Path analysis reveals combined winter climate and pollution effects on the survival of a marine top predator

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 aspects of biological organisation
(behaviour, individual traits, demographic rates). Determining the
combined effects of stressors, through different biological pathways, is
key to predicting the consequences for the viability of populations
threatened by 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 population dynamics is
scarce. Using a path analysis, we quantify the effects of climate
change and pollution on the survival of adult great black-backed gulls,
both directly and through the effects of individuals’ body mass. 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 the indirect negative effects of organochlorines, highly toxic
pollutants to seabirds, on survival which acted, in part, 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 path models to improve
predictions of population variability under multiple stressors.