Demography of two lemming species on Bylot Island, Nunavut, Canada

Lemmings play a key role in the tundra food web and their widely reported cyclic oscillations in abundance may have a strong effect on other components of the ecosystem. We documented seasonal and annual variations in population density, reproductive activity, survival, and body mass of two sympatric species, the brown (Lemmus trimucronatus) and collared lemmings (Dicrostonyx groenlandicus), over a 2-year period on Bylot Island, Nunavut, Canada. We live trapped and marked lemmings on two grids throughout the summer and we estimated demographic parameters using three different capture–recapture methods. All three methods are based on robust estimators and they yielded similar population density estimates. The density of brown lemmings declined markedly between the 2 years whereas that of collared lemmings was relatively constant. For brown lemmings, 2004 was a peak year in their cycle and 2005 a decline phase. Density of brown lemmings also decreased during the summer, but not that of collared lemmings. The recruitment of juvenile brown lemmings in the population increased during the summer and was higher in the peak year than in the year after, but no change was detected in collared lemmings. Survival rates of both species tended to be lower during the peak year than in the following year and body mass of brown lemmings was higher in the peak year than in the following year. We conclude that both changes in adult survival and juvenile recruitment occur during the population decline of brown lemmings.     

Habitat use by lemmings near Barrow, Alaska

Brown lemmings reach much higher densities than collared lemmings near Barrow, Alaska, and captures from 19 summers of snap trapping confirm previous reports that brown lemmings prefer lower, wetter habitats than do collared lemmings. Data also support the hypothesis that brown lemmings concentrate in higher habitats during early summer when melt water floods lower habitats, then shift to lower habitats where preferred food is more available as the waters recede. This pattern appears similar to seasonal shifts in habitat use reported for Norwegian lemmings. Two hypotheses were not supported by our data: (1) Unlike Norwegian lemmings, brown lemmings did not expand their use of suboptimal habitats at higher population densities. Rather, absolute densities changed in concert so that the relative densities among habitats remained unchanged. (2) Preferential use of polygon troughs during winter, as indicated by patterns of winter grazing, was not simply a function of snow depth. Instead it appeared to be related to shoot density of preferred foods. Nearly all patterns of habitat use seemed to be linked to food availability. Other factors, such as protection from predators by vegetative cover in summer and increased insulation from deeper snow in winter, did not appear to influence the distribution of lemmings as strongly.     

Habitat selection,reproduction and predation of wintering lemmings in the Arctic

Snow cover has dramatic effects on the structure and functioning of Arctic ecosystems in winter. In the tundra, the subnivean space is the primary habitat of wintering small mammals and may be critical for their survival and reproduction. We have investigated the effects of snow cover and habitat features on the distributions of collared lemming (Dicrostonyx groenlandicus) and brown lemming (Lemmus trimucronatus) winter nests, as well as on their probabilities of reproduction and predation by stoats (Mustela erminea) and arctic foxes (Vulpes lagopus). We sampled 193 lemming winter nests and measured habitat features at all of these nests and at random sites at two spatial scales. We also monitored overwinter ground temperature at a subsample of nest and random sites. Our results demonstrate that nests were primarily located in areas with high micro-topography heterogeneity, steep slopes, deep snow cover providing thermal protection (reduced daily temperature fluctuations) and a high abundance of mosses. The probability of reproduction increased in collared lemming nests at low elevation and in brown lemming nests with high availability of some graminoid species. The probability of predation by stoats was density dependent and was higher in nests used by collared lemmings. Snow cover did not affect the probability of predation of lemming nests by stoats, but deep snow cover limited predation attempts by arctic foxes. We conclude that snow cover plays a key role in the spatial structure of wintering lemming populations and potentially in their population dynamics in the Arctic.     

