Postglacial Dispersal Research Articles

Historical demography and colonization pathways of the widespread intertidal seaweed Hormosira banksii (Phaeophyceae) in southeastern Australia.

The palaeoceanography of southern Australia has been characterized by fluctuating sea levels during glacial periods, changing temperature regimes and modified boundary currents. Previous studies on genetic structuring of species in southeastern Australia have focused mainly on the differentiationof eastern and western populations while thepotential role of Bass Strait as a region of overlap for three biogeographic provinces (Peronia, Maugea, and Flindersia) has been largely ignored. This study aimed to explore the likely roles of historic and contemporary factors in determining divergence patterns in the habitat-forming intertidal seaweed Hormosira banksii in southeastern Australia with a special focus on postglacial dispersal into Bass Strait. We examined the genetic diversity of 475 Hormosira specimens collected from 19 sites around southern Australia using DNA sequence analysis of cytochrome oxidase 1. Three major haplotype groups were identified (western, centre and eastern) corresponding with the three existing biogeographical provinces in this region. Historic break points appeared to be retained and reinforced by modern day dispersal barriers. Phylogeographic grouping of Hormosira reflected a combination of historic and contemporary oceanography. As western and eastern group haplotypes were largely absent within Bass Strait, re-colonization after the last glacial maximum appeared to have originated from refuges within or near present day Bass Strait. Patterns of genetic structure for Hormosira are consistent with other marine species in this region and highlight the importance of biogeographical barriers in contributing to modern genetic structure.

Postglacial dispersal patterns and mitochondrial genetic structure of the Pyrenean desman (Galemys pyrenaicus) in the northwestern region of the Iberian Peninsula.

The genetic structure of small semiaquatic animals may be influenced by dispersal across both rivers and land. The relative importance of these two modes of dispersal may vary across different species and with ecological conditions and evolutionary periods. The Pyrenean desman (Galemys pyrenaicus) is an endemic mammal of the Iberian Peninsula with a strong phylogeographic structure and semiaquatic habits, thus making it an ideal model to study the effects of river and overland dispersal on its genetic structure. Thanks to different types of noninvasive samples, we obtained an extensive sampling of the Pyrenean desman from the northwestern region of the Iberian Peninsula and sequenced two mitochondrial DNA fragments. We then analyzed, using an isolation‐by‐distance approach, the correlation between phylogenetic distances and geographical distances measured along both river networks and land to infer the relative importance of river and overland dispersal. We found that the correlations in the whole area and in a large basin were consistent with an effect of overland dispersal, which may be due to the postglacial colonization of new territories using terrestrial corridors and, possibly, a more extensive fluvial network that may have been present during the Holocene. However, in a small basin, likely to be less influenced by the impact of ancient postglacial dispersal, the correlations suggested significant overall effects of both overland and river dispersal, as expected for a semiaquatic mammal. Therefore, different scales and geographical regions reflect different aspects of the evolutionary history and ecology of this semiaquatic species using this isolation‐by‐distance method. The results we obtained may have crucial implications for the conservation of the Pyrenean desman because they reinforce the importance of interbasin dispersal for this species in the studied area and the need to protect the whole riverine ecosystem, including rivers, upland streams and terrestrial corridors between basins.

The Blackwater gold-spessartine-pyrolusite glacial dispersal train, British Columbia, Canada: Influence of sampling depth on indicator mineralogy and geochemistry

A comprehensive program of indicator mineral and geochemical sampling of both oxidized and unoxidized till at the glaciated Blackwater Au-Ag deposit in the interior of south-central British Columbia, Canada has provided key insights into glacial and post-glacial dispersal patterns that can be used to explore other glaciated regions more effectively. The Blackwater deposit is located on the northeastern slope of Mount Davidson and is completely covered by till that was deposited by NE-flowing ice. Only the sulphide-depleted supergene cap of the deposit was exposed to the ice. Indicator mineral sampling of C-horizon till on the mountain slope identified a 1.3 km wide gold grain dispersal train that extends c. 3 km glacially down-ice and 200 vertical metres down-slope from the deposit. The till within this train contains from 10 to 641 gold grains, dominantly of 25 µm size, per 10 kg of its <2 mm matrix. As well, the 0.25 – 0.5 mm fraction contains up to 25 000 grains per sample of spessartine garnet derived from the hypogene alteration envelope of the Blackwater deposit, increasing the probable detectable length of the train to 10 km. Geochemically, in the <0.063 mm fraction of the till, the detectable length of the train is reduced to <1 km, with the best indicator elements being Au and Zn. In drill holes, the host till is oxidized to a depth of 2 – 5 m. It thickens markedly downslope from a few metres to c. 100 m. The gold-spessartine dispersal train thickens sympathetically to nearly 30 m and also ascends progressively within the till section. Gold grain and spessartine levels in the unoxidized till at depth are similar to those in the oxidized till at surface but the unoxidized till also contains up to 60 000 grains per sample of metal-rich pyrolusite derived from supergene cap of the Blackwater deposit.

