Gene flow on ice: the role of sea ice and whaling in shaping Holarctic genetic diversity and population differentiation in bowhead whales (Balaena mysticetus)

Sea ice is believed to be a major factor shaping gene flow for polar marine organisms, but it remains unclear to what extent it represents a true barrier to dispersal for arctic cetaceans. Bowhead whales are highly adapted to polar sea ice and were targeted by commercial whalers throughout Arctic and subarctic seas for at least four centuries, resulting in severe reductions in most areas. Both changing ice conditions and reductions due to whaling may have affected geographic distribution and genetic diversity throughout their range, but little is known about range‐wide genetic structure or whether it differed in the past. This study represents the first examination of genetic diversity and differentiation across all five putative stocks, including Baffin Bay‐Davis Strait, Hudson Bay‐Foxe Basin, Bering‐Beaufort‐Chukchi, Okhotsk, and Spitsbergen. We also utilized ancient specimens from Prince Regent Inlet (PRI) in the Canadian Arctic and compared them with modern stocks. Results from analysis of molecular variance and demographic simulations are consistent with recent and high gene flow between Atlantic and Pacific stocks in the recent past. Significant genetic differences between ancient and modern populations suggest PRI harbored unique maternal lineages in the past that have been recently lost, possibly due to loss of habitat during the Little Ice Age and/or whaling. Unexpectedly, samples from this location show a closer genetic relationship with modern Pacific stocks than Atlantic, supporting high gene flow between the central Canadian Arctic and Beaufort Sea over the past millennium despite extremely heavy ice cover over much of this period.     

The impact of past climate change on genetic variation and population connectivity in the Icelandic arctic fox

Previous studies have suggested that the presence of sea ice is an important factor in facilitating migration and determining the degree of genetic isolation among contemporary arctic fox populations. Because the extent of sea ice is dependent upon global temperatures, periods of significant cooling would have had a major impact on fox population connectivity and genetic variation. We tested this hypothesis by extracting and sequencing mitochondrial control region sequences from 17 arctic foxes excavated from two late-ninth-century to twelfth-century AD archaeological sites in northeast Iceland, both of which predate the Little Ice Age (approx. sixteenth to nineteenth century). Despite the fact that five haplotypes have been observed in modern Icelandic foxes, a single haplotype was shared among all of the ancient individuals. Results from simulations within an approximate Bayesian computation framework suggest that the rapid increase in Icelandic arctic fox haplotype diversity can only be explained by sea-ice-mediated fox immigration facilitated by the Little Ice Age.     

Late quaternary variations in oxygen and carbon isotopic compositions in Canadian Arctic marine bivalves

The oxygen and carbon isotopic composition of arctic marine bivalves Mya truncata, Hiatella arctica, and Mytilus edulis are reported on samples from raised marine deposits in Hudson Bay and eastern Baffin Island. The shells range in age from modern, through the Holocene, to “old” marine units. During the Holocene the ¹⁸O/¹⁶O ratio in shells rose to a maximum about 3,500 B.P. which coincides in time with the period of maximum growth rates of bivalves, maximum size and maximum faunal diversity. The change is interpreted to indicate that about 3,500 years ago arctic waters may have reached a salinity ≈ 1–2% greater than present. Comparison of Holocene shell-carbonate isotopic compositions with those from the “old” marine shells (that are characteristically extremely thick) suggests that during the early Wisconsin advance on eastern Baffin Island, surface and near-surface waters were more saline than at present. This may have been related to low meltwater discharge. Paradoxically, positive values of ¹⁸O/¹⁶O and ¹³O/¹²C in marine shells occurred during the Holocene marine optimum and during the early Wisconsin ice advance.     

The great Cenozoic ice sheet

Ocean-floor sediments contain a record of Quaternary continental glaciations in which global cooling prevails over a 100,000 year cycle and the intensity of the last glacial maximum is as strong as any of the earlier glaciations. Yet, the areal extent of Quaternary glacial drift in North America and Eurasia during the last glaciation is much less extensive than during several earlier glaciations. In an attempt to explore this problem, two versions of a Great Cenozoic Ice Sheet were reconstructed. The minimum version transformed a frozen bed beneath high interior ice domes and along southern margins during the last glaciation into a thawed bed during more extensive earlier glaciations. This allowed thinner ice to cover a larger area with little change in ice volume, thereby satisfying both the continental and oceanic records of Quaternary glaciation. The maximum version extended frozen basal conditions beneath high interior ice domes and along southern margins, so that a greater areal extent could only be accommodated by thicker ice. A greater ice volume is required in this case, and would seem to put the Great Cenozoic Ice Sheet in the late Tertiary, where some evidence for global cooling on a 400,000 year cycle exists and might provide the longer timespan needed to produce a larger ice sheet.     