Habitat selection and the scale of ghostly coexistence among Arctic rodents

Competition between coexisting species existing near their stable equilibrium can be obscured if they occupy separate habitats. Theories of habitat selection promise an ability to reveal the underlying ghost of competition by using isodars to infer the behavioural map of habitat selection. We tested the theory with two years of data on abundance and habitat preference by three Arctic rodent species living at low density along a gradient of wet to dry tundra on Herschel Island in Canada's western Arctic. Generalist brown lemmings exhibited a constant partial preference toward wet tundra whereas specialist collared lemmings and voles occupied the driest and wettest zones respectively. Although both lemming species compete for habitats elsewhere in the Canadian Arctic, isodar analyses suggest that the three species occupy wet and dry habitats independently of one another on Herschel Island. Competition at this large scale may be hidden at low densities, however, if the wet‐dry dichotomy is too coarse. Analyses at a finer subdivision of habitat revealed that these species coexist by using different microhabitats. Collared lemmings shifted their niche towards even drier habitat as the abundance of brown lemmings increased. We were thus able to reveal the ghost of competition lurking at large scales through a more refined analysis at smaller scales of density‐dependent habitat use.     

The effect of snow cover on lemming population cycles in the Canadian High Arctic

Rising temperatures and changes in the precipitation regime will have a strong impact on the quality of the snow cover in the Arctic. A snow cover of good quality protecting lemmings from cold temperatures and predators is thought to be an important factor for maintaining the cyclic dynamic of their populations in the tundra. We examined if the characteristics of annual fluctuations (amplitude and shape of phases) in brown lemming (Lemmus trimucronatus) density could be determined by snow depth, snow density, sub-nivean temperature and persistence of snow. Using an 18-year time series of brown lemming abundance on Bylot Island in the Canadian Arctic, we tested if snow variables could explain the residual variation between the observed lemming density and the one predicted by models where cyclicity had been accounted for. Our analysis provides support for the hypothesis that snow cover can affect the amplitude and possibly also the periodicity of lemming population cycles in the High Arctic. Summer abundance of brown lemmings was higher following winters with a deep snow cover and a low-density snow pack near the ground but was unaffected by the date of establishment or melting and duration of the snow cover. Two snow variables showed a temporal trend; mean winter snow depth tended to increase and date of establishment of the hiemal threshold occurred earlier over time. These temporal trends, which should be favourable to lemmings, may explain why healthy population cycles have apparently been maintained at our study site contrary to other Arctic sites.     

Long-term genetic monitoring of a translocated population of collared brown lemurs

Post-release monitoring is important to improve translocation success because it provides an opportunity to identify factors relevant to the survival of local populations. We studied a population of the endangered collared brown lemur (Eulemur collaris) translocated from a degraded forest fragment to a nearby littoral forest within the Mandena Conservation Area in southeast Madagascar from 2000–2011. We compared genetic surveys of mitochondrial and nuclear markers with the genetic profile of nearby populations to examine the dispersal capacity of the collared brown lemur. We also performed a landscape analysis to assess changes in connectivity between forest fragments. There was a fluctuating trend characterized by a phase of demographic and genetic stability shortly after translocation, followed by an increase in genetic diversity coinciding with a population decrease and a gradual recovery of initial conditions. These results demonstrated the ability of the collared brown lemur to disperse through unfavorable landscapes and to recover after translocation. Our study revealed the importance of monitoring translocated populations over time using a multidisciplinary approach.     

Photoperiod differentially affects immune function and reproduction in collared lemmings (Dicrostonyx groenlandicus)

Many nontropical rodent species experience predictable annual variation in resource availability and environmental conditions. Individuals of many animal species engage in energetically expensive processes such as breeding during the spring and summer but bias investment toward processes that promote survival such as immune function during the winter. Generally, the suite of responses associated with the changing seasons can be induced by manipulating day length (photoperiod). Collared lemmings (Dicrostonyx groenlandicus) are arvicoline rodents that inhabit parts of northern Canada and Greenland. Despite the extreme conditions of winter in their native habitat, these lemmings routinely breed during the winter. In the laboratory, collared lemmings have divergent responses to photoperiod relative to other seasonally breeding rodents; short day lengths can stimulate, rather than inhibit, the reproductive system. Male and female collared lemmings were maintained for 11 weeks in 1 of 3 photoperiods (LD 22:2, LD 16:8, or LD 8:16) that induce markedly different phenotypes. Following photoperiod treatment, cell-mediated immune function as assessed by delayed-type hypersensitivity reactions was elevated in lemmings housed in LD 16:8 and LD 8:16 relative to LD 22:2. However, antibody production to a novel antigen was unaffected by photoperiod. Exposure to LD 8:16 induced weight gain, molt to a winter pelage, and in contrast to previous studies, regression of the male, but not the female, reproductive tract. In conclusion, these data indicate that components of immune function among collared lemmings are responsive to changes in day length.     

