Long-term monitoring programs reveal the impacts of weather and climate change on the functional diversity of birds on islands

Climate change is altering biodiversity, but the relative importance of short-term weather changes and long-term climate trends remains unclear, particularly regarding their impacts on functional diversity. In this study, we compare trends in species and functional diversity from two complementary long-term bird monitoring programs in the Wadden Sea. One program focuses on breeding birds in spring and summer, while the other focuses on waterbirds and waders throughout the year. We combined abundances from each standardised bird observation dataset with ecomorphological traits to analyse the spatio-temporal variation of functional diversity across six East Frisian Wadden Sea islands over 25 years. While species diversity increased over time in both datasets, functional diversity declined, suggesting progressive functional homogenisation despite taxonomic gains. Functional diversity dynamics of breeding birds was more strongly associated with short-term weather fluctuations, whereas long-term climate trends had a stronger influence on the functional diversity patterns of waterbirds and waders. Our results demonstrate that long-term climate change and short-term weather dynamics impact different aspects of biodiversity depending on the species' life history and the monitoring design. We show that functional metrics may reveal ecological restructuring processes that cannot be detected by species counts alone. We emphasise the importance of trait-based biodiversity monitoring approaches and suggest that functionally explicit indicators can provide valuable insights into the resilience and vulnerability of ecosystems in the context of climate change.