Phylogenetic Lineages and Postglacial Dispersal Dynamics Characterize the Genetic Structure of the Tick, Ixodes ricinus, in Northwest Europe.

Dispersal and gene flow are important mechanisms affecting the dynamics of vectors and their pathogens. Here, patterns of genetic diversity were analyzed in many North European populations of the tick, Ixodes ricinus. Population sites were selected within and between areas separated by geographical barriers in order to evaluate the importance of tick transportation by birds in producing genetic connectivity across open sea and mountain ranges. The phylogenetic analyses of the mitochondrial control region and the cytochrome b gene revealed two distinct clades with supported sub-clades, with three genetic lineages: GB and WNo associated with Great Britain and western Norway respectively, and Eu with a wider distribution across continental Europe in agreement with much lower efficiency of tick dispersal by birds than by large mammals. The results suggest different ancestry of I. ricinus colonizing Britain and the rest of northern Europe, possibly from different glacial refuges, while ticks from western Norway and continental Europe share a more recent common ancestry. Demographic history modeling suggests a period of strong increase in tick abundance coincident with progression of the European Neolithic culture, long after their post-glacial colonization of NW Europe.

Morphological peculiarities as indicators of natural history of sable (Martes zibellina) in North-Asia Pacific coast

Few studies exist on phenotypic composition of the sable populations in the Russian Far East, China and Japan. We present new data on the morphological diversification of sable for 20 mainland and island populations in an effort to document existing variability and produce baseline data that would be useful in constructing phylogenies for this species. Сlinal variability of skull size, cranial phene, foramen in fossa condyloidei inferior (FFCI), and fur coloration is revealed. One district, Central Sikhote-Alin, is characterized by sables with the minimum cranial size and the maximum phene expression. The area between the Uda River and Goryun and Amgun rivers is inhabited by sables of the darkest fur coloration. Moving out from those areas in all directions we were detected a trend of increasing skull size, reduction of cranial phene, and decreasing color intensity of fur. The skull size of sable is consistent with Bergmann’s rule. Special attention paid to the particular morphological properties of insular populations in specific climatic conditions and under isolation from the continent. They are characterized by minimal expression phene FFCI (Bolshoi Shantar, Iturup, and Hokkaido islands) and larger skull size, especially in sable of Kamchatka Peninsula. It is hypothesized that the area of the lower Amur Basin was a source of sable postglacial dispersal on the Eurasian Continent.

Subterranean termite phylogeography reveals multiple postglacial colonization events in southwestern Europe.

A long‐standing goal of evolutionary biology is to understand how paleoclimatic and geological events shape the geographical distribution and genetic structure within and among species. Using a diverse set of markers (cuticular hydrocarbons, mitochondrial and nuclear gene sequences, microsatellite loci), we studied Reticulitermes grassei and R. banyulensis, two closely related termite species in southwestern Europe. We sought to clarify the current genetic structure of populations that formed following postglacial dispersal from refugia in southern Spain and characterize the gene flow between the two lineages over the last several million years. Each marker type separately provided a fragmented picture of the evolutionary history at different timescales. Chemical analyses of cuticular hydrocarbons and phylogenetic analyses of mitochondrial and nuclear genes showed clear separation between the species, suggesting they diverged following vicariance events in the Late Miocene. However, the presence of intermediate chemical profiles and mtDNA introgression in some Spanish colonies suggests ongoing gene flow. The current genetic structure of Iberian populations is consistent with alternating isolation and dispersal events during Quaternary glacial periods. Analyses of population genetic structure revealed postglacial colonization routes from southern Spain to France, where populations underwent strong genetic bottlenecks after traversing the Pyrenees resulting in parapatric speciation.

