Eiders, nutrients and eagles: Bottom-up and top-down population dynamics in a marine bird

Area of Science:

• Ecology
• Wildlife Conservation
• Marine Biology

Background:

• The Baltic/Wadden Sea has experienced declining trends in eider (Somateria mollissima) populations.
• Understanding the ecological drivers of these declines is crucial for conservation efforts.

Purpose of the Study:

• To investigate the bottom-up effects of nutrient levels on mussel stocks and subsequent impacts on eider reproduction and abundance.
• To quantify the top-down effects of white-tailed eagle (Haliaeetus albicilla) predation on eider populations.
• To model the combined influence of these factors on eider population dynamics.

Main Methods:

• Long-term study spanning 1978-2016.
• Structural equation modelling to analyze population variance.
• Quantification of nutrient effects on marine food webs and predator-prey interactions.

Main Results:

• White-tailed eagle predation directly impacts incubating female eiders.
• Seawater nutrient levels indirectly affect eider populations by influencing mussel stocks and breeding success.
• A significant portion of the variance in eider abundance was explained by these bottom-up and top-down factors.

Conclusions:

• Eider populations in the Baltic/Wadden Sea are regulated by a combination of direct eagle predation and indirect nutrient-driven effects.
• The observed long-term population decrease is likely due to the additive and interactive effects of top-down and bottom-up factors.
• These findings provide critical insights into the complex ecological dynamics affecting marine bird populations.