When species are lost but functions persist: a trait‐based perspective on Wadden Sea bird diversity dynamics

Biodiversity faces several global threats and communities are likely to show complex delayed responses. Accurately measuring these lags is critical to properly understand diversity dynamics. Here, we investigated these delays with minimal data availability and found critical mismatches between the temporal dynamics of taxonomic and functional diversity in Wadden Sea bird communities. By analysing 17 long‐term time series spanning an average of 30+ years, we quantified the net imbalance between colonisations and extinctions (NICE) to measure delays in taxonomic (tNICE) and functional (fNICE) diversity. Our approach used empirically measured (trophic and morphological) traits, and in parallel, traits inferred through diffusion maps, allowing to quantify species traits only based on their co‐occurrence patterns over time. First, we found that diffusion maps are a relevant tool to quantify species traits when trait data are scarce. Moreover, we found that, while initial colonisations outnumbered extinctions, the taxonomic balance shifted dramatically toward local species losses, deviating significantly from neutral expectations. Yet despite this concerning trend, functional diversity was stable. This stability likely stems from functional redundancy among declining species, temporarily preventing functional changes. However, despite current functional resilience, the on‐going loss of taxonomically distinct species threatens to erode unique functional roles, potentially triggering abrupt shifts and/or collapse in ecosystem functions. Most critically, our study show how seemingly stable functional diversity can mask accelerating taxonomic losses, highlighting the urgent need for multi‐faceted biodiversity monitoring. As global changes intensify, combining taxonomic and functional assessments through advanced analytical methods becomes essential for detecting early warning signals of ecosystem deterioration and implementing effective conservation strategies.