Population genetics of Penstemon albomarginatus (Plantaginaceae), a rare Mojave Desert species of conservation concern

Penstemon albomarginatus is a psammophytic endemic of the Mojave Desert, found only in deep sand and dune habitats of San Bernardino County, California, Mohave County, Arizona, and Clark and Nye Counties in Nevada. We used six microsatellite loci to assess genetic differentiation and diversity for 228 individuals across the 12 known populations of this rare species. A slight heterozygote deficiency was found in two populations, but most populations show no signs of inbreeding. Results show a geographic pattern of northern populations being more closely related to one another compared to all other geographic regions. Genetic diversity was greatest in the southern populations, with decreasing amounts of diversity observed with latitude. In general, the geographic pattern of genetic diversity among all populations suggests a post-glacial dispersal from south-to-north. Our results are discussed in the framework of anthropogenic pressures on deep sand habitats of the Mojave Desert.

Interpretation of postglacial recolonization of Formica aquilonia (Hymenoptera, Formicidae) in Fennoscandia according to workers’ color variation

The results of studying the color variation of the red wood ant F. aquilonia in Northwestern Russia and Fennoscandia are presented. Pronounced phenotypic differentiation among the ant populations was observed, which may be interpreted in the light of postglacial dispersal of the species. Ants from Karelia and the environs of Moscow showed high similarity in their color variation. The share of the light morph Pn3 increased in more northern localities, namely the Pasvik Reserve, and also in more western ones, namely St. Petersburg and Espoo. Thus, there were two vectors of ant dispersal: the northern and the western ones, coinciding with the known routes of postglacial recolonization by small mammals. Analysis of color variation is a very promising approach to the study of intraspecific structure and ways of its formation.

Connectivity and vagility determine spatial richness gradients and diversification of freshwater fish in North America and Europe

The latitudinal species richness gradient (LRG) has been the subject of intense interest and many hypotheses but much less consideration has been given to longitudinal richness differences. The effect of postglacial dispersal, determined by connectivity and vagility, on richness was evaluated for the species-poor European and North American Pacific and species-rich Atlantic regional freshwater fish faunas. The numbers of species, by habitat, migration and distributional range categories, were determined from regional species lists for these three realms. The current orientation and past connections of drainage channels indicate that connectivity is greatest in the Atlantic and least in the Pacific. With increasing connectivity across realms, endemism decreased and postglacial recolonization increased, as did the LRG slope, with the greatest richness difference occurring between southern Atlantic and Pacific regions. Recolonizing species tended to be migratory, habitat generalists and from families of marine origin. Diversification, as indicated by species/genus ratios, probability of diversification, taxonomic distinctness and endemicity, declined with increasing latitude in all realms and was least in Europe. Richness patterns are consistent with an LRG driven by the time available for postglacial recolonization and by differences in dispersal ability, with richness differences across realms reflecting differences in dispersal and diversification.

Post-glacial colonization of eastern Europe from the Carpathian refugium: evidence from mitochondrial DNA of the common voleMicrotus arvalis

There is now considerable evidence for the survival of temperate species within glacial refugia that were situated at relatively high latitudes, notably the Carpathian Basin and Dordogne region in Europe. However, the prevalence of fossil remains in such locations is rarely matched by molecular evidence for their contribution to subsequent geographical and demographic expansion of the species in question. One obstacle to this has been insufficient analysis of modern samples from the relevant areas, in particular the parts of eastern Europe that surround the Carpathian refugium. In the present study, we examine the patterns of variation in mitochondrial DNA of the common vole (Microtus arvalis), obtained from existing museum specimens and from newly-collected samples obtained in this area. We show that common voles from one of six extant mitochondrial DNA lineages have colonized most of the species' range in eastern Europe. We contend that the post-glacial dispersal of this lineage most likely originated from the Carpathian refugium, adding support to the argument that such northern refugia made an important contribution to existing genetic diversity in Europe.

