Contrasting effects of climate and population density over time and life-stages in a long-lived seabird
Résumé
1. Although population responses to environmental variability have been extensively studied
for many organisms, few studies have considered early-life stages owing to the inherent difficulties
in tracking the fate of young individuals. However, young individuals are expected to be
more sensitive to environmental stochasticity owing to their inexperience and lower competitive
abilities. Thus, they are keys to understand demographic responses of an age-structured
population to environmental variability.
2. In this study, we used capture–recapture modelling, based on a 49 year-long individualbased
longitudinal monitoring dataset, to investigate climatic and population density effects on
immature demographic parameters in a long-lived seabird, the wandering albatross.
3. We provide evidence that climate and population size affected both survival and recruitment
age of young individuals although in different ways according to the trait. We found that
early-life survival was mainly affected by population density, whereas recruitment age variation
appeared to be better explained by climatic conditions, with a surprising long-term effect of
climate. While population size explained 60% of the variation in juvenile survival, the average
Southern Annular Mode over the five previous years explained 52% of variation in
recruitment age.
4. In addition, although early-life survival was consistently negatively affected by population
size, the relationship between recruitment age and population size shifted from negative to
positive over time from the 1970s to 2000s, showing that density dependence mechanisms can
temporarily disappear.
5. Finally, we found that similar climatic conditions may affect individual performances in
opposite ways according to the life stage of individuals. This result underlines the critical need
to assess age-specific functional responses to environmental variability to allow accurate
demographic predictions. By revealing the poorly known demographic process of younger age
classes, the results of this study improve our understanding of population dynamics of
long-lived marine species.