A new study from the Stockholm Resilience Centre reveals unexpected links between the Baltic Sea ecosystem and land-living birds. It was the unusually large population of house martins at Stora Karlsö, a small bird island in the Baltic Sea, that triggered an investigation by centre scientists and a Scottish research group, looking specifically at the origin of the food source of these small songbirds.
The researchers sampled feathers from newly hatched chicks, nesting on the lighthouse of this seabird colony, and performed a stable-isotope analysis.
This type of analysis shows the chemical composition of different chemical elements, and can be used to track the origin of the food eaten. The results surprised the scientists:
"We found that the protein source of these land birds was 100 % of marine origin," says centre scientist Jonas Hentati-Sundberg.
"We compared the isotopic signal with fish-eating seabirds breeding on the island and found that the carbon signal was identical, but very different from land birds breeding on the nearby island of Gotland."
Birds good indicators for change
The house martins appear to feed on small insects that are hatched on the sea bottom surrounding the island. A possible explanation to the extreme concentration of house martins is the large and growing seabird colony on Stora Karlsö which every day brings tonnes of fish back to the island, thereby enriching the nearby waters with nutrients.
"This is a unique example of a trophic cascade between ecosystems in the sea and on land"
Henrik Österblom, co-author
Links between land and the sea as well as between different spatial scales (e.g. the Baltic Sea and small islands) are intuitively obvious, but science and management has often focused on single scales, which may mask important dynamics in social-ecological systems.
"Understanding cross-scale effects may be increasingly important as human impact and dependence on ecosystem services are growing. Birds are known to be good indicators for change in ecosystems and this study from the island of Stora Karlsö may also contribute in establishing the importance of birds linking together different ecosystems,"
Common Guillemots (Uria aalge) on Stora Karlso. After Yellowstone National Park in the US, Stora Karlsö is the second oldest natural reserve in the world, first and foremost known for its abundant bird life, in particular the many colonies of breeding guillemots and razorbills. Photo: E. Kylberg/Azote
Cross, A. D. P., Hentati-Sundberg, J., Österblom, H., McGill, R. A. R., Furness, R. W. (2014), Isotopic analysis of island House Martins Delichon urbica indicates marine provenance of nutrients. Ibis, 156: 676–681. doi: 10.1111/ibi.12150
Jonas Hentati-Sundberg is a PhD student at the centre. His research focuses on the social-ecological dynamics of marine system, with particular focus on fisheries.
Henrik Österblom is Deputy Science Director at Stockholm Resilience Centre. He is also joint theme leader for the centre theme Governance and ecosystem management of coastal and marine systems.
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