Spatial structure of lemming populations (Dicrostonyx groenlandicus) fluctuating in density

The pattern and scale of the genetic structure of populations provides valuable information for the understanding of the spatial ecology of populations, including the spatial aspects of density fluctuations. In the present paper, the genetic structure of periodically fluctuating lemmings (Dicrostonyx groenlandicus) in the Canadian Arctic was analysed using mitochondrial DNA (mtDNA) control region sequences and four nuclear microsatellite loci. Low genetic variability was found in mtDNA, while microsatellite loci were highly variable in all localities, including localities on isolated small islands. For both genetic markers the genetic differentiation was clear among geographical regions but weaker among localities within regions. Such a pattern implies gene flow within regions. Based on theoretical calculations and population census data from a snap-trapping survey, we argue that the observed genetic variability on small islands and the low level of differentiation among these islands cannot be explained without invoking long distance dispersal of lemmings over the sea ice. Such dispersal is unlikely to occur only during population density peaks.     

Population cycles of lemmings near Barrow,Alaska: a historical review

Current hypotheses regarding the causes of population cycling of brown lemmingsLemmus trimucronatus (Richardson, 1828), developed during long-term studies from 1950–1974. We maintain that three factors largely determine the timing and amplitude of population cycles in brown lemmings. First, a basic interaction between lemmings and vegetation sets the stage because dense populations of lemmings severely damage the vegetation, at which point lemming populations decline and remain low until the vegetation recovers. Second, opportunistic predators, mainly jaegers and owls, assemble as the snow melts during peak years and drive already declining populations to extremely low densities. Weasels are effective predators under the snow, but they appear irregularly. If weasel populations increase early in the cycle, lemming populations that normally increase dramatically during the winter in a peak year can be decimated before the snow melts. Finally, both wet summers that result in extensive flooding of the preferred habits of lemmings and freezing rains or winter thaws that cause ice formation reduce food availability, disrupt the cycle and extend the period between peak densities. Numerous observations, experimental results and simulation models support these views. Similar results regarding the importance of the available food supply, particularly in winter, and of the predation regime also have been reported for arvicoline populations at lower latitudes.     

Comparative phylogeography of brown (Sula leucogaster) and red-footed boobies (S. sula): The influence of physical barriers and habitat preference on gene flow in pelagic seabirds

To test the hypothesis that both physical and ecological barriers to gene flow drive population differentiation in tropical seabirds, we surveyed mitochondrial control region variation in 242 brown boobies (Sula leucogaster), which prefer inshore habitat, and 271 red-footed boobies (S. sula), which prefer pelagic habitat. To determine the relative influence of isolation and gene flow on population structure, we used both traditional methods and a recently developed statistical method based on coalescent theory and Bayesian inference (Isolation with Migration). We found that global population genetic structure was high in both species, and that female-mediated gene flow among ocean basins apparently has been restricted by major physical barriers including the Isthmus of Panama, and the periodic emergence of the Sunda and Sahul Shelves in Southeast Asia. In contrast, the evolutionary history of populations within ocean basins differed markedly between the two species. In brown boobies, we found high levels of population genetic differentiation and limited gene flow among colonies, even at spatial scales as small as 500 km. Although red-footed booby colonies were also genetically differentiated within ocean basins, coalescent analyses indicated that populations have either diverged in the face of ongoing gene flow, or diverged without gene flow but recently made secondary contact. Regardless, gene flow among red-footed booby populations was higher than among brown booby populations. We suggest that these contrasting patterns of gene flow within ocean basins may be explained by the different habitat preferences of brown and red-footed boobies.     

