Global macroecology of bird assemblages in urbanized and semi-natural ecosystems

Aim: Despite the increasing pace of urbanization, little is known about how this process affects biodiversity globally. We investigate macroecological patterns of bird assemblages in urbanized areas relative to semi-natural ecosystems. Location: World-wide. Methods: We use a database of quantitative bird surveys to compare key assemblage structure parameters for plots in urbanized and semi-natural ecosystems controlling for spatial autocorrelation and survey methodology. We use the term 'urbanized' instead of 'urban' ecosystems as many of the plots were not located in the centre of towns but in remnant habitat patches within conurbations. Results: Some macroecological relationships were conserved in urbanized landscapes. Species-area, species-abundance and species-biomass relationships did not differ significantly between urbanized and non-urbanized environments. However, there were differences in the relationships between productivity and assemblage structure. In forests, species richness increased with productivity; in both forests and open habitats, the evenness of species abundances declined as productivity increased. Among urbanized plots, instead, both species richness and the evenness of species abundances were independent of variation in productivity. Main conclusions: Remnant habitats within urbanized areas are subject to many ecological alterations, yet key macroecological patterns differ remarkably little in urbanized versus non-urbanized plots. Our results support the need for increased conservation activities in urbanized landscapes, particularly given the additional benefits of local experiences of biodiversity for the human population. With increasing urbanization world-wide, broad-scale efforts are needed to understand and manage the effects of this driver of change on biodiversity.