Climate warming is pronounced in the Arctic and migratory birds are expected to be among the most affected species. We examined the effects of local and regional climatic variations on the breeding phenology and reproductive success of greater snow geese (Chen caerulescens atlantica), a migratory species nesting in the Canadian Arctic. We used a long-term dataset based on the monitoring of 5447 nests and the measurements of 19 234 goslings over 16 years (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004) on Bylot Island. About 50% of variation in the reproductive phenology of individuals was explained by spring climatic factors. High mean temperatures and, to a lesser extent, low snow cover in spring were associated with an increase in nest density and early egg-laying and hatching dates. High temperature in spring and high early summer rainfall were positively related to nesting success. These effects may result from a reduction in egg predation rate when the density of nesting geese is high and when increased water availability allows females to stay close to their nest during incubation recesses. Summer brood loss and production of young at the end of the summer increased when values of the summer Arctic Oscillation (AO) index were either very positive (low temperatures) or very negative (high temperatures), indicating that these components of the breeding success were most influenced by the regional summer climate. Gosling mass and size near fledging were reduced in years with high spring temperatures. This effect is likely due to a reduced availability of high quality food in years with early spring, either due to food depletion resulting from high brood density or a mismatch between hatching date of goslings and the timing of the peak of plant quality. Our analysis suggests that climate warming should advance the reproductive phenology of geese, but that high spring temperatures and extreme values of the summer AO index may decrease their reproductive success up to fledging.

Arctic wildlife is often presented as being highly at risk in the face of current climate warming. We use the long-term (up to 24 years) monitoring records available on Bylot Island in the Canadian Arctic to examine temporal trends in population attributes of several terrestrial vertebrates and in primary production. Despite a warming trend (e.g. cumulative annual thawing degree-days increased by 37% and snow-melt date advanced by 4–7 days over a 23-year period), we found little evidence for changes in the phenology, abundance or productivity of several vertebrate species (snow goose, foxes, lemmings, avian predators and one passerine). Only primary production showed a response to warming (annual above-ground biomass of wetland graminoids increased by 123% during this period). We nonetheless found evidence for potential mismatches between herbivores and their food plants in response to warming as snow geese adjusted their laying date by only 3.8 days on average for a change in snow-melt of 10 days, half of the corresponding adjustment shown by the timing of plant growth (7.1 days). We discuss several reasons (duration of time series, large annual variability, amplitude of observed climate change, nonlinear dynamic or constraints imposed by various rate of warming with latitude in migrants) to explain the lack of response by herbivores and predators to climate warming at our study site. We also show how length and intensity of monitoring could affect our ability to detect temporal trends and provide recommendations for future monitoring.

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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.

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Under varying prey abundance, generalist consumers should be less affected than specialists due to their more diverse diet. Nonetheless, when prey availability declines, interspecific competition among consumers should increase and could lead to increased intraguild predation. We examined these potential effects in a generalist predator of the tundra, the Glaucous Gull (Larus hyperboreus), over a 7-yr period characterized by large fluctuations in lemming abundance, a potential prey item for gulls. We studied diet by analyzing regurgitated pellets collected at nests and blood nitrogen and carbon stable isotopes, and we monitored annual nesting density and reproductive success on Bylot Island, Nunavut, Canada. Stable isotopes revealed a relatively similar contribution of terrestrial and marine food sources to the gull diet, although the terrestrial contribution increased in the year of high lemming abundance. According to analysis of pellets, diet during incubation was dominated by geese and lemmings, whereas geese were the main prey during chick-rearing. As expected for a generalist predator, annual variation in diet during incubation reflected overall lemming abundance, and the increased consumption of geese during chick-rearing was associated with an influx of goose families into the study area at that time. Gull nest density, laying date, clutch size, and mass gain of chicks did not vary with lemming population fluctuations. Hatching success, on the other hand, was positively related to lemming abundance and was greater for nests located on islets than for those along the shores of ponds and lakes. Intraguild predation on gull eggs by predators such as Arctic foxes, which primarily feed on lemmings during the summer, was probably the main cause of nest failure. Although a generalist predator like the Glaucous Gull has a diversified diet and may feed only opportunistically on lemmings, our results suggest that large cyclical fluctuations in lemming abundance may still affect gull reproductive success through intraguild predation.Régime alimentaire et succès reproducteur d'un prédateur généraliste de l'Arctique: interaction entre des variations dans l'abondance de proies, la localisation du site de nidification et la prédation intraguilde RÉSUMÉ Lorsque soumisà une abondance variable de proies, les consommateurs généralistes devraientêtre moins affectés que les spécialistesà cause de leur régime alimentaire plus varié. Toutefois, lorsque la disponibilité des proies diminue, la compétition interspécifique entre les consommateurs devrait augmenter et ceci pourrait conduireà une augmentation de la prédation intraguilde. Nous avons examiné ces effets potentiels chez un prédateur généraliste de la toundra, Larus hyperboreus, sur une période de sept ans caractérisée par de grandes fluctuations d'abondance des lemmings, une proie potentielle des goélands. Nous avonsétudié le régime alimentaire en analysant des boulettes de régurgitation récoltées aux nids et des isotopes stables d'azote et de carbone dans le sang. Nous ...

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