How did they get here?

The Southern Ocean, encircling Antarctica with its predominantly westerly winds and currents, hosts numerous isolated oceanic islands. Its biological landscape has long intrigued researchers due to its susceptibility to large-scale climate-induced changes. Throughout history, climatic variations, particularly during glacial–interglacial cycles like those of the Pleistocene Ice Age, have driven significant shifts in species distributions across various regions globally.

Major deglaciation events provide significant ecological opportunities for surviving species. Genetic studies suggest that some highly dispersive species, like the southern elephant seal, had rapid responses to these changes, swiftly colonising newly available habitats following deglaciation but were later extirpated as ice sheets returned.

The Blue-eyed shag, a rich group of seabirds found across the Southern Ocean, presents a compelling case for understanding the evolutionary dynamics and biogeography of species in this region. With its circumpolar distribution encompassing numerous isolated islands, the Blue-eyed shag offers valuable insights into the impact of climatic shifts on species dispersal and speciation.

Figure 1: Evolutionary history of blue-eyed shags (Leucocarbo spp.) in the Southern Ocean. The map depicts inferred postglacial colonization routes (arrows). The extent of winter sea ice (cross-hatched pattern) and land area (dark green) during the Pleistocene Last Glacial Maximum 29–19 Kya is indicated. Rapid circumpolar expansion and founder-speciation hypotheses are supported by temporal phylogenetic and ancestral biogeographic reconstructions. The time-calibrated species tree is derived from 8.2 kilobases of DNA sequence data (five mitochondrial and five nuclear genes) with Nannopterum as the outgroup. Node bars on the phylogeny are 95% HPD of divergence times as indicated on the scale bar (millions of years before present). Colours and abbreviations are as follows: Orange: South America; purple/red: High-latitude sub-Antarctic islands; yellow: Antarctic Peninsula; blue: New Zealand region; olive: North America; dark green: South America; FA: Falkland/Malvinas Islands; SA: South America; SG: South Georgia; SO: South Orkney Islands: AP: Antarctic Peninsula; MA: Marion Island; CZ: Crozet Island; HE: Heard Island; MQ: Macquarie Island; KG: Kerguelen Island; CA: Campbell Island; AU: Auckland Island; NZ: Mainland New Zealand; ST: Stewart Island; CH: Chatham Islands; BO: Bounty Islands; N: North America; NS: North and South America. Clade and arrow colours: Orange: South America, South Georgia, South Orkney Islands; purple: Antarctic Peninsula, high-latitude sub-Antarctic islands; blue: New Zealand region.

The biogeography of blue-eyed shags can be representative of taxa with highly or poorly dispersive characteristics at the same time. While they exhibit wide distribution across the Antarctic islands, like highly dispersive species, they also display high levels of endemism, within each island, typical of less dispersive taxa. This paradoxical situation raises questions about the mechanisms driving speciation in these birds, which prompted this study.

The survival and colonization of Blue-eyed shags on remote islands during periods of glaciation likely depended on favourable climatic conditions and the availability of suitable habitats. Due to Shag’s lack of dispersal capability, the authors point at the Antarctic Circumpolar Current and westerly winds encircling Antarctica as the facilitator for accidental dispersal events, allowing these birds to colonize new territories.

By performing genetic analyses, the authors found that Blue-eyed shags have a remarkably shallow evolutionary history among endemic lineages breeding on scattered islands across the vast Southern Ocean. Suggesting recent radiation events, likely influenced by climatic fluctuations, such as the ones occurring during the Late Pleistocene (Figure 1).

The phylogeographic patterns observed in Blue-eyed shags suggest multiple waves of dispersal from South America to different regions, including the sub-Antarctic and New Zealand. The taxa in the New Zealand region are believed to have descended from an earlier migration wave, later strained because of a new glacial period, while those in the sub-Antarctic likely colonized more recently, when the sea ice coverage retreats again, enabling these migrations (Figure 1).

Furthermore, genetic evidence suggests that many Blue-eyed shag taxa arose through founder-event speciation, where populations descended from only a small number of founding individuals. This highlights the importance of stochastic processes in shaping the genetic diversity and distribution of Southern Ocean seabirds.

Overall, the Blue-eyed shag serves as a fascinating model for studying the interplay between evolutionary history, dispersal mechanisms, and environmental factors in shaping species distributions and diversity in the Southern Ocean.

Reference: Rawlence, N. J., Salis, A. T., Spencer, H. G., Waters, J. M., Scarsbrook, L., Mitchell, K. J., … & Kennedy, M. (2022). Rapid radiation of Southern Ocean shags in response to receding sea ice. Journal of Biogeography49(5), 942-953.

Author: Lucas Bastos