The evolution and distribution of Calyptozetes sarekensis (Acari: Oribatei)

Calyptozetes sarekensis , an oribatid mite found predominantly, though not exclusively in lichens, inhabits regions within the arctic circle and subarctically between latitudes 60^o and 70^o (Swedish Lapland, Norway, Bear Island, Spitzbergen, Jan Mayen, Iceland, Greenland, northern Canada). In recent years, however, the species has been recorded from a number of montane sites in Great Britain and southern Europe. There would appear to be only two possible explanations for die discontinuous distribution of this mite. (1) The species has been transported from its main arctic and subarctic area of distribution to its southern montane outposts by some agent (driftwood, birds, wind), or (2) in Britain and southern Europe it represents a relict species of the Ice Age, left stranded on high ground after the final retreat of the ice. Evidence is presented in support of the second hypothesis. In addition, it is suggested that its presence on the different land masses of its main arctic and subarctic range, which are separated from each other by wide stretches of ocean, could be explained by die hypothesis that it is a very ancient species daring from the time Laurasia was in existence some 60 or more million years ago. The alternative hypothesis of transportation of the species between one or more of these areas, however, is by no means ruled out.     

Behavioral and Physiological Characteristics of the Antarctic Krill, Euphausia superba

The antarctic krill, Euphausia superba, is considered a successin the intensely seasonal environment of the Southern Oceanbecause of its abundance and central role as an important fooditem for many of the larger carnivores in the ecosystem. Thebehavioral and physiological characteristics that foster thissuccess are: (1) the ability to find concentrations of foodin several types of habitat and efficiently exploit whateverfood is available; (2) the close correspondence of the lifecycle with seasonal cycles of food availability; and (3) a combinationof physiological mechanisms that enable krill to survive thelong winter period of low food availability. We evaluated therelative importance of the following four major winter-overmechanisms that have been proposed for adult krill west of theAntarctic Peninsula. The three-fold reduction in metabolic rateis the most important winter-over mechanism for these adults,although lipid utilization and shrinkage also help satisfy energyrequirements in the winter. Alternate food sources did not appearto contribute significantly as a winter energy source. However,the extent, predictability and complexity of the ice cover ina region during winter may have a great influence on the relativeimportance of these winter-over mechanisms for different populations.Ice cover in the waters west of the Antarctic Peninsula is unpredictableand smooth surfaced when it occurs, providing the krill withlittle refuge from predation. In multi-year pack ice of theWeddell Sea, however, ice cover is predictable and extensive,and there is a complex undersurface that provides hiding places.In this multi-year ice, adult krill have been observed underthe ice feeding, whereas west of the Antarctic Peninsula mostadult krill are in the water column in the winter and are notfeeding. The balance between acquiring energy and avoiding predationmay be different in these two regions in the winter becauseof differences in predictability and complexity of the ice cover.     

Vegetation establishment, succession and microsite frost disturbance on glacier forelands within patterned ground chronosequences