Strong intraspecific variation in genetic diversity and genetic differentiation in Daphnia magna: the effects of population turnover and population size

Theory predicts that genetic diversity and genetic differentiation may strongly vary among populations of the same species depending on population turnover and local population sizes. Yet, despite the importance of these predictions for evolutionary and conservation issues, empirical studies comparing high‐turnover and low‐turnover populations of the same species are scarce. In this study, we used Daphnia magna, a freshwater crustacean, as a model organism for such a comparison. In the southern/central part of its range, D. magna inhabits medium‐sized, stable ponds, whereas in the north, it occurs in small rock pools with strong population turnover. We found that these northern populations have a significantly lower genetic diversity and higher genetic differentiation compared to the southern/central populations. Total genetic diversity across populations was only about half and average within‐population diversity only about a third of that in southern/central populations. Moreover, an average southern population contains more genetic diversity than the whole metapopulation system in the north. We based our analyses both on silent sites and microsatellites. The similarity of our results despite the contrasting mutation rates of these markers suggests that the differences are caused by contemporary rather than by historical processes. Our findings show that variation in population turnover and population size may have a major impact on the genetic diversity and differentiation of populations, and hence may lead to differences in evolutionary processes like local adaptation, hybrid vigour and breeding system evolution in different parts of a species range.     

Genetic population structure in an equatorial sparrow: roles for culture and geography

Female preference for local cultural traits has been proposed as a barrier to breeding among animal populations. As such, several studies have found correlations between male bird song dialects and population genetics over relatively large distances. To investigate whether female choice for local dialects could act as a barrier to breeding between nearby and contiguous populations, we tested whether variation in male song dialects explains genetic structure among eight populations of rufous‐collared sparrows (Zonotrichia capensis) in Ecuador. Our study sites lay along a transect, and adjacent study sites were separated by approximately 25 km, an order of magnitude less than previously examined for this and most other species. This transect crossed an Andean ridge and through the Quijos River Valley, both of which may be barriers to gene flow. Using a variance partitioning approach, we show that song dialect is important in explaining population genetics, independent of the geographic variables: distance, the river valley and the Andean Ridge. This result is consistent with the hypothesis that song acts as a barrier to breeding among populations in close proximity. In addition, songs of contiguous populations differed by the same degree or more than between two populations previously shown to exhibit female preference for local dialect, suggesting that birds from these populations would also breed preferentially with locals. As expected, all geographic variables (distance, the river valley and the Andean Ridge) also predicted population genetic structure. Our results have important implications for the understanding whether, and at what spatial scale, culture can affect population divergence.     

Population dynamics of the collared lemming and the tundra vole at Pearce Point,Northwest Territories,Canada

From 1987 to 1989 we monitored population changes during summer of the collared lemming (Dicrostonyx groenlandicus) and the tundra vole (Microtus oeconomus) at Pearce Point, Northwest Territories, Canada (69° 48 N, 122° 40 W). Populations on four study areas did not cycle but remained at low density (<3/ha) each year and continued at low numbers for the following 3 years (Reid et al. 1995). Lemming numbers often declined throghout the summer in spite of continous reproduction, and population recovery occurred overwinter. Heavy predation losses of radio-collared lemmings occurred each summer and this lemming population may be trapped in a predator-pit. Collared lemmings breed in winter and only because of winter population growth do these populations persist. Tundra vole numbers increased rapidly in most summers but usually declined overwinter. Tundra voles do not seem able to sustain winter reproduction in this extreme environment and this prevents them from reaching high density because of the short summer. Population growth in both these rodents could be prevented by poor food or by predation losses, and landscape patchiness may also help to prevent population growth. For lemmings we do not think that a shortage of shelter or intrinsic limitations could be restricting population increase at Pearce Point. This is the first detailed study of a non-cyclic collared lemming population.     

