close to ice edges in spring in advance of the peak availability of edges. Also, seasonal selection of sea ice by polar bears was generally of a larger amplitude than cycles in ice and is best explained by intensive use of specific ice types in spring and summer, and sparse use during the remaining year. During spring and summer, Archipelago bears used landfast ice more intensively, whereas Baffin bears used moving ice, defined as thick first-year ice found in large floes. Both ice types likely represent areas where most seal pupping occurred in spring for each region. Bears from both regions selected first-year ice in winter when new ice was forming and multiyear ice in autumn when maximum ice melt had occurred. Overall, polar bear selection of ice habitat was similar between regions despite major differences in seasonal ice characteristics. Polar bear density may not directly relate to prey density, due to the limited ability of bears to track the extreme seasonal fluctuations in ice extent found in more productive environments.

Modelling the mating system of polar bears: a mechanistic approach to the Allee effect

Allee effects may render exploited animal populations extinction prone, but empirical data are often lacking to describe the circumstances leading to an Allee effect. Arbitrary assumptions regarding Allee effects could lead to erroneous management decisions so that predictive modelling approaches are needed that identify the circumstances leading to an Allee effect before such a scenario occurs. We present a predictive approach of Allee effects for polar bears where low population densities, an unpredictable habitat and harvest-depleted male populations result in infrequent mating encounters. We develop a mechanistic model for the polar bear mating system that predicts the proportion of fertilized females at the end of the mating season given population density and operational sex ratio. The model is parametrized using pairing data from Lancaster Sound, Canada, and describes the observed pairing dynamics well. Female mating success is shown to be a nonlinear function of the operational sex ratio, so that a sudden and rapid reproductive collapse could occur if males are severely depleted. The operational sex ratio where an Allee effect is expected is dependent on population density. We focus on the prediction of Allee effects in polar bears but our approach is also applicable to other species.

A tale of two polar bear populations: Ice habitat, harvest, and body condition

One of the primary mechanisms by which sea ice loss is expected to affect polar bears is via reduced body condition and growth resulting from reduced access to prey. To date, negative effects of sea ice loss have been documented for two of 19 recognized populations. Effects of sea ice loss on other polar bear populations that differ in harvest rate, population density, and/or feeding ecology have been assumed, but empirical support, especially quantitative data on population size, demography, and/or body condition spanning two or more decades, have been lacking. We examined trends in body condition metrics of captured bears and relationships with summertime ice concentration between 1977 and 2010 for the Baffin Bay (BB) and Davis Strait (DS) polar bear populations. Polar bears in these regions occupy areas with annual sea ice that has decreased markedly starting in the 1990s. Despite differences in harvest rate, population density, sea ice concentration, and prey base, polar bears in both populations exhibited positive relationships between body condition and summertime sea ice cover during the recent period of sea ice decline. Furthermore, females and cubs exhibited relationships with sea ice that were not apparent during the earlier period (1977–1990s) when sea ice loss did not occur. We suggest that declining body condition in BB may be a result of recent declines in sea ice habitat. In DS, high population density and/or sea ice loss, may be responsible for the declines in body condition.

Implications of the Circumpolar Genetic Structure of Polar Bears for Their Conservation in a Rapidly Warming Arctic

We provide an expansive analysis of polar bear (Ursus maritimus) circumpolar genetic variation during the last two decades of decline in their sea-ice habitat. We sought to evaluate whether their genetic diversity and structure have changed over this period of habitat decline, how their current genetic patterns compare with past patterns, and how genetic demography changed with ancient fluctuations in climate. Characterizing their circumpolar genetic structure using microsatellite data, we defined four clusters that largely correspond to current ecological and oceanographic factors: Eastern Polar Basin, Western Polar Basin, Canadian Archipelago and Southern Canada. We document evidence for recent (ca. last 1–3 generations) directional gene flow from Southern Canada and the Eastern Polar Basin towards the Canadian Archipelago, an area hypothesized to be a future refugium for polar bears as climate-induced habitat decline continues. Our data provide empirical evidence in support of this hypothesis. The direction of current gene flow differs from earlier patterns of gene flow in the Holocene. From analyses of mitochondrial DNA, the Canadian Archipelago cluster and the Barents Sea subpopulation within the Eastern Polar Basin cluster did not show signals of population expansion, suggesting these areas may have served also as past interglacial refugia. Mismatch analyses of mitochondrial DNA data from polar and the paraphyletic brown bear (U. arctos) uncovered offset signals in timing of population expansion between the two species, that are attributed to differential demographic responses to past climate cycling. Mitogenomic structure of polar bears was shallow and developed recently, in contrast to the multiple clades of brown bears. We found no genetic signatures of recent hybridization between the species in our large, circumpolar sample, suggesting that recently observed hybrids represent localized events. Documenting changes in subpopulation connectivity will allow polar nations to proactively adjust conservation actions to continuing decline in sea-ice habitat.

