Abstract/Summary

Marine ecosystems are experiencing growing pressure from multiple threats caused by human activities, with far-reaching consequences for marine food webs. Determining the effects of multiple stressors is complex, in part, as they can affect different levels of biological organisation (behaviour, individual traits, demographic rates). Knowledge of the cumulative effects of stressors is key to predict the consequences for threatened populations’ viability under global change. Due to their position in the food chain, top predators such as seabirds are considered more sensitive to environmental changes. Climate change is affecting the prey resources available for seabirds, through bottom-up effects, while organic pollutants can bioaccumulate in food chains with the greatest impacts on top predators. However, knowledge of their combined effects on seabird population dynamics is lacking. Using a path analysis, we quantify the effects of both climate change and pollution, via an effect on body mass, on the survival of adult great black-backed gulls. Warmer ocean temperatures in gulls’ winter foraging areas in the North Sea were correlated with higher survival, potentially explained by shifts in prey availability associated with global climate change. We also found support for indirect negative effects of organochlorines, highly toxic pollutants to seabirds, on survival acting through a negative effect on body mass. The results from this path analysis highlight how, even for such long-lived species where variance in survival tends to be limited, two stressors still have had a marked influence on adult survival and illustrate the potential of such models to improve predictions of population variability under multiple 36 stressors.