Critical constraints to ecological buffering in a rapidly changing Arctic: A long-term seabird study

Long-term ecological studies testing global change impacts are rare yet essential in the Arctic, a region warming nearly four times faster than the global average. These rapid environmental changes reshape ecosystem functioning across the pan-Arctic and affect the livelihoods of local communities. Seabirds are widely recognised indicators of environmental change. Identifying species-specific critical constraints is therefore essential for detecting early warning signals of population decline and ecosystem reorganisation. Little auks (Alle alle), the most abundant Arctic seabirds, are key ecosystem engineers shaping coastal environments through for example food-web interactions and guano deposition. We present a 20-year study (2005-2024) of an East Greenland little auk population, combining long-term demographic monitoring with behavioural, physiological and ecotoxicological data. We quantified fitness-related traits (adult body condition, overwinter survival and chick growth) alongside potential constraints, including parental foraging effort, trophic ecology, winter distribution, and mercury contamination in adults, eggs and chicks. Over the study period, climate-driven ocean warming, sea-ice decline, altered zooplankton communities, and increasing mercury exposure reshaped environmental conditions. Faced with these changes, little auks exhibited marked behavioural and dietary plasticity, allowing them to maintain relatively stable fitness metrics in most years. However, sustained monitoring revealed key constraints that undermine this apparent resilience: (a) Elevated mercury burdens may reduce aerobic diving capacity and thermal tolerance in adults and were associated with reduced reproductive performance. (b) Chick growth was strongly impaired during years with increased summer storm activity, which disrupted adult foraging trips and provisioning rates. Winter storms are also likely to negatively affect adult overwinter survival. Together, these findings show that little auks can partially buffer rapid Arctic change through behavioural flexibility, but multiple interacting climatic and contaminant stressors impose increasing, possibly cumulative constraints on performance. Identifying such critical constraints is crucial for understanding the limits of resilience in wild populations and for improving predictions of ecosystem responses to ongoing climate change.