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Contrasting effects of climate and population density over time and life-stages in a long-lived seabird
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International audience. Although population responses to environmental variability have been extensively studiedfor many organisms, few studies have considered early-life stages owing to the inherent difficultiesin tracking the fate of young individuals. However, young individuals are expected to bemore sensitive to environmental stochasticity owing to their inexperience and lower competitiveabilities. Thus, they are keys to understand demographic responses of an age-structuredpopulation to environmental variability.In this study, we used capture–recapture modelling, based on a 49 year-long individualbasedlongitudinal monitoring dataset, to investigate climatic and population density effects onimmature demographic parameters in a long-lived seabird, the wandering albatross.We provide evidence that climate and population size affected both survival and recruitmentage of young individuals although in different ways according to the trait. We found thatearly-life survival was mainly affected by population density, whereas recruitment age variationappeared to be better explained by climatic conditions, with a surprising long-term effect ofclimate. While population size explained 60% of the variation in juvenile survival, the averageSouthern Annular Mode over the five previous years explained 52% of variation inrecruitment age.In addition, although early-life survival was consistently negatively affected by populationsize, the relationship between recruitment age and population size shifted from negative topositive over time from the 1970s to 2000s, showing that density dependence mechanisms cantemporarily disappear.5. Finally, we found that similar climatic conditions may affect individual performances inopposite ways according to the life stage of individuals. This result underlines the critical needto assess age-specific functional responses to environmental variability to allow accuratedemographic predictions. By revealing the poorly known demographic process of younger ageclasses, the results of this study improve our understanding of population dynamics oflong-lived marine species.
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