Aim The impact of microscale frost disturbance on vegetation colonization and successionary trends was examined within patterned ground features of Little Ice Age chronosequences. The goal was to investigate and compare vegetation response to micro‐site frost disturbance with that of previous studies done at a coarser landscape scale. Location The study sites occur on Little Ice Age glacier forelands within Jotunheimen, Norway (61°–62° N). The forelands of the glaciers Slettmarkbreen, Styggedalsbreen and Vestre Memurubreen have been well studied providing chronological controls for landscape studies. Sorted patterned ground features are found within the chronosequences, typically declining with frost intensity and disturbance with increasing terrain age. Methods Micro‐plots (8.3 × 8.3 cm) were placed at the inner borders and centres of patterned ground features. Species were identified and per cent species cover and per cent cover of life‐form category were noted. Nonparametric Kruskal–Wallis and Mann–Whitney U‐tests were used to test for differences between percent cover of life‐form categories within patterned ground features as well as to identify thresholds of successional change across the chronosequences. Results Significant relationships between life‐from groups and patterned ground positions of varying ages were deduced using nonparametric statistics. Findings were then used to discuss trends of succession within patterned ground features and across the chronosequences. Vegetation establishment occurs at the border positions of young (< 30 years) patterned ground features. With time and distance from the ice margin, vegetation encroaches inwards toward the disturbed centres. Succession within patterned ground exhibits several stages: (1) bryophytes/crusts and lichens, (2) grasses/sedges and (3) woody shrubs. The occurrence of forbs was sporadic and generally non‐significant. Main conclusions Frost disturbance in patterned ground appears to delay successional trends of vegetation communities when compared with previous studies on ‘stable’ terrain, producing micro‐site lag effects. These small patches of disturbed ground are therefore important regarding vegetation assemblages across the landscape and are unlikely to be detected at the landscape scale.     

Terrestrial and aquatic responses to climate change and human impact on the southeastern Tibetan Plateau during the past two centuries

Rapid population growth and economic development have led to increased anthropogenic pressures on the Tibetan Plateau, causing significant land cover changes with potentially severe ecological consequences. To assess whether or not these pressures are also affecting the remote montane‐boreal lakes on the SE Tibetan Plateau, fossil pollen and diatom data from two lakes were synthesized. The interplay of aquatic and terrestrial ecosystem response was explored in respect to climate variability and human activity over the past 200 years. Nonmetric multidimensional scaling and Procrustes rotation analysis were undertaken to determine whether pollen and diatom responses in each lake were similar and synchronous. Detrended canonical correspondence analysis was used to develop quantitative estimates of compositional species turnover. Despite instrumental evidence of significant climatic warming on the southeastern Plateau, the pollen and diatom records indicate very stable species composition throughout their profiles and show only very subtle responses to environmental changes over the past 200 years. The compositional species turnover (0.36–0.94 SD) is relatively low in comparison to the species reorganizations known from the periods during the mid‐ and early‐Holocene (0.64–1.61 SD) on the SE Plateau, and also in comparison to turnover rates of sediment records from climate‐sensitive regions in the circum arctic. Our results indicate that climatically induced ecological thresholds are not yet crossed, but that human activity has an increasing influence, particularly on the terrestrial ecosystem in our study area. Synergistic processes of post‐Little Ice Age warming, 20th century climate warming and extensive reforestations since the 19th century have initiated a change from natural oak‐pine forests to seminatural, likely less resilient pine‐oak forests. Further warming and anthropogenic disturbances would possibly exceed the ecological threshold of these ecosystems and lead to severe ecological consequences.     

Satellite tracking of a killer whale (Orcinus orca) in the eastern Canadian Arctic documents ice avoidance and rapid, long-distance movement into the North Atlantic

Killer whales (Orcinus orca) occur in the eastern Canadian Arctic during the open-water season, but their seasonal movements in Arctic waters and overall distribution are poorly understood. During August 2009, satellite transmitters were deployed onto two killer whales in Admiralty Inlet, Baffin Island, Canada. A whale tracked for 90 days remained in Admiralty and Prince Regent Inlets from mid-August until early October, when locations overlapped aggregations of marine mammal prey species. While in Admiralty and Prince Regent Inlets, the whale traveled 96.1 ± 45.3 km day⁻¹ (max 162.6 km day⁻¹) and 120.1 ± 44.5 km day⁻¹ (max 192.7 km day⁻¹), respectively. Increasing ice cover in Prince Regent Inlet in late September and early October was avoided, and the whale left the region prior to heavy ice formation. The whale traveled an average of 159.4 ± 44.8 km day⁻¹ (max 252.0 km day⁻¹) along the east coast of Baffin Island and into the open North Atlantic by mid-November, covering over 5,400 km in approximately one month. This research marks the first time satellite telemetry has been used to study killer whale movements in the eastern Canadian Arctic and documents long-distance movement rarely observed in this species.     