Landscape-level spatial genetic structure in Quercus acutissima (Fagaceae)

Quercus acutissima (Fagaceae), a deciduous broad-leaved tree, is an important forest element in hillsides of South Korea. We used allozyme loci, Wright's F statistics, and multilocus spatial autocorrelation statistics to examine the distribution of genetic diversity within and among three local populations and the spatial genetic structure at a landscape scale (15 ha, 250 × 600 m) on Oenaro Island, South Korea. Levels of genetic diversity in Q. acutissima populations were comparable to mean values for other oak species. A moderate but significant deficit of heterozygotes (mean F(IS) = 0.069) was detected within local populations and low but significant differentiation was observed among populations (F(ST) = 0.010). Spatial autocorrelation analyses revealed little evidence of significant genetic structure at spatial scales of 100-120 m. The failure to detect genetic structure within populations may be due to intraspecific competition or random mortality among saplings, resulting in extensive thinning within maternal half-sib groups. Alternatively, low genetic differentiation at the landscape scale indicates substantial gene flow among local populations. Although wind-borne pollen may be the primary source of gene flow in Q. acutissima, these results suggest that acorn movement by animals may be more extensive than previously anticipated. Comparison of these genetic data for Oenaro Island with a disturbed isolated inland population suggests that population-to-population differences in internal genetic structure may be influenced by local variation in regeneration environment (e.g., disturbance).     

Genetic structure of brown trout, salmo trutta l., at the southern limit of the distribution range of the anadromous form

Genetic variation at 33 protein loci was investigated in 41 wild brown trout populations from four river basins in Galicia (northwest Spain) to analyse the amount and distribution of genetic diversity in a marginal area, located in the distribution limit of the anadromous form of this species. The genetic diversity detected within populations (H between 0 and 6%) lies within the range quoted for this species in previous reports. The Mino, the most southern river basin analysed, showed a significantly lower genetic diversity and the highest genetic differentiation among the river basins studied. The hierarchical gene diversity analysis showed high population differentiation in a restricted area (GST = 27%), mostly due to differences among populations within basins (GSC = 22%). The reduction of GST observed when the isolated samples were excluded from the analysis (GST = 17%) showed the importance of habitat fragmentation on the heterogeneity detected. Gene flow among populations was comparatively evaluated by three indirect methods, which in general revealed low figures of absolute number of migrants per generation, slightly higher than 1. The gene flow among basins reflected a positive relationship with geographical distance. This trend was confirmed by the significant correlation observed between geographical and genetic distances, including all population pairs, which suggests a component of isolation by distance in brown trout genetic structure. Nevertheless, the nonsignificant intrabasin correlation demonstrates the complexity of genetic relationships among populations in this species. The model of genetic structure in brown trout is discussed in the light of the results obtained.     

Landscapes of fear or competition? Predation did not alter habitat choice by Arctic rodents

In systems where predation plays a key role in the dynamics of prey populations, such as in Arctic rodents, it is reasonable to assume that differential patterns of habitat use by prey species represent adaptive responses to spatial variation in predation. However, habitat selection by collared (Dicrostonyx groenlandicus) and brown (Lemmus trimucronatus) lemmings depends on intra- and inter-specific densities, and there has been little agreement on the respective influences of food abundance, predators, and competition for habitat on lemming dynamics. Thus, we investigated whether predation affected selection of sedge-meadow versus upland tundra by collared lemmings in the central Canadian Arctic. We first controlled for the effects of competition on lemming habitat selection. We then searched for an additional signal of predation by comparing habitat selection patterns between 12 control plots and one large grid where lemmings were protected from predators by fencing in 1996 and 1997, but not during 5 subsequent years when we monitored habitat use in the grid as well as in the control plots. Dicrostonyx used upland preferentially over meadows and was more numerous in 1996 and 2011 than in other sample years. Lemmus was also more abundant in 1996 than in subsequent years, but its abundance was too low in the exclosure to assess whether exclusion of predators influenced its habitat selection. Contrary to the effects of competition, predation had a negligible impact on the spatial dynamics of Dicrostonyx, at least during summer. These results suggest that any differences in predation risk between the two habitats have little direct influence on the temporal dynamics of Dicrostonyx even if induced through predator–prey cycles.     