Circumpolar phylogeography of the northern pike (Esox lucius) and its relationship to the Amur pike (E. reichertii)

BackgroundFreshwater fishes of the genus Esox are found throughout the Holarctic region. The northern pike (E. lucius) has a circumpolar distribution whereas the assumed sister species the Amur pike (E. reichertii) is only found in the Amur region. The genetic structure and post-glacial dispersal of these species are not well known. Here, we use sequence variation at two mitochondrial DNA regions (cytb and D-loop) to investigate the phylogeography, infer location of glacial refugia and investigate the time of divergence and potential demographic expansion of the various clades detected.ResultsThe two species did not share haplotypes implying long-term isolation with no gene flow, and divergence of the taxa were estimated at 4.55 Myr. The northern pike mtDNA haplotypes revealed three main lineages. One of the northern pike mtDNA lineages was found throughout the entire Holarctic region suggesting transcontinental dispersal from a single refugium. The three lineages exhibited a star phylogeny, indicating population expansion following isolation in separate glacial refugia. Estimated time of divergence of these lineages was 0.18 - 0.26 Myr.ConclusionsThe precise location of the glacial refugia is uncertain, but our data suggests an Asian origin. The expansion of the circumpolar lineage is estimated to be around the end of the second glacial, implying that the current distribution is due to a recent recolonization from an east-Asian refugium. All three northern pike mtDNA lineages occurred sympatrically in Europe, possibly due to secondary contact. Two of the lineages probably originated from different European refugia, one in the Danube-region and one in Western Europe, the latter seems to be the primary source for recolonization of northern Europe.

Morphological and genetic analyses identify a new record of a glacial relict: pygmy whitefish (Prosopium coulterii) from northwestern Ontario

The pygmy whitefish (Prosopium coulterii (Eigenmann and Eigenmann, 1892)) is a glacial relict species with a disjunct North American distribution that, apart from its most easterly known location in Lake Superior, is predominantly found in northern and western regions of Canada. Here we report on a new finding of pygmy whitefish from Winnange Lake in northwestern Ontario that extends the range of this species ∼320 km from its most easterly distribution in Lake Superior and almost 1500 km east of the closest previously known western localities. Genetic analyses confirmed that the fish from Winnange Lake were most closely related to the lineage that includes fish from Lake Superior and likely also originated via postglacial dispersal from a refugium in the upper Mississippi – Missouri river system.

Recent expansion led to the lack of genetic structure of Sargassum aquifolium populations in Southeast Asia

Phylogeographical study of the brown macroalga, Sargassum aquifolium using nuclear internal transcribed spacer 2, plastidal RuBisCo spacer, and mitochondrial cytochrome oxidase subunit-III revealed the populations in Southeast Asia to be homogeneous. On the other hand, genetic differences were detected between populations from Southeast Asia and western Pacific Islands/Guam, suggesting the presence of genetic break between these regions. This further suggests that populations of S. aquifolium may have survived east of Sunda Shelf during the Last Glacial Maximum and recent recolonization led to homogeneity of the populations in the Sunda Shelf region. Recolonization could be facilitated by year-round reproduction of the populations and dispersal of germlings on floating thalli by coastal currents. Restricted current flow across Maluku Sea and directional equatorial current flows could have isolated the Pacific Island and Guam populations from those of Southeast Asia. Our results support the presence of multiple refugia as the source of different lineages of S. aquifolium populations with a lack of secondary contact in the post-glacial dispersal between Southeast Asia and western Pacific as the mechanisms behind the phylogeographical patterns observed.

Population history of the two carnivorous plants Drosera peltata var. nipponica and Drosera rotundifolia (Droseraceae) in Korea

Drosera peltata var. nipponica, an element of the East Asia warm-temperate vegetation, and D. rotundifolia, a widely distributed boreal species, reach one of their northernmost and southernmost limits, respectively, on the Korean Peninsula. Because the Last Glacial Maximum (LGM)-Holocene dynamics of warm-temperate and boreal paleovegetation differed considerably on the Peninsula, the population history of these two sundews is expected to be different, leaving differential imprints in their genetic structure. We investigated population genetic structure of D. peltata var. nipponica and D. rotundifolia in South Korea (10 populations of each for 20 allozyme loci) to infer their population history in this region. In addition, we compared the genetic variation harbored in the two sundews to those reported for other carnivorous and wetland plants. Drosera peltata var. nipponica showed no genetic diversity, whereas D. rotundifolia exhibited extremely low within-population variation (He = 0.005) and considerable among-population divergence (FST = 0.817). Our results suggest that extant populations of D. peltata var. nipponica likely originated from a single ancestral population from southern Japan or southern China through postglacial dispersal. On the contrary, D. rotundifolia probably survived the LGM in situ, with extant populations derived from either one or several small source populations. We argue that separate conservation strategies should be employed, given that the two taxa have different ecological and demographic traits and harbor different levels of genetic diversity.