Distribution and population structure of the anther smut Microbotryum silenes‐acaulis parasitizing an arctic–alpine plant

Cold‐adapted organisms with current arctic–alpine distributions have persisted during the last glaciation in multiple ice‐free refugia, leaving footprints in their population structure that contrast with temperate plants and animals. However, pathogens that live within hosts having arctic–alpine distributions have been little studied. Here, we therefore investigated the geographical range and population structure of a fungus parasitizing an arctic–alpine plant. A total of 1437 herbarium specimens of the plant Silene acaulis were examined, and the anther smut pathogen Microbotryum silenes‐acaulis was present throughout the host's geographical range. There was significantly greater incidence of anther smut disease in more northern latitudes and where the host locations were less dense, indicating a major influence of environmental factors and/or host demographic structure on the pathogen distribution. Genetic analyses with seven microsatellite markers on recent collections of 195 M. silenes‐acaulis individuals revealed three main genetic clusters, in North America, northern Europe and southern Europe, likely corresponding to differentiation in distinct refugia during the last glaciation. The lower genetic diversity in northern Europe indicates postglacial recolonization northwards from southern refugia. This study combining herbarium surveys and population genetics thus uniquely reveals the effects of climate and environmental factors on a plant pathogen species with an arctic–alpine distribution.     

The biogeography of aquatic macrophytes in North America since the Last Glacial Maximum

Aim To document the post‐glacial migration of the major aquatic macrophytes of North America. Location North America north of Mexico. Methods Aquatic macrophyte pollen were extracted from the North American Pollen Database. The modern pollen distribution was mapped and related to the climate to document the geographical and climatic constraints on these taxa. The fossil pollen were mapped at 2‐ka intervals for the past 21 ka. Results Numerous genera were present in ice‐free Alaska during the Last Glacial Maximum, and south of the Laurentide Ice Sheet in the southeast. Those taxa with the widest modern climatic ranges migrated rapidly into ice‐marginal areas, first in the west and then in the east of North America. Subsequent changes in the range and abundance were smaller. Main conclusions There were four migration routes of aquatic macrophytes during the late‐glacial and post‐glacial periods: a southward migration from Alaska between 14–13 and ka, a northern migration in the west at the same time into the ice‐free Cordilleran region, and movements east and west of Appalachia as early as 19 ka for some taxa into the lower Mississippi and into the upper Mississippi and Great Lakes by 11 ka. As the Laurentide ice sheet wasted, aquatic taxa with the broadest contemporary temperature tolerances rapidly occupied ice‐marginal environments.     

Natal den selection by sympatric arctic and red foxes on Herschel Island,Yukon, Canada

In the twentieth century, red fox (Vulpes vulpes) expanded into the Canadian Arctic, where it competes with arctic fox (Vulpes lagopus) for food and shelter. Red fox dominates in physical interactions with the smaller arctic fox, but little is known about competition between them on the tundra. On Hershel Island, north Yukon, where these foxes are sympatric, we focused on natal den choice, a critical aspect of habitat selection. We tested the hypothesis that red fox displaces arctic fox from dens in prey-rich habitats. We applied an approach based on model comparisons to analyse a 10-year data set and identify factors important to den selection. Red fox selected dens in habitats that were more prey-rich in spring. When red foxes reproduced, arctic fox selected dens with good springtime access, notably many burrows unblocked by ice and snow. These provided the best refuge early in the reproductive season. In the absence of red foxes, arctic foxes selected dens offering good shelter (i.e. large isolated dens). Proximity to prey-rich habitats was consistently less important than the physical aspects of dens for arctic fox. Our study shows for the first time that red foxes in the tundra select dens associated primarily with prey-rich areas, while sympatric arctic foxes do not. These results fit a model of red fox competitively interfering with arctic fox, the first detailed study of such competition in a true arctic setting.     

Molecular phylogeography and hybridization in members of the circumpolar Potentilla sect. Niveae (Rosaceae)

Glacial events and the formation of ice-free areas serving as refugia for plants and animals are important in shaping present patterns of genetic diversity in arctic areas. Beringia, situated in northeastern Russia and Alaska, has been pointed out as a major refugium. This study focuses on the historical biogeography of the circumpolar taxon Potentilla sect. Niveae. The taxonomy of the group is complex, most likely highly influenced by hybridization and apomixis. cpDNA microsatellites together with AFLP fragments were used to map the genetic variability in the section, from Beringia across the Canadian Arctic to Greenland. The data support the hypothesis that Beringia, as well as parts of adjacent arctic Canada, served as refugia during the Wisconsinan glaciation, and there is some evidence for a northern and a southern migration route out of Beringia. The hair type groups within sect. Niveae are more or less genetically distinct, and hybridization, especially with sect. Multifida, takes place. Haplotype diversity as well as frequency is at its maximum close to the Last Glacial Maximum ice cap edge. This pattern can be explained by merging of previously isolated refugia, by repeated extinction/colonization events close to the ice edge, and by hybridization among sympatric taxonomical lineages.     