Variation in pika (Ochotona collaris,O. princeps) vocalizations within and between populations

Understanding geographic call variation can resolve evolutionary and behavioural questions, yet the factors influencing divergent acoustic signals remain poorly understood in mammals. We explored call variation between collared pikas in Yukon and Alaska and American pikas in Alberta, and between individuals within a population of collared pikas. Classification trees were used to determine the extent of call divergence between populations and the elements of calls driving these differences. Pika populations had significant differences in call structure, and individual pikas were classified to their correct populations with up to 94% accuracy. To investigate possible mechanisms responsible for interspecific variability, we tested the acoustic adaptation hypothesis by using a playback experiment to explore whether American and collared pika calls transmit with less degradation across their own species' habitat than the habitat of their congener. We found no support for the acoustic adaptation hypothesis. Geographic call variation in these two species of pikas likely reflects genetic divergence, and may be a result of separate evolutionary histories. We calculated the potential for individual coding for both time and frequency measurements of calls. High frequency harmonics showed greater between‐ than within‐individual variation, and may act as sources of information regarding individual identity.     

Does lemming winter grazing impact vegetation in the Canadian Arctic?

In low-productivity environments such as the tundra, it has been proposed that regular, multi-annual population cycles of lemmings could be driven by winter food depletion in years of peak abundance. If lemming population dynamics is controlled by food resources, we predict that (1) winter grazing should negatively impact the abundance of food plants, (2) this impact should be proportional to lemming density and (3) high lemming winter grazing pressure should result in reduced plant growth during the following summer. We tested these predictions on Bylot Island, Nunavut, Canada, where two species of lemmings are present: the brown (Lemmus trimucronatus) and collared lemming (Dicrostonyx groenlandicus). We installed 16 exclosures in their preferred wintering habitat (snowbeds) and annually sampled plant biomass inside and outside exclosures at snow melt and at peak growth during the summers of 2009–2012, covering a full population cycle. Winter grazing had no impact on total vascular plant or moss biomass at snow melt in all years. Among plant families, only Caryophyllaceae, which was uncommon, showed a decline. In moss taxa, a negative effect was found on Polytrichum in only 1 year out of three. Overall, plant regrowth during the subsequent summer showed annual variation and tended to be reduced in the 2 years of high lemming abundance. However, this could be a consequence of summer grazing. Overall, the impact of lemming winter grazing on plants was weak and short-lived, even in years of high lemming abundance. Therefore, our results are not consistent with the hypothesis that food depletion during winter was the cause of the lemming decline following peak abundance at our study site. Other factors may limit lemming populations and prevent them from reaching densities high enough to exhaust their food resources.     

Genetic differentiation across a latitudinal gradient in two co-occurring butterfly species: revealing population differences in a context of climate change

Genetic differentiation within a species' range is determined by natural selection, genetic drift, and gene flow. Selection and drift enhance genetic differences if populations are sufficiently isolated, while gene flow precludes differentiation and local adaptation. Over large geographical areas, these processes can create a variety of scenarios, ranging from admixture to a high degree of population differentiation. Genetic differences among populations may signal functional differences within a species' range, potentially leading to population or ecotype-specific responses to global change. We investigated differentiation within the geographical range of two butterfly species along a broad latitudinal gradient. This gradient is the primary axis of climatic variation, and many ecologists expect populations at the poleward edge of this gradient to expand under climate change. Our study species inhabit a shared ecosystem and differ in body size and resource specialization; both also find their poleward range limit on an island. We find evidence for divergence of peripheral populations from the core in both taxa, suggesting the potential for genetic distinctiveness at the leading edge of climate change. We also find differences between the species in the extent of peripheral differentiation with the smaller and more specialized species showing greater population divergence (microsatellites and mtDNA) and reduced gene flow (mtDNA). Finally, gene flow estimates in both species differed strongly between two marker types. These findings suggest caution in assuming that populations are invariant across latitude and thus will respond as a single ecotype to climatic change.