Bayesian phylogeographic inferences reveal contrasting colonization dynamics among European groundwater isopods

The potentially important role of northern microrefugia during postglacial dispersal is challenging the view of southern Europe as a refuge and source area of European biota. In groundwaters, large geographic ranges of presumably good dispersers are increasingly suspected to consist of assemblages of cryptic species with narrow ranges. Moreover, a large species range, even when confirmed by molecular evidence, tells us little about the spatiotemporal dynamics of dispersal. Here, we used phylogenetic inferences, species delineation methods and Bayesian phylogeographic diffusion models to test for the likelihood of postglacial colonization from distant refugia among five morphospecies of Proasellus (Isopoda, Asellidae). All morphospecies except one were monophyletic, but they comprised a total of 15-17 cryptic species. Three cryptic species retained ranges that spanned a distance >650km, similar to that of the nominal morphospecies. Bayesian diffusion models based on mitochondrial markers revealed considerable spatiotemporal heterogeneity in dispersal rates, suggesting that short-time dispersal windows were instrumental in shaping species ranges. Only one species was found to experience a recent, presumably postglacial, range expansion. The Jura and Alpine foothills probably played a major role in maintaining diversity within Proasellus in northern regions by acting both as diversification hotspots and Pleistocene refugia. Gaining insight into the spatiotemporal heterogeneity of dispersal rates revealed contrasting colonization dynamics among species that were not consistent with a global postglacial colonization of Europe from distant refugia.

A Bayesian approach to conservation genetics of Blanding’s turtle (Emys blandingii) in Ontario, Canada

Blanding's turtle, Emys blandingii, is a globally endangered species with a range centred on the Great Lakes of North America. Several disjunct populations also occur along the East Coast of North America. Previous studies suggest that the Great Lakes portion of the species' range exhibits panmixia. However, E. blandingii is restricted to relatively small populations in many areas around the Great Lakes. Therefore, panmixia across large geographic dis- tances in this area is unlikely. Here, we apply Bayesian analyses of population structure to samples collected across southern Ontario (N = 97) to test a null hypothesis of pan- mixia and assess possible management units (MUs), and to estimate rates of gene flow across the study area. Sampled sites in Ontario represent a minimum of four distinct genetic clusters of E. blandingii, which we recommend should be considered as independent MUs. Preliminary evidence suggests that further structure may be present in less robustly sampled areas, which deserve further consideration. Genetic diversity at sampled sites is comparable to that reported for other freshwater turtles. Our comparison between this study and previous work confirms that genetic diversity in E. blandingii is reduced in disjunct eastern populations com- pared to populations centred on the Great Lakes. Genetic diversity in E. blandingii is not correlated with latitude, and instead may reflect post-glacial dispersal of this species from multiple Pleistocene glacial refugia.

Phylogeography and subspecies status of Black Grouse

The cold periods of the Pleistocene have had a striking impact on the diversification of most organisms in temperate regions. Phylogeographic patterns and postglacial expansion are poorly understood in the Black Grouse (Tetrao tetrix). This species is widely distributed across Eurasia, and has been divided into a number of subspecies on the basis of morphological differences and geographic isolation. To investigate the evolutionary history of the species, 143 samples from different regions were examined for a mtDNA control region fragment. Overall, analyses of mtDNA gave support for the divergence between Tetrao tetrix tetrix, T. t. ussuriensis and T. t. mongolicus. The analyses reveal the effects of colonisation out of glacial refugia on the genetic diversity and genetic structure of Black Grouse. The phylogeographical results are consistent with a demographic population expansion following a bell-shaped mismatch distribution, a star-shaped phylogeny and low nucleotide diversity. Patterns of postglacial dispersal imply that Black Grouse from southern Europe have been restricted to this area, and did not contribute to the genetic diversity of northern Europe. Instead, Black Grouse spread out to northern Europe from a refugium in the east and a possible one in western Europe, following the retreat of glacial ice sheets, although both refugia remain unidentified. We suggest that T. t. britannicus and T. t. viridanus correspond to northern T. t. tetrix, and that this lineage has diverged from the other subspecies. This division is tentative due to limited sampling, but it will facilitate the management of different evolutionary significant units of the species.

