Urban gradient resolution matters! Avian diversity patterns in a boreal green city

Abstract Urbanization is a prominent driver of environmental change worldwide, profoundly impacting biodiversity dynamics. While much research has originated from large urban areas in temperate regions, medium-sized boreal cities remain relatively understudied. By following a citywide survey approach, we studied the impact of urbanization intensity gradient resolution on bird diversity in Lahti, Finland, a green boreal city. While one of the urbanization intensity gradients (four-category gradient) showed no significant differences in species richness, the more detailed resolution (ten-category gradient) revealed a decline in species richness within the most urbanized categories, albeit such decline was not pronounced, probably due to the extensive green cover of the city. Species identity analyses confirmed distinct bird community compositions at 50% built cover threshold for both gradient resolutions. Our analyses identified key variables related to bird species richness on the site level, highlighting the positive correlation with tree cover and negative correlations between built cover and noise level with recorded bird species richness. These findings expose the intricate interplay of habitat variables in shaping bird diversity in urban environments. Moreover, the results of this study emphasize the importance of detailed resolutions when assessing urbanization gradients, particularly in less biodiverse regions. Finally, we show the pivotal role of citywide surveys in providing a comprehensive understanding of urban biodiversity, allowing for precise categorization and comparison of conditions within the city. This approach provides valuable insights for urban planning and conservation efforts in boreal climatic regions.