RELATIONSHIPS BETWEEN DENNING OF POLAR BEARS AND CONDITIONS OF SEA ICE

Abstract We examined shelter and maternity dens used by 97 adult female polar bears (Ursus maritimus) in relation to conditions of sea ice. Obligate use of maternity dens for pregnancy, birth, and lactation varied little with latitude or area. In contrast, timing of facultative use of shelter dens switched from autumn in the southern area (<70°N) to winter in the northern area (>75°N). For the southern area, 13 of 16 female polar bears used shelter dens in autumn versus winter (median dates of entry and exit, 11 September and 2 November; total = 56 days), whereas in the northern area, 11 of 17 bears used shelter dens in winter versus autumn (median dates, 24 December and 2 March; total = 65 days). Difference in facultative use of shelter dens was associated with conditions of sea ice. Southern regions had no sea ice when polar bears used shelter dens. In contrast, northern areas had more constant ice conditions that included presence of ice throughout the year. Southern regions seem to have greater primary productivity and more seals as a result of a pronounced seasonal cycle of annual ice. Polar bears in northern areas responded to the more constant ice conditions and less productive environment with use of shelter dens during the period with lowest seal accessibility.

Sex-selective harvesting of polar bears Ursus maritimus

Abstract We explored limits and consequences of male-biased harvesting of polar bears Ursus maritimus using a simulated population based on empirically-derived estimates of age-specific rates of survival and reproduction. The maximum sustainable yield (MSY) was identified as the total kill in which the number of females that could be taken resulted in ≤5% change in females older than 50 years. MSY depended on the proportion of males in the harvest, although the effect of male selection on the post-harvest population was to reduce the mean age and number of males. A practical limit to the increase in MSY possible from male-selective harvesting was identified at the 3 : 1 (M/F) sex ratio. At 3 : 1 (M/F), all males were eventually harvested as 2-year olds, and males were reduced to 25% of pre-harvest levels. A more conservative harvest strategy of 2 : 1 (M/F) resulted in a 30% reduction of males and a reduction of the mean age of males from 10.0 to 7.7 years post-harvest. We thus recommend that sex-selective harvesting of polar bears do not exceed 67% males (i.e. a harvest ratio of 2 : 1), a demonstrably safe and sustainable harvest strategy, to avoid depletion of males and possibly reduce recruitment by having too few sexually mature males in the population. When females are harvested below MSY, then harvest strategies that select for males at rates > 2 : 1 (M/F) can be conservative because the increase in females also increases the reproductive performance of the population. In the absence of information on density effects, managers should be conservative in their expectations of increases in the female population.

Demographic and traditional knowledge perspectives on the current status of Canadian polar bear subpopulations

Abstract Subpopulation growth rates and the probability of decline at current harvest levels were determined for 13 subpopulations of polar bears (Ursus maritimus) that are within or shared with Canada based on mark–recapture estimates of population numbers and vital rates, and harvest statistics using population viability analyses (PVA). Aboriginal traditional ecological knowledge (TEK) on subpopulation trend agreed with the seven stable/increasing results and one of the declining results, but disagreed with PVA status of five other declining subpopulations. The decline in the Baffin Bay subpopulation appeared to be due to over‐reporting of harvested numbers from outside Canada. The remaining four disputed subpopulations (Southern Beaufort Sea, Northern Beaufort Sea, Southern Hudson Bay, and Western Hudson Bay) were all incompletely mark–recapture (M‐R) sampled, which may have biased their survival and subpopulation estimates. Three of the four incompletely sampled subpopulations were PVA identified as nonviable (i.e., declining even with zero harvest mortality). TEK disagreement was nonrandom with respect to M‐R sampling protocols. Cluster analysis also grouped subpopulations with ambiguous demographic and harvest rate estimates separately from those with apparently reliable demographic estimates based on PVA probability of decline and unharvested subpopulation growth rate criteria. We suggest that the correspondence between TEK and scientific results can be used to improve the reliability of information on natural systems and thus improve resource management. Considering both TEK and scientific information, we suggest that the current status of Canadian polar bear subpopulations in 2013 was 12 stable/increasing and one declining (Kane Basin). We do not find support for the perspective that polar bears within or shared with Canada are currently in any sort of climate crisis. We suggest that monitoring the impacts of climate change (including sea ice decline) on polar bear subpopulations should be continued and enhanced and that adaptive management practices are warranted.