Changes in species composition of South Georgia seabirds and mammals 1985–2023: decrease in opportunities for local enhancement?

Seabirds cue into other birds and marine mammals to amplify their foraging success. Consequently, changes to the community (e.g. as the climate warms) can impact foraging success and potentially fitness. For example, flying birds that use groups of seals or penguins as cues might decline if the seal or penguin population declines, owing to fewer opportunities for positive foraging associations with one another. To test this idea, we leveraged data from transect surveys spanning 38 years (data collected in 1985, 1991, 1993 and 2023) in waters surrounding the subantarctic island South Georgia. We asked: ‘Has the frequency with which seabirds form statistically significant pairings with other species declined over time?’ We found such changes have occurred: network connectivity (the percentage of species pairings out of all possible pairs) decreased from approximately 40 to 7%, although the overall impact of warming on the community in 2023 was complicated by the arrival of a 100 km iceberg from the Weddell Sea. Possible fitness impacts from reduced associations could have broad-ranging consequences as the ocean continues to warm and the community changes. The results presented here represent one of just a handful of at-sea surveys documenting the winter seabird community at South Georgia. This article is part of the theme issue ‘Multi-predator feeding aggregations in pelagic waters’.