Long-term monitoring reveals contrasting effects of weather and climate on taxonomic and functional diversity of birds on the East Frisian Islands in the Wadden Sea

Climate change is reshaping biodiversity, yet it remains challenging to distinguish the respective influences of short-term weather variability and long-term climate trends, particularly with regard to functional diversity. We analysed two long-term monitoring programmes in the Wadden Sea: one surveying breeding birds in spring and early summer and the other surveying waterbirds and waders throughout the year. Analyses were conducted separately for each programme to reflect their seasonal context. Across the East Frisian Islands (1996–2021), we combined standardised counts with ecomorphological traits to quantify species richness and diversity (using the Shannon Index) as well as functional diversity, expressed as standardised effect sizes of functional richness (FRic) and functional dispersion (FDis). We additionally computed mean species contributions for FDis and FRic. First, we estimated island-specific temporal trends in these metrics and then related them to short-term weather variability and long-term climate trends using mixed-effects models. Although taxonomic diversity generally increased over time, functional diversity often decreased, particularly in the case of FDis. This is consistent with functional homogenisation despite taxonomic gains. FDis was dominated by a few abundant taxa, whereas FRic was driven by less abundant species with distinct traits. The associations with environmental predictors varied between monitoring programmes: breeding-bird functional diversity exhibited stronger links with short-term weather variability, whereas waterbird and wader patterns were more closely linked with long-term climate trends. Overall, these results emphasise the value of trait-based indicators in long-term monitoring and highlight that functional metrics can reveal community restructuring that is not captured by taxonomic metrics alone. We assess taxonomic and functional diversity change in bird communities on the East Frisian Islands over 25 years. Taxonomic diversity increased in both breeding birds and waterbirds and waders, whereas functional diversity often declined. Declines in functional dispersion indicate functional homogenisation despite gains in species richness and Shannon diversity. Breeding-bird diversity showed stronger associations with short-term weather variability, while waterbird and wader diversity was more closely associated with long-term climate trends. Species-level contribution analyses showed that functional richness and functional dispersion were driven by different sets of taxa.