Disentangling trophic relationships in a High Arctic tundra ecosystem through food web modeling.

Determining the manner in which food webs will respond to environmental changes is difficult because the relative importance of top-down vs. bottom-up forces in controlling ecosystems is still debated. This is especially true in the Arctic tundra where, despite relatively simple food webs, it is still unclear which forces dominate in this ecosystem. Our primary goal was to assess the extent to which a tundra food web was dominated by plant-herbivore or predator-prey interactions. Based on a 17-year (1993-2009) study of terrestrial wildlife on Bylot Island, Nunavut, Canada, we developed trophic mass balance models to address this question. Snow Geese were the dominant herbivores in this ecosystem, followed by two sympatric lemming species (brown and collared lemmings). Arctic foxes, weasels, and several species of birds of prey were the dominant predators. Results of our trophic models encompassing 19 functional groups showed that <10% of the annual primary production was consumed by herbivores in most years despite the presence of a large Snow Goose colony, but that 20-100% of the annual herbivore production was consumed by predators. The impact of herbivores on vegetation has also weakened over time, probably due to an increase in primary production. The impact of predators was highest on lemmings, intermediate on passerines, and lowest on geese and shorebirds, but it varied with lemming abundance. Predation of collared lemmings exceeded production in most years and may explain why this species remained at low density. In contrast, the predation rate on brown lemmings varied with prey density and may have contributed to the high-amplitude, periodic fluctuations in the abundance of this species. Our analysis provided little evidence that herbivores are limited by primary production on Bylot Island. In contrast, we measured strong predator-prey interactions, which supports the hypothesis that this food web is primarily controlled by top-down forces. The presence of allochthonous resources subsidizing top predators and the absence of large herbivores may partly explain the predominant role of predation in this low-productivity ecosystem.

Historical and ecological determinants of genetic structure in arctic canids.

Wolves (Canis lupus) and arctic foxes (Alopex lagopus) are the only canid species found throughout the mainland tundra and arctic islands of North America. Contrasting evolutionary histories, and the contemporary ecology of each species, have combined to produce their divergent population genetic characteristics. Arctic foxes are more variable than wolves, and both island and mainland fox populations possess similarly high microsatellite variation. These differences result from larger effective population sizes in arctic foxes, and the fact that, unlike wolves, foxes were not isolated in discrete refugia during the Pleistocene. Despite the large physical distances and distinct ecotypes represented, a single, panmictic population of arctic foxes was found which spans the Svalbard Archipelago and the North American range of the species. This pattern likely reflects both the absence of historical population bottlenecks and current, high levels of gene flow following frequent long-distance foraging movements. In contrast, genetic structure in wolves correlates strongly to transitions in habitat type, and is probably determined by natal habitat-biased dispersal. Nonrandom dispersal may be cued by relative levels of vegetation cover between tundra and forest habitats, but especially by wolf prey specialization on ungulate species of familiar type and behaviour (sedentary or migratory). Results presented here suggest that, through its influence on sea ice, vegetation, prey dynamics and distribution, continued arctic climate change may have effects as dramatic as those of the Pleistocene on the genetic structure of arctic canid species.

Lifetime selection on heritable life-history traits in a natural population of red squirrels.

Despite their importance in evolutionary biology, heritability and the strength of natural selection have rarely been estimated in wild populations of iteroparous species or have usually been limited to one particular event during an organism's lifetime. Using an animal-model restricted maximum likelihood and phenotypic selection models, we estimated quantitative genetic parameters and the strength of lifetime selection on parturition date and litter size at birth in a natural population of North American red squirrels, Tamiasciurus hudsonicus. Litter size at birth and parturition date had low heritabilities (h2 = 0.15 and 0.16, respectively). We considered potential effects of temporal environmental covariances between phenotypes and fitness and of spatial environmental heterogeneity in estimates of selection. Selection favored early breeders and females that produced litter sizes close to the population average. Stabilizing selection on litter size at birth may occur because of a trade-off between number of offspring produced per litter and offspring survival or a trade-off between a female's fecundity and her future reproductive success and survival.

Anticipatory parental care: acquiring resources for offspring prior to conception.

Many organisms acquire and defend resources outside the breeding season and this is thought to be for immediate survival and reproductive benefits. Female red squirrels (Tamiasciurus hudsonicus) acquire traditional food cache sites up to four months prior to the presence of any physiological or behavioural cues associated with mating or offspring dependency. They subsequently relinquish these resources to one of their offspring at independence (ten months later). We experimentally show that acquisition of these cache sites cannot be explained by conventional arguments such as enhanced survival of the parent or increased reproductive output. Instead this behaviour functions as a form of parental care.

Seasonal and interindividual variation in field water metabolism of female meadow voles Microtus pennsylvanicus.

