Unraveling Biodiversity Change: A multi-scale decomposition of changes in European breeding bird diversity

AimDetecting and describing temporal changes in biological communities is fundamental to biodiversity research and applied ecology. Species richness remains a widespread metric in long-term monitoring, yet it obscures underlying processes since changes in species richness are often only the result of turnover, homogenisation and/or shifts in relative abundances. Furthermore, biodiversity trends and their drivers can vary across spatial scales, demanding spatially explicit approaches. This study aims at unravelling how changes in community structure shape trends in richness across spatial scales, offering a more mechanistic view on biodiversity trend detection. LocationEurope Time period1975 - 2023 Major taxa studiedBirds MethodsWe first assessed trends in breeding bird richness on local (site-level) and national scale for 25 European countries or sub-divisions using linear models. Next, we applied the multi-scale Measures of Biodiversity (MoB) framework in a temporal context to decompose changes in species richness into contributions from individual density, species-abundance distribution, and con-specific spatial aggregation. We then quantify how these components drive species richness from local plots to national extents. Analyses were further conducted separately for farmland and forest guilds, as well as across ecoregions. ResultsThree general patterns emerged beyond variation among countries and functional guilds: Aggregation dominates local richness dynamics, evenness governs broad-scale trends, and density plays an intermediate role. Findings of distinct local and national trends in bird richness agree with previous findings, albeit we find more heterogeneous average trends among countries on local scales. Distinct trends and components patterns vary among ecoregions within countries, highlighting the need for sub-national analyses. Main conclusionsThis scale-explicit, component-based approach reveals how changes in community structure shape trends in species richness from local to national scales. Such mechanistic insights of biodiversity change might enable more precisely targeted conservation strategies and identification of external drivers.