Coexistence and origin of trophic ecotypes of pygmy whitefish,Prosopium coulterii, in a south‐westernAlaskan lake

Ecologically, morphologically and genetically distinct populations within single taxa often coexist in postglacial lakes and have provided important model systems with which to investigate ecological and evolutionary processes such as niche partitioning and ecological speciation. Within the Salmonidae, these species complexes have been well studied, particularly within the Coregonus clupeaformis-C. laveratus (lake and European whitefish, respectively) group, but the phenomenon has been less well documented in the other whitefish genera, Prosopium and Stenodus. Here, we examined the morphology, feeding biology and genetic structure of three putative forms of the pygmy whitefish, Prosopium coulterii (Eigenmann & Eigenmann, 1892), first reported from Chignik Lake, south-western Alaska, over 40 years ago. Field collections and morphological analyses resolved a shallow water (< 5 m depth) low gill raker count form (< 15 first arch gill rakers), a deepwater (> 30 m), low gill raker form and a deepwater, high gill raker count (> 15 gill rakers) form. The two low gill raker count forms fed almost exclusively on benthic invertebrates (mostly chironomids), while the deepwater, high gill raker count form fed almost exclusively on zooplankton; differences in diet were also reflected in differences both in δ(13) C and δ(15) N stable isotopes. All three forms were characterized by the same major mitochondrial DNA clade that has been associated with persistence in, and postglacial dispersal from, a Beringian glacial refugium. Analysis of variation at nine microsatellite DNA loci indicated low, but significant differentiation among forms, especially between the two low gill raker count forms and the high gill raker count form. The extent of differentiation along phenotypic (considerable) and genetic (subtle) axes among the Chignik Lake forms is similar to that found among distinct taxa of Prosopium found in pre-glacial Bear Lake (Utah-Idaho, USA) which is probably at least ten times older than Chignik Lake. Our analyses illustrate the potential for the postglacial differentiation in traits subject to divergent natural selection across variable environments.

Allopatric distribution of generalist parasites: interplay between postglacial dispersal and host species

Dispersal of freshwater mussels occurs when the larvae parasitize fishes. In northeastern North America, distributions of floater mussels ( Pyganodon spp.) are allopatric and arranged in an east–west pattern that corresponds poorly with the biogeographic patterns of the regional fish fauna. This study aims at determining whether associations with distinct fish species can explain the distribution of floaters. Here, we devised a framework based on a series of novel distributional null models to elucidate dispersal of species that are dependent on host species. The distributional patterns of floaters were tested by controlling for associations with fishes, while host–parasite associations were assessed with null models based on co-occurrence patterns. The disjoint distribution of the giant floater ( Pyganodon grandis ) and the eastern floater ( Pyganodon cataracta ) could not be explained by their association with distinct host fish and is likely the result of associations with distinct host populations rather than distinct species. In contrast, the distribution of the Newfoundland floater ( Pyganodon fragilis ) could be explained by its association with euryhaline fishes. Such associations may have also promoted differentiation with its sister taxa, the eastern floater. This study demonstrated the effects of positive biological associations on distributional patterns and the utility of a null model framework to uncover them.

Gene flow and effective population size in two life‐history types of broad whitefish Coregonus nasus from the Canadian Arctic

In this study, the magnitude and direction of gene flow and estimates of effective population sizes (N(e) ) were quantified among two life-history types (lacustrine and anadromous) of broad whitefish Coregonus nasus in the lower Mackenzie River system. The data suggest that dispersal and subsequent gene flow occurs between these groups, with the former appearing to be asymmetrical. Gene flow may potentially be directionally biased as well, a result attributed to source-sink population dynamics and the ongoing process of post-glacial colonization and contemporary range expansion. Additionally, average N(e) estimates were consistently lower for lacustrine populations of C. nasus although confidence intervals for both contemporary and historical estimates broadly overlapped. The lower average estimates of N(e) for lacustrine populations was suggested to be the result of more recent founding events following post-glacial dispersal. This study provides one of the first assessments of gene flow and N(e) in an Arctic coregonine, results that may be relevant to other freshwater and anadromous Arctic species persisting in systems near the periphery of their range.