We analyzed variations in water flux rates on a large sample of meadow voles (Microtus pennsylvanicus) to quantify the effect of season on water metabolism of individuals and to examine patterns of intra- and interindividual variability. Voles were nonreproductive females maintained in outdoor enclosures where they fed on natural vegetation. They were injected one to three times with doubly labeled water, which resulted in one to six measures of daily water flux rate per individual. Summer water flux rates of voles were 258% of the predicted values for herbivorous eutherian mammals of similar size. To date, very few studies have focused on mammals with such high water flux rates. Body water volume of individuals was higher in summer than in winter (75.6% vs. 72.5%), and water flux rate of animals was 12.5% higher in the winter season (0.99 vs. 0.88 mL H2O g-1 d-1). Between-season differences in water fluxes were proportional to differences in energy expenditures, hence the water economy index remained constant across seasons (0.30 mL H2O kJ-1). Intraindividual variability of water flux rate was high compared to interindividual variability (repeatability, r<0.30), which will make it difficult to study natural selection of water metabolism in a microevolutionary framework, at least in meadow voles.

The use of hydroxyapatite-coated implants in revision surgery for acetubular component loosening.

Since 1989,the authors have used the Hydroxyapatite (HA)-coated ABG prosthesis for both primary and revision hip replacement. Hydroxyapatite is osteoconductive, and therefore speeds up bone formation and ensures a direct contact between the implant and the host bone, without any interposed fibrous tissue. The cup can thus be firmly fixed in the bony acetabulum, with eventual osseointegration into the host bone. The quality of the results achieved, at primary hip replacement, with bioactively coated devices has been highlighted in a large number of publications.

[Severe strains of the cervical spine operated on by a posterior approach (author's transl)]. / Entorses graves du rachis cervical. Traitement par voie postérieure.

The authors have operated on 18 cases of severe strain of the cervical spine (six cases of C1-C2 and 12 cases of level cervical spine). They point out the difficulty of an accurate diagnosis in emergency. They emphasize the importance of flexion-extension X-rays when conventional X-rays are not strictly normal. After an analysis of the results they suggest that recent cases should be treated by posterior grafting at the level of C1 and C2, posterior plating at lower levels. Old and irreducible cases should be treated by osteotomy and anterior grafting: banding with grafting at C1 and C2, posterior screwed plating at the inferior cervical level.

[The treatment of diaphyso-epiphyseal tumours by local resection with the preservation of joint movement. A report of four cases (author's transl)]. / Résection avec conservation de la mobilité dans les tumeurs diaphyso-épiphysaires. A propos de 4 cas.

Four cases of epiphyseal tumours were treated by the authors by local resection. In two cases, there was a giant-cell tumour of the lower end of the femur, in one case a giant-cell tumour of the lower end of the radius and in one case a juxta-cortical sarcoma of the lower end of the radius. In the two cases involving the knees, resection preserved the articular surface and the subchondral bone. Reconstruction was achieved by a cortical autograft. In all cases, painless joint movement was preserved with satisfactory muscle power.

[Severe ligamentous injury due to trauma of the mobile spinal segment of the cervical column (author's transl)]. / Entorse grave par lésion traumatique du segment mobile rachidien (S.M.R.) de la colonne cervicale.

The mobile spinal segment includes anatomically, all the soft parts which lie between two neighbouring vertebrae. Its involvement in trauma is not always obvious on initial X Ray. However, the course may lead to complete vertebral dislocation. The initial diagnosis is based on opening of the joint spaces, study of the mobility between two vertebrae by dynamic radiography. The distinction between a benign and a severe ligamentous sprain may thus be made. The attitude in cases of mobile spinal segment involvement, i.e. severe ligamentous injury, should include arthrodesis of the mobile segment affected and this is discussed here.

[Results of Papineua's operation. Apropos of 46 cases]. / Résultats de l'intervention de Papineau. A propose de 46 cas

The authors analyse about one and a half years later, the results of 46 Papineau operations divided in three groups depending of the initial excision. The complete healing was obtained for 44 patients and by the sole Papineau operation in 70 p. 100 of the cases, with relatively short delays: 6 months without a solution of bone continuity, 9 months with a small solution of bone continuity, 11 months in the case of major solution of bone continuity. They insist on the absence of amputation, the small number of complications and in particular of interative fractures in this series.

[Technic and natural history of Papineus's operation. Excision of cancellous graft without skin closure]. / Technique et histoire naturelle de l'intervention de Papineau. Excision-greffe de spongieux à l'air libre

The authors present a new treatment technique of osteitis, which consists of an excision of necrotic bone followed by cancellous graft bone in two operations without a skin closure. This allows a simultaneous drying, cicatrization and consolidation. The post-operative treatment necessary in order to obtain a rapid healing is particularly detailed.