Unravelling biodiversity change: a multi-scale decomposition of changes in European breeding bird diversity

•Aim: Biodiversity trends often vary across spatio‐temporal scales and are influenced by complex community structural changes beyond species richness alone, such as homogenisation and/or shifts in relative abundances. This study aims to explore how changes in community structure shape temporal trends in richness across spatial scales, offering a more mechanistic view of how biodiversity is changing.
•Location: Europe.
•Time Period: 1975–2023.
•Major Taxa Studied: Birds.
•Methods: We first assessed temporal trends in breeding bird richness at local and national scales for 25 European countries. Next, we applied the multi‐scale ‘Measurement of Biodiversity’ framework in a temporal context to decompose changes in species richness into contributions from shifts in bird density (number of individuals per unit area), Species Abundance Distribution (SAD) and conspecific spatial aggregation. We quantified how these community components drive species richness across a gradient of sampling effort and explored the consistency of emerging patterns across Europe. Analyses were repeated for farmland and forest guilds, as well as across ecoregions within countries.
•Results: Bird richness showed heterogeneous trends among countries at local scales, whereas at national scales we detected general increases in richness. Despite some variation across countries and functional guilds, three general patterns emerged: density and conspecific aggregation dominated richness dynamics at small to mid‐scale, both with effects varying in direction among countries, whereas shifts towards increasingly even SADs and/or larger effective species pools governed broad‐scale trends. Partly, trends in richness and component's patterns displayed contrasting trends across ecoregions within countries, highlighting the need for sub‐national analyses.
•Main Conclusions: This scale‐explicit, component‐based approach elucidates how community structural changes shape species richness patterns across spatial scales. Importantly, it allows disentangling concurrent processes influencing richness, including those that may reinforce or offset one another. By clarifying how communities are restructured through time, such a perspective can guide more targeted conservation actions and help identify underlying external drivers.