Environment-triggered demographic changes cascade and compound to propel a dramatic decline of an Antarctic seabird metapopulation

While seabirds are well-known for making a living under some of the harshest conditions on the planet, their capacity to buffer against unfavourable conditions can be stretched in response to ecosystem change. During population increases, overlap between conspecifics can limit population growth through competition for breeding or feeding resources. What is less well understood is the role that intrinsic processes play during periods of population decline or under a changing environment. We interrogate key demographic parameters and their biophysical drivers to understand the role of intrinsic and extrinsic drivers during a recent near halving of a large Adelie penguin (Pygoscelis adeliae) metapopulation. The loss of 154,000 breeding birds along the 100-km East Antarctic coastline centred around 63 degrees E over the last decade diverges from a sustained increase over preceding decades and is contrary to recent models that predict a continued increase. The decline was initially triggered by changed environmental conditions: more extensive near-shore sea ice caused a reduction in breeding success. The evidence suggests this decline was exacerbated by feedback processes driving an inverse density-dependent decrease in fledgling survival in response to smaller cohort size. It appears that the old adage of safety in numbers may shape the fledgling penguins' chances of survival and, if compromised over multiple years, could exacerbate difficulties during population decline or if feedback processes arise. The likely interplay between demographic parameters meant that conditions were more unfavourable and negative effects more rapid than would be expected if demographic processes acted in isolation or independently. Failure to capture both intrinsic and extrinsic drivers in predictive population models may mean that the real impacts of climate change on species' populations are more severe than projections would lead us to believe. These results improve our understanding of population regulation during periods of rapid decline for long-lived marine species.

Automated summary

While seabirds are known for their ability to survive in harsh conditions, their capacity to adapt may be challenged by ecosystem changes. This study examines the impact of intrinsic and extrinsic factors on the decline of a large Adelie penguin metapopulation. The results suggest that the decline was triggered by environmental changes and exacerbated by feedback processes, highlighting the importance of considering both intrinsic and extrinsic factors in population models.