Environmental Predictors of Ice Seal Presence in the Bering Sea

Ice seals overwintering in the Bering Sea are challenged with foraging, finding mates, and maintaining breathing holes in a dark and ice covered environment. Due to the difficulty of studying these species in their natural environment, very little is known about how the seals navigate under ice. Here we identify specific environmental parameters, including components of the ambient background sound, that are predictive of ice seal presence in the Bering Sea. Multi-year mooring deployments provided synoptic time series of acoustic and oceanographic parameters from which environmental parameters predictive of species presence were identified through a series of mixed models. Ice cover and 10 kHz sound level were significant predictors of seal presence, with 40 kHz sound and prey presence (combined with ice cover) as potential predictors as well. Ice seal presence showed a strong positive correlation with ice cover and a negative association with 10 kHz environmental sound. On average, there was a 20–30 dB difference between sound levels during solid ice conditions compared to open water or melting conditions, providing a salient acoustic gradient between open water and solid ice conditions by which ice seals could orient. By constantly assessing the acoustic environment associated with the seasonal ice movement in the Bering Sea, it is possible that ice seals could utilize aspects of the soundscape to gauge their safe distance to open water or the ice edge by orienting in the direction of higher sound levels indicative of open water, especially in the frequency range above 1 kHz. In rapidly changing Arctic and sub-Arctic environments, the seasonal ice conditions and soundscapes are likely to change which may impact the ability of animals using ice presence and cues to successfully function during the winter breeding season.     

Origins of the rabies viruses associated with an outbreak in Newfoundland during 2002-2003

After being free of rabies of terrestrial mammals since 1988, an outbreak of rabies occurred on the Island of Newfoundland in December 2002 and continued into the middle of 2003. Twenty-one cases, all due to the arctic fox strain of rabies virus, were reported. To explore the immediate origins of this outbreak, viruses from the Newfoundland epizootic were genetically compared to two other rabies viruses recovered in mid-2002 from Cartwright, a mainland coastal community near the Island. While all Island isolates from the 2002-03 outbreak were genetically very similar, consistent with a single introduction from the mainland, they were phylogenetically quite distant from the two samples from Cartwright. A broader-based study examined the relationships between the Island viruses and arctic fox strain viruses originating from across Canada over a period of 14 yr. This analysis indicated that the Newfoundland outbreak viruses were most similar to a variant identified in Labrador in 2004 but also widely distributed in northern Quebec both before and after the Newfoundland incursion. The eastern coastline of mainland Labrador has harbored a particularly large number of variants during the study period, some of which have not been detected elsewhere. A small number of Greenland isolates included in this study were dispersed within the clades of the Canadian samples rather than forming a discrete cluster, an observation that may underline the relative ease of movement of the rabies arctic lineage between these two countries as a result of animal movements over pack ice.     

Tree-line dynamics in Ungava peninsula,northern Quebec

The characteristics of the present tree-line in continental Northern Quebec appear to be related to modern and past ecological conditions. ¹⁴C dating of fossil trees and charcoal, and age structure of tree populations are used as evidence of tree-line dynamics over the last 400 yr. As inferred from the reproductive strategies of the two tree species, black spruce Picea mariana (Mill.) BSP, and Tamarack Larix laricina (DuRoi) K. Koch, forming the present tree-line populations, major changes during postglacial time in Northern Quebec are not only concerned with tree-line position, but also with tree-line composition. The present tree-line is made by larch (or tamarack) and corresponds to its natural seed regeneration limit. Black spruce probably reached its northernmost holocene latitudes during a warmer period and it then formed the tree-line. During the Little Ice Age, fires destroyed large tracks of forest and black spruce krummholz (400, and 250–200 yr ago); afterwards, tree species reestablished only in protected sites, and exposed sites were not reseeded. At some places, fires did not burn completely the forest cover, and preserved isolated trees and small groves probably became locus of reafforestation. Differential forest regeneration and tree age structure suggest that fire and climate are intimately associated in controlling tree population dynamics. Favourable and less favourable forest regeneration periods are inferred from these data and it is suggested that tree-line displacements in Northern Quebec during the last 400 yr were less important than in the Northwest Territories.     

Food web changes in arctic ecosystems related to climate warming

Sedimentary records from three Canadian High Arctic ponds on Ellesmere Island, spanning the last several thousand years, show major shifts in pond communities within the last ～200 years. These paleolimnological data indicate that aquatic insect (Diptera: Chironomidae) populations rapidly expanded and greatly increased in community diversity beginning in the 19th century. These invertebrate changes coincided with striking shifts in algal (diatom) populations, indicating strong food‐web effects because of climate warming and reduced ice‐cover in ponds. Predicted future warming in the Arctic may produce ecological changes that exceed the large shifts that have already occurred since the 19th century.     

From ‘third pole’ to north pole: a Himalayan origin for the arctic fox

The ‘third pole’ of the world is a fitting metaphor for the Himalayan–Tibetan Plateau, in allusion to its vast frozen terrain, rivalling the Arctic and Antarctic, at high altitude but low latitude. Living Tibetan and arctic mammals share adaptations to freezing temperatures such as long and thick winter fur in arctic muskox and Tibetan yak, and for carnivorans, a more predatory niche. Here, we report, to our knowledge, the first evolutionary link between an Early Pliocene (3.60–5.08 Myr ago) fox, Vulpes qiuzhudingi new species, from the Himalaya (Zanda Basin) and Kunlun Mountain (Kunlun Pass Basin) and the modern arctic fox Vulpes lagopus in the polar region. A highly hypercarnivorous dentition of the new fox bears a striking resemblance to that of V. lagopus and substantially predates the previous oldest records of the arctic fox by 3–4 Myr. The low latitude, high-altitude Tibetan Plateau is separated from the nearest modern arctic fox geographical range by at least 2000 km. The apparent connection between an ancestral high-elevation species and its modern polar descendant is consistent with our ‘Out-of-Tibet’ hypothesis postulating that high-altitude Tibet was a training ground for cold-environment adaptations well before the start of the Ice Age.     

Polyploidy in arctic plants

The Arctic is an excellent model system for the study of polyploidy. It is one the Earth's most polyploid‐rich areas, in particular of high‐level and recently evolved polyploids. Here we re‐address previous hypotheses on arctic polyploidy based on a new analysis of the circumarctic flora, and review recent molecular, cytological and reproductive studies. The frequency and level of polyploidy strongly increase northwards within the Arctic. We found no clear‐cut association between polyploidy and the degree of glaciation for the arctic flora as a whole, which contains many widespread species. However, for ‘arctic specialist’ taxa with restricted distributions, the frequency of diploids is much higher in the Beringian area, which remained largely unglaciated during the last ice age, than in the heavily glaciated Atlantic area. This result supports the hypothesis that polyploids are more successful than diploids in colonizing after deglaciation. There is abundant molecular evidence for recurrent formation of arctic polyploids at different scales in time and space. Examples are given of low‐level polyploids formed after the last glaciation and of repeated and successively more high‐level polyploidizations throughout the Quaternary. Recurrent polyploid origins, followed by interbreeding within and across ploidal levels, provide a major explanation for the taxonomic complexity of the arctic flora. In the well‐studied, recently deglaciated archipelago of Svalbard, most species are mainly self‐fertilizing or clonal. All Svalbard polyploids examined so far are genetic allopolyploids with fixed heterozygosity at isozyme loci. The level of heterozygosity in 65 taxa increases dramatically from diploids to high‐level polyploids. In the circumarctic area, there is evidence for numerous recently evolved sibling species within diploid taxonomic species. Rapid evolution of crossing barriers at the diploid level promotes further diversification after expansion from different refugia, and may provide new raw materials for allopolyploid formation. We conclude that the evolutionary success of polyploids in the Arctic may be based on their fixed‐heterozygous genomes, which buffer against inbreeding and genetic drift through periods of dramatic